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Chapter Three

The Audio Patents

By late 1928, Standard Telephones & Cables had certainly had their worth out of Alan Blumlein. He had applied for seven Patents during his time with them, invented an entire signalling system, improved several elements of their submarine cabling system and designed various electrical circuits. Most notable of these was the AC bridge circuit of course, and all of which would earn ST&C money for many years to come. However, Alan Blumlein was restless, perhaps even bored.

There is no way of knowing now, at this distance, exactly when and why Alan Blumlein made up his mind that it was time for a change of direction and career. Suffice to say that, by the end of 1928, he felt (even if his employers did not) that he had gone as far as he considered he might within the field of telegraphy. What was needed was a new challenge, something that would stimulate his mind. As luck would have it in early 1929 such an opportunity came along. Blumlein took his chance, changed career and never looked back.

Early in February 1929, Alan Blumlein arrived at the offices of Isaac Shoenberg, General Manager, The Columbia Graphophone Company in 'Petit France', London, for a job interview. Shoenberg had known about Blumlein for some time, first hearing of him during his days at Marconiphone (who developed Columbia's range of domestic receivers and gramophones) when Blumlein was making quite a name for himself as a bright young circuit designer in telegraphy and telephony circles. Though Shoenberg had heard that Blumlein had expressed an interest in furthering his career in a different field, he was not aware of his growing impatience to leave Standard Telephones & Cables and 'get on' with some new direction in his life.

At the age of 25, Blumlein had in fact decided some time before this that he needed to change and move on to new challenges. He felt that he had exhausted all of his options within the field of telephony and submarine cabling. He had (even) applied for several positions, which he hoped were available. These were mostly with companies that dealt with the relatively new science of recording sound. Writing in one letter, he explained "I believe my ability lies more in a sound understanding of physical and engineering principals (sic) than in the knowledge of telephone engineering obtained by meticulous reading"

When Shoenberg had been promoted to General Manager at Columbia in January 1929, he had been given the task of finding a way, any way, of overcoming the Bell Telephone Laboratories Patent held by Joseph P. Maxfield and H. C. Harrison. This was incorporated within the Western Electric semi-mechanical recording system (first introduced in 1925, the Patents were Maxfield/Harrison/Bell's and Western manufactured the recorder/cutter), which imposed a royalty of between 0.875d (pence) and 1.25d on every record that was cut using it. In 1929, the combined sales of The Gramophone Company, and The Columbia Graphophone Company, were just over 30 million units. Every one of these had to have a royalty on it paid to a rival manufacturer, so it was imperative that a way be found as quickly as possible to overcome this.

In fact Columbia had not one, but two high-priority projects that were now the responsibility of Shoenberg. The first was the overcoming of the Western Patent and the development of a moving iron recording cutter, and the second was the development of a high frequency, high quality microphone. As Shoenberg took stock of the situation, he became aware very quickly that if he did not recruit some new engineers, the tasks simply would not be completed. Of course, what he needed most of all was a top-flight engineer, someone that could tackle the entire project. The question was who?

Blumlein was not the only candidate that Shoenberg interviewed. There were in fact two to be considered; the job had been originally offered to one of Blumlein's colleagues at ST&C, E. K. Sandeman, for whom Blumlein and J. B. Kaye were working at the time. Sandeman however, while attending the interview with Shoenberg, had also been offered a more attractive position with the BBC and so turned down the opportunity of working for Columbia. He did, however, inform Shoenberg about Blumlein's interest, pointing out to him that, "He will be the most expensive, but he will be the best". It was almost certainly Sandeman who, returning to ST&C following the interview, told Blumlein that the position at Columbia was available and that he should apply for it.

Shoenberg, in the meantime, approached Eric Nind for information. He was one of the new young men at Columbia and had been one of the students that Blumlein had helped when he had been an assistant demonstrator at City & Guilds. Nind remembered Blumlein as a "Delightful man, very human indeed, and very good at explaining anything. He didn't get exasperated if you did not understand, and would go through a point again and again. He had plenty of inexhaustible patience and a great facility for converting quite complicated mathematics into very simple circuit elements". Shoenberg was convinced and invited Alan Blumlein to attend an interview.

Figure 3.01 - Isaac Shoenberg (later Sir) who would become Blumlein's employer, mentor and friend (Courtesy of EMI)

At his interview, after the usual pleasantries and once Blumlein had been given the essence of the task that Shoenberg needed him for, he was asked if he thought he could do the job. Without any detailed understanding of the magnitude of the project, Blumlein told Shoenberg he was sure he could. Shoenberg then offered Blumlein the position and asked him what salary he required.

Blumlein told Shoenberg that he might not want him after he had said how much he wanted. "How much do you want then?" Shoenberg asked. Blumlein, put on the spot as he now was, named a figure that he felt certain was too much, but probably worth a try. After all, he had nothing to lose. Shoenberg made his decision there and then. "You are engaged", was his classic response. Shoenberg then offered an annual salary somewhat in excess of what he had just asked. Shoenberg had made his point and was sure he had his man. He would not be disappointed in his choice. Blumlein, perhaps realising he could have asked for more than he had - and still got it (none the less) accepted the position, returned to Standard Telephones & Cables to hand in his notice, and say his good-byes to the many friends he had made during the five years he had been there. It was time to move to his new challenge.

The Columbia Recording System

In March 1929, Alan Blumlein joined The Columbia Graphophone Company Limited, reporting directly to Isaac Shoenberg, the man to whom he would report for the rest of his life. Blumlein was ready to face the not inconsiderable task that was ahead of him. Columbia, however, was not the only company that had decided that there must be a way of getting around the Bell Patent. HMV (The Gramophone Company), had also decided that it would invest in a team of engineers to design and build a completely new system. The HMV team was headed by A. W. Whitaker, working on a mechanical cutting system. Shoenberg however was certain that, with the team he was putting together and with Blumlein's expertise, if anybody could produce the right apparatus it was Columbia.

Figure 3.02 - The Works Designs, Research & Development Building at EMI, Hayes (Courtesy of EMI)

The task was ideal for a man like Alan Blumlein. Not only did he possess all the technical and electrical attributes to ensure that the actual day-to-day running of the project would go well, but his insatiable desire to complete the job at hand meant that much of this enthusiasm would rub off on the men working with him. Blumlein was also intensely interested in music, listening to all manner of recordings over and over again. He especially liked Beethoven and analysed the music in a mathematical way as many deep thinking men have done in the past. Beethoven's music lends itself to that form of thought. Though there is absolutely no evidence that Blumlein's love of music had any bearing on his decision to accept the job at Columbia (or that the task he was employed to solve had anything to do with producing better quality music recordings), it is very probable that these considerations did cross his mind at the time.

When Blumlein arrived at the laboratory at Columbia, he found that the electric recording apparatus was rudimentary at best, though it has to be said, not uncommon for the time. It had only been a matter of a few years since recording had made the enormous leap from acoustic recordings, that is, effectively shouting into a horn driven direct-cutting system straight onto the wax record. At Columbia, Blumlein found a capacitive microphone, an amplifier based on a public address system and a moving iron cutter, similar to that which Western Electric were manufacturing.

Figure 3.03 - Herbert Edward Holman whose initial 'H' would become one half of the 'HB1A' microphone (Courtesy of EMI)

Figure 3.04 - Henry Arthur Maish Clark, always known to his colleagues as 'HAM' (Courtesy of EMI)

Figure 3.05 - The kind of recording system that was being used at Columbia in 1929 when Blumlein joined

A team of engineers had been assembled by Shoenberg already and they had made a start on the principles of a system to overcome the Bell Patents, but lacked that electrical expertise that Blumlein now brought. Among his new colleagues were Herbert Edward Holman, Henry Arthur Maish Clark, Peter William Willans, and Geoffrey F. Dutton, all of whom would become engineers, scientists and friends to Blumlein for many years to come. It was almost certainly a member of this team, who had drawn up the provisional designs for the system, dated 1 May 1929.

Figure 3.06 - An early recording system based around a telephone earpiece

Figure 3.07 - The type of carbon microphone being used by Holman in 1929

Blumlein did not at first work on the problem of the cutter. The first record of his work for Columbia appears in a memorandum dated 1 May 1929, showing him working on the several projects which had already been started by Holman and Clark. Among these was a high frequency, high quality microphone project. In June, when the complex construction work on this had been completed, Blumlein turned his attention to the designing of the coils and diaphragms for the microphone and the estimation of stray capacities. Herbert Holman did much of the mechanical design for the construction of the first prototype system which was eventually to be given the designation 'MC1A' or Master Cutter 1A (Holman, who was Blumlein's senior by eleven years, was a first rate mechanical engineer who had been educated privately and at Redhill Technical School. He been employed by Columbia in February 1924, to develop a simple form of moving coil microphone, and was therefore ideal for a project such as this). Recalling Blumlein's arrival at Columbia, Holman said "In 1929, we were joined by A. D. Blumlein, a young engineer of whom I cannot speak too highly. I was closely associated with him in many projects, and his capacity for original thought, coupled with a thorough knowledge and appreciation of basic engineering principles considerably impressed us. Blumlein's abilities as a circuit designer here came to the fore; equalisers not only had to produce an ideal response curve, but had also to provide for wide deviations by simple strapping of terminals if such were required. This was a characteristic of A. D. B., nothing must be left if an improvement could be made. He spared no effort himself and inspired all those who worked with him"

By August when test runs were first done, Blumlein was definitely working on the cutter project, but the results, which had been carried out using a Western condenser microphone calibrated by the National Physical Laboratory (as a standard), determined that there was a pronounced peak at 100Hz and a dip around 1kHz. On 27 August, the first comparative tests were made with the prototype system rig and the Western system in wax. Blumlein and Holman noted that there did not seem to be any discernible difference between the two systems.

At the same time as Blumlein and Holman were labouring away with their prototype system in the laboratory in Hayes, Columbia had sent Eric Nind to work in Tokyo, Japan, with one of the systems which had been licensed to the Japanese record label 'Nipponophone'. These cutters were of the Western Electric type which Louis Sterling, Chairman of Columbia, had been very keen to licence as soon as Western and Columbia had agreed a contract earlier in 1929. The Japanese, at Nipponophone, had been among the first to receive a system because of this. Eric Nind had been sent to Japan to oversee the installation and act as an on-site guide for the Japanese recording engineers. Nind was also encountering problems associated with cutter overload, he reported that the amplitude of the cut was not proportional to the signal, complaining that low frequency waves became 'peaky' and that Japanese music seemed particularly difficult to record.

Columbia decided that Blumlein should try to solve this problem also and a series of measurements were carried out, from which it was discovered that there was indeed considerable non-linearity, giving rise to this peaky waveform, and probably made worse with the Japanese practice of recording at very high levels. The waveform of the Japanese records (which were regularly being sent back to Columbia for comparison) was analysed by Blumlein's team under a microscope. From the Western Electric cutter in the laboratory, as well as Eric Nind's recordings made in Japan it was discovered that, when these were compared with recordings made on the prototype system on which the team had been working, the Columbia cutter, with a sine wave input of 375Hz (roughly the frequency of the bass resonance for the system), the device was producing 150% second harmonic and 100% third and fourth harmonics. The Western cutter was only producing 25% second harmonic and 5% third.

On 30 August, Eric Nind recorded another Japanese record to compare it with the Western system, but again once it reached Blumleins team and was compared to their new rig in the laboratory, they had to concede that the Western system was better. In Blumlein's opinion, these problems were arising from at least two causes, the first being a reduction in the reluctance of the field magnetic circuit as the armature moved from its rest position which made the cutter more sensitive. In an extreme case, if the cutter was overloaded for example, the armature could adhere to the pole piece; indeed Blumlein had noted that this too was happening with the Western system.

Secondly, the rubber diaphragms, which were used to damp the bass resonance frequency of 375Hz, were also causing non-linearity problems. The armature was controlled by springs which had been shaped to increase the restoring force more than linearly with displacement. This was intended to compensate for the rise in field strength and, despite several attempts by Blumlein who carried out a series of experiments to try to increase this effect, he had little success. Finally, in an attempt to gloss over the problem, Blumlein designed a circuit that would weaken the field as the signal increased. This did help during loud passages of music, but the effect could not be brought into action in less than 1/20th second and so did not help with transients.

In September 1929, frustrated with the lack of success, Blumlein took stock of the problem, and reconsidered the fundamental elements of what they were trying to do. The first decision that had to be made was the principle on which the Columbia system might be made to work. For the first four months, Blumlein had worked with the apparatus that he had found upon his arrival at the laboratory. Now he decided upon a completely new approach; they would attempt to make the recorder/cutter work using electromagnetic damping.

Blumlein, with his knowledge of electronic circuits had been aware, as had many before him, of the potential of the moving coil. Indeed, only a few years earlier in 1925, Rice and Kellogg had produced the world's first moving coil loudspeaker, where the speaker cone itself was moved in and out due to the action of an electrical current upon a moving coil. The current acting on the coil was directly representative of the amplitude of the frequencies being fed to it. If a moving coil could be used in such a manner to drive a loudspeaker, could not the same principle could be used to drive a suspended cutting head (through a master recording)? Certainly nobody had done anything like it before, which Blumlein found strange; the technology was neither new nor unknown.

In order to decide whether it would work, Blumlein first had to detail the method by which the Bell Patents worked. The Bell system, which Western Electric was manufacturing, used a balanced armature arrangement (for the cutting technique). This produced a satisfactory cut in the groove of the master recording from which the pressings would be made, but Blumlein had already seen the results of the armature's reciprocal action on the depth of actual cutting head. This was difficult to control and that the direct attenuation of the cut was damped with a totally mechanical action. In other words, the cut made in the groove did not directly reflect the amplitude of the audio that was being recorded. Instead, a uniform cut produced what can only be described as an adequate, if rather un-dynamic recording, due to the main resonance of the cutting head itself.

Early in September 1929, the team gathered in the laboratory and Blumlein explained to them the basis of his decision to try electro-mechanical damping to see if they could make it work. Having decided that a moving coil transducer system would not only overcome the Bell Patents, but would almost certainly produce a better recording (as the movement of the cutter would more accurately represent the audio). Blumlein now received the go ahead from Shoenberg to start engineering a recorder as this was the first critical element of any new system.

Blumlein, who had by now been placed in charge of the entire research and development team for the project, designed and then produced the circuitry, with Holman mainly responsible for producing any electro-mechanical apparatus required to construct the recorder. The records from the archives give us a reasonable account of the week-by-week, and on occasion, day-by-day activity during this period, and the problems they encountered. The first of these was the power-to-weight ratio. A coil that generated enough power to move the cutting stylus precisely enough and yet capable of moving through the wax master disc, would be so heavy as to be impractical. Blumlein came up with the idea of having a wound coil enclosed completely in a solid aluminium casing (being the most lightweight yet durable material available at the time), with a separate multi-turn signal coil rotating in a strong magnetic field from the currents induced into it from the audio. These currents in turn induced currents in the signal coil which, when passed through the driving impedance would lose power and thus provide the necessary damping action upon the cutting head.

That September, significant progress was made and Blumlein's original notes give an insight into his train of thought. Dated 11 September 1929 and entitled 'Recording Levels', the notes reveal some of the calculations that were being made. The next day the varying fields of the Western Electric system were recorded as cutting the bass by 4dB at 100Hz. On 24 September, they were working on the automatic field control of the Columbia cutter. Henry Clark (always known to his colleagues as 'ham' Clark, not only because his initials were H. A. M., but also because he happened to be a leading light in the ham radio field) recalled how Blumlein's arrival and work at Columbia had changed all their lives "I had the good fortune to work as A. D. Blumlein's assistant and our experiments spread into the field of moving coil microphones and cutters, the main object of which was the reduction of non-linear distortion present in all the then known moving iron devices. H. E. Holman was a tower of strength in all these projects as he was responsible for most of the mechanical design. In these early days I learned that the overall performance of even such complicated devices could be calculated in advance before the device was made. Blumlein's wrath when the measured resonant frequency of a new model did not match the pre-calculated one is still a vivid memory"

By 3 October, more test records were being cut with the auto field control now working on the Nipponophone system and, on the 18 October, Blumlein produced a large scale drawing of a representation of the 'Columbia Field System', with a series of calculations on the following pages which show the predicted figures for the probable field density. On 25 October 1929, the notes refer to the fact that a version of the Columbia recorder had arrived in Japan for Eric Nind to carry out comparison tests. Problems were also being encountered with overloads and non-linearity. The work by now had moved away from straight comparison tests with other systems such as the Western Electric cutter, to direct non-linearity analysis of the Columbia system. Originally the axis of the moving coil had been constructed horizontally and the field was provided by a single coil. However, to cut wax, it was decided that this might not be sufficiently strong to give an adequate performance at high frequencies, so Blumlein, while calculating the main characteristics of the cutter himself, gave the design of the field control to Henry Clark.

Working on the basis of a field flux density of 10,000 lines/sq.cm, (though a lower field of 6,000 lines/sq.cm was also considered), 'ham' Clark did the necessary calculation and found that the power dissipation would indeed be too high for a single coil. They determined on several alternative arrangements before the eventual double-coil system with two parallel coils on a 'U' shaped core, which was tilted upwards at an angle of about 22.5° degrees was adopted. This immediately led to the realisation that there would be a significant reduction in the moment of inertia of the moving coil assembly, mainly from the reduction in the length of the stylus bar, because they had effectively changed from a horizontal to a vertical axis.

Much of the design work and construction was now carried out by Holman who had decided that the moving coil should have a hard steel bar along its axis. In this, two knife-edges were formed which rested on two hard steel plates, which in turn gave low friction and, equally important, striction. Steel wires in torsion held the knife edges onto the plates and absorbed much of the thrust. The stylus bar was then held in a slot at the lower end of the axis rod. With a cork shim of 0.003 inches thickness between the stylus bar and the coil axis with clearances around the clamping boltholes to avoid direct metallic contact between the two components.

In early November, the first manuscript was drawn up of the results of this work and the analysis showing how the rubber damping was holding up with the application to the balanced armature. The tests were to continue from 29 November 1929, through to 6 February 1930. There was still a lot of work to be done, though by now the team was reasonably satisfied with the results they were obtaining. The effective mass of the recorder at the stylus was calculated at 22 ounces, whereas the Western system was only 12 ounces, which meant that a totally new way of suspending the cutter would have to be found.

These matters aside, Blumlein felt by 10 January 1930 that sufficient work had been completed to write a comprehensive description of the cutter and it was this that would eventually form the basis of the Patents which came later in the year. On 17 January, they completed the first run of the cutter and on 20 February 1930, Blumlein and Holman were again testing the impedance of the Western Electric cutter but now with their own system working well for comparison. Blumlein decided that the change in the axis of the moving coil from horizontal to vertical was sufficient to rename the apparatus MC1B and completed this version in the first week of March. He also introduced a much larger field coil which of course made the entire design much heavier than the MC1A with a higher centre of gravity, but this did allow the cutting head to cut a uniform groove in the wax master. By 5 March, they were engaged in simulation impedance tests and on 8 March 1930, continuing through to 11 March, when modification were made, they were concerning themselves with designing the transformer for the system. These modifications were also carried out on the Japanese system on 10 March, to give it a high frequency boost.

Blumlein's attention now turned to the amplifier. It had originally been a low-power example which, while it gave reasonable equalisation and had a good smooth frequency response, had a definite spike and dip at around 3.5kHz. It was discovered that a small resonance in the cutter was being caused by the bending of the steel shaft which was suspended at two points and which formed the axis of the moving coil. It seems that the shaft was unable to vibrate in a whirling mode. So the balance conditions for the shaft were carefully calculated, the problem being solved by replacing the steel nuts which had been used to hold the stylus arm in place with platinum ones (these being more flexible). This modification went some way toward solving the frequency spike problem in the amplifier.

In April a new push-pull, class A amplifier was devised using GEC transmitting triodes (DEM3s), running at 1000 volts on their anodes each at 250mA current. This was known as the '500 Watt' amplifier and the output was transformer-coupled via an equaliser to the cutter. This work on the coupling of the amplifier to the cutter is one of Blumlein's most impressive achievements at this time and it illustrates his incredible grasp of the complexity of the situation and its solution. High frequency boost was inserted between the driver and the power amplifier, which meant that at high frequencies the coil was being driven at a constant velocity. This, in turn, meant that the driving force required was now proportional to the frequency being applied. By inserting high frequency equalisation at the input to the power stage, low frequency level was kept low in an attempt to minimise microphony, which was a major problem with triode valves at that time.

When the first trial cuts using the MC1B were made, the team found that it had a constant resonant frequency of around 250Hz, exactly the frequency that had been predicted by Blumlein. Further tests showed that temperature runs were satisfactory and, although the performance of the system seemed to be acceptable, Blumlein was still not satisfied and continued to return to various elements of the system, constantly adjusting them until he was happy.

On 26 June 1930, Blumlein wrote to Peter Willans, the Head of the laboratory, and suggested to him that they should manufacture another six cutters each with 100 Watt amplifiers. He also pointed out that extensive tests still needed to be carried out involving perhaps two or three other amplifiers of differing power outputs, as well as a series of recordings made to ascertain the results of oriental music, dance bands as well as classical music. This was to ensure that the amplifier could maintain a sufficient power margin for all musical genres. He drew up a flow chart for the process of manufacture and pointed out that, although the cutter was now satisfactory, more development would need to be done on the floater. Blumlein had calculated that the power needed to drive the Columbia cutter was 2.26 times as high as that of the Western Cutter at low frequencies and 22.6 times as high at high frequencies.

Though much of the electric circuitry is easy to attribute directly to Blumlein himself, as is the original idea for using electro-magnetic damping in the system, much of the mathematical calculation such as the reluctance of the magnetic circuit and the resistance and movement of inertia of the various designs of the moving coil was carried out by Clark and other members of the Columbia team. Blumlein did however work out the impedance of the wax to enable the required damping to be calculated, and it was only his and Holman's names who appeared on the Patents for the new system when they were applied for in March.

The Columbia Recording System, Patent No. 350,954 & Patent No. 350,998

The new system had been built, tested and had been shown to work. Now it was presented to Shoenberg, who had been keeping a close eye on the progress that had been made by the team he had put together. The prototype MC1B would become the basis of the new Columbia Recording System and, as it overcame the Western Electric system with a totally new concept of electro-mechanical damping, records cut from it would not have to pay a royalty. The system would save Columbia a fortune and at the same time earn them a considerable income. The tables had now been turned; any records that were cut on the new Columbia system would mean that a royalty would have to be paid to them. Shoenberg was delighted.

Patent No. 350,954, and No. 350,998, were Alan Blumlein's first for his new employer, but his eighth and ninth overall. They both date from Monday, 10 March 1930, when the applications were lodged with The Patents Office and are entitled 'Improvements in Electro-mechanical Sound Recording Devices more especially of the Moving Coil Type' (No. 350,954), & 'Improvements in Apparatus for Recording Sounds upon Wax or other Discs or Blanks.' (No. 350,998). It was just one year after he had started with Columbia and gives some indication of the pace at which he could work.

There are 34 'claims' (a claim within a Patent being that which the inventor wishes to be considered as entirely new compared to anything which has previously been invented), made by Blumlein and Holman in Patent No. 350,954, the outline of which is as follows: "The Patent describes a moving coil system in which the cutting tool is connected to the drive motor by a linkage system comprising, or kinetically equivalent to, a shaft inclined or perpendicular to the wax surface. This shaft extends down from the motor and stylus arm assembly placed perpendicular to the shaft. The recorder is comprised of an electric coil mounted so as to pivot in a magnetic field about an axis in its own plane. The coil can then move upon the induction of an applied electric current by means of one or more adjacent electric coils. These are mounted about a common core, which forms the primary and secondary windings of a transformer.

"The magnetic flux of the coil, and the same of the transformer, are arranged so that they are at right angles to each other. The moving coil itself was to be cut from a solid block of suitable material such as aluminium. This coil would be substantially triangular in shape the sides of which would increase in width and thickness as the pivotal axis is approached. The steady magnetic field of the moving coil is achieved by providing magnetic pole pieces adjacent to the coil at right angles to the transformer core. In this way, the air gap within the moving coil assembly is reduced to a minimum, compatible with the free movement of the coil itself. The neutral position of the moving coil is maintained with a suspension system, locating the core by means of springs giving an elastic-like connection"

Figure 3.08 - Detail of Patent No. 350,998 (1930) The Columbia Recorder.Cutter designed by Blumlein and Holman

An additional 48 claims are made in Patent No. 350,998, again the outline of which read as follows: "The Patent describes an electro-mechanical device for the recording or reproduction of sound records in either wax or other such discs or blanks. The device is also however applicable to any other moving coil device such as a microphone which could be employed in a sound recording system, or also of a moving coil pick-up or moving coil loudspeaker. It could also be used for optical recording devices such as those employed in the cinema for sound recording and reproduction.

"The moving coil element is suspended in a magnetic field and connected to an external circuit and caused to vibrate by mechanical means. This in turn generates an electromotive force (EMF) across the coil, and will cause a current to flow through any external circuit. This current, will in turn, produce a Back EMF which will tend to modify the movement of the coil. If the impedance of the loading circuit is made frequency selective, the Back EMF so produced will also be frequency selective. This can therefore be easily optimised to damp any natural mechanical or electrical resonance within the type of electro-mechanical system herein. In order to get the greatest damping effect from the application of this principle, three factors are necessary; first, that the impedance of the moving coil circuit should be as small as possible (for a given configuration and numbers of turns, hence the DC resistance), second, that the purely electrical impedance shall be as pure resistive as possible, and lastly that the magnetic flux density in the moving coil shall be as high as possible.

"If efficient power transfer is to be obtained between a moving coil and the circuit within which it operates, it is necessary that the impedance of the operating circuit be of the same order as the electrical impedance of the moving coil to which it is connected. But, in order that the movement of the moving coil itself may be well damped at frequencies close to its resonant frequency, it is necessary that the impedance connected to the moving coil be as low as possible. The basis of the invention then, is to connect a moving coil to an operating circuit whose impedance is comparable with the electrical impedance of the moving coil, at frequencies where an efficient transfer of power is required, and thereby obtaining easily controllable electrical damping"

On 30 August 1930, the circuits for monitoring and driving the moving coil cutter were completed. Arrangements were made for waxes of 22 titles to be cut using both the Western Electric and the Columbia cutters simultaneously; the signals to both being fed from a Western CT microphone. The recordings, all of which were of popular singers, included Will Fyffe singing 'Daft Sunday', The Four Bright Sparks singing 'That Night in Vienna', and Billy Elliot singing 'Home is Heaven'. The resulting pressings were listened to by a number of people who were asked their opinion as to which they preferred, though there is sadly no information as to the listening conditions under which the tests were conducted. In the end, there was no strong preference for either, though there was a small majority in favour of the Columbia cutter.

Further trial cuts were made with reduced high tension voltage on the DEM3 valves in the power amplifier and it was on this basis that Blumlein had calculated that 100 Watt amplifiers would be adequate. Somewhere along the line however something went amiss and the amplifier that was eventually used had a single DEM3 triode and ran at 250 Watts. Blumlein by now was concerned about the cost of all this development and, in his notes, made comment of his desire to modify the equaliser between the amplifier and the cutter to give greater efficiency and so reduce the power needs. For a single amplifier the components had cost £92-6s-9d, the HT generator £46-19s-2d, and sundries (not listed) £13-15s-1d; a total of £153-0s-4d, quite a considerable sum in 1930, and not far off an annual wage for many of the engineers working on the project.

An Entire System Evolves

Having produced the first design details of the new recording and reproduction system as early as October 1929, much of the year following was spent producing first, a prototype (February 1930), and later, production models initially for Columbia to use itself and later to license to others. It soon became obvious that the accurate control of electrical damping, coupled with electro-acoustic microphones and electromagnetic loudspeakers could indeed produce very high-fidelity recording and reproduction sound systems. Part of the resonance of an electro-mechanical system, such as the Columbia cutter, depended upon the elasticity of the needle and the record surface initiating a vibration, resonant with that of the effective mass of the whole device. If this was not controlled, unwanted resonance could occur, especially at the moment of inertia. Blumlein's next Patent, No. 361,468 (Improvements in Sound Reproducing and Recording Devices), applied for on Friday, 12 September 1930, was designed to overcome this by taking into account the pivotation of the gramophone needle relative to the remainder of the assembly, in this case the recording apparatus as previously described.

Figure 3.09 - Detail from Patent No. 361,468 (1930) Co-written by Blumlein and Willans

The Patent, which was co-written with Peter William Willans, and initially appears to be simple in design, though it does for the first time in a Blumlein Patent, resort to mathematics. Blumlein's mathematics ability, as has been documented, only went as far as limited calculus and, while the mathematics in the Patent are not overly complicated, it is likely that Peter Willans was responsible for most of it.

"The invention describes the mounting of an electromagnetic pick-up on a rigid body (which Blumlein and Willans call an 'armature') which is rotated about a given axis. Attached to this is a lever, on which is mounted the needle. The needle point is at a given distance from the axis of rotation of the armature, and is adapted to receive impulsive forces in its direction of displacement. According to the invention it is desirable to mount these members in such a way that the centre of rotation of the combined needle and armature assembly lies on the axis of rotation. Assuming that the needle is fixed perpendicular to the axis, the following applies:

I1 = The moment of inertia of the armature about the axis of rotation

I2 = The moment of inertia of the needle about its centre of gravity

m = The mass of the needle a = The distance between the point and the centre of gravity of the needle

x = The distance of axis of rotation from the centre of gravity of the needle (on the side remote from the needle point (right))

The desired objective is achieved if: Equation 3.01

"In accordance with this equation, the needle can be fixed in such a position relative to the armature, that the value of x corresponds to the value calculated (by this means) from the known or measurable quantities of the right hand side. The net result of the invention is a sound recording (or reproduction) device where the disposition of the various operating elements about their rotating axis is so arranged that the upper resonant frequency of the device has a maximum value. As defined here, the device comprises a rigid member adapted for rotation about a given axis, and a second member rigidly connected to the first in order to receive impulsive forces at an extremity remote from the axis of the first. In this combination, the instantaneous centre of gravity of the assembly lies on the axis of rotation. In this way, an electric pick-up in which the distance x from the axis of rotation of the operating system, to the centre of gravity is calculated from the equation above"

The next Patent that Blumlein applied for, No. 362,472, deals with the idea that a system such as the Columbia recorder needed to produce predictable attenuation characteristics across the entire frequency range of the disc being recorded. This attenuation had to be carried out regardless of the number of input signals being transmitted, or whether they were balanced or un-balanced lines. Only in this way was it possible to produce a uniformly recorded sound. Blumlein designed one of his most important circuits, one that would have far reaching uses then and is still used now, for the Patent describes what is known today as 'The p-line attenuator'.

Applied for on 30 July 1930, Patent No. 362,472, 'Improvements in Electrical Transmission Devices', outlines an attenuation device that consists of a number of networks constituting an artificial line of any type, balanced or un-balanced, which can attenuate electric currents of all frequencies uniformly. The device would have one or more sections consisting of a network of predetermined frequency characteristics, together with sections of uniform attenuation with frequency, in order that a given frequency preference or selection can be obtained with a uniform variation of attenuation over all the frequencies.

The degree of frequency selection, or preference obtainable, could furthermore be varied at will by the operation of a moving contact switch. These sections, with their given frequency characteristics and definite impedance relationships, were to work over any of the known equaliser networks and were to have constant impedance characteristic of any wave filter provided they were sufficiently constant over the transmitting frequency to work in a system such as the Columbia recorder.

The invention was further flexible enough to allow it to work with one or more sources to feed the same load, in such a way that control over the relative attenuation and impedance relationships between the various loads could still be controlled. In this way the efficiency of transmission of an electric potential from one piece of apparatus to another by means generally referred to as an 'artificial line', can be improved and varied in a known and defined manner.

Essentially what had been invented here was a method of transmission of electrical potential where the input to the system gave a constant impedance at the generator or the load, regardless of how many or how few terminations were present. This meant that a constant volume could be controlled using a minimum of moving contacts, thereby reducing noise and effectively becoming more efficient. The system has become known as the p-line attenuator and its use as a constant impedance volume control in professional audio equipment is now commonplace.

At around this time, late summer 1930, Blumlein began the relationship that would eventually lead to his marriage. His mother, Jessie was giving up her flat in Linden Road (just off Muswell Hill Road) and going to see her relatives in South Africa and wanted to store a rather nice Bechstein piano. Alan had, some 20 years before, been a pupil at Gothic House School and went to see Miss Chataway, the headmistress and owner of the school to see if she would be able to store the piano at Gothic House or, at least be able to use it. Miss Chataway did indeed want a piano for the school and so asked one of her teachers, Miss Doreen Lane, who played a little, to come with her to the Blumlein residence to see the piano and to try it out. Evidently Alan Blumlein must have been at his mother's flat during the visit and was introduced to Doreen who must have made quite an impression on him.

Some years later, Doreen recalled the events of those first meetings: "As a matter of fact I played very badly, but I used to play for the children you know, and she wanted me to go with her to see this piano. So, we went up, though we didn't see much of the piano, but had quite a nice evening, and Alan sat with me while Mrs. Blumlein and Miss Chataway chatted away, and he took some pipe-cleaners, and made me three little dogs. And one was the father, the mother, and the co-respondent put behind. That was Alan's sense of humour; great sense of fun you know. And that was that. We said we would have the piano"

A few days later, Blumlein called on Miss Chataway at Gothic House, on the basis of arranging something about the piano and, while there he asked after Doreen: "Well, a few days after, we were having dinner that night, and Miss Chataway said, 'There's Alan in his car again', so in he came, and he chatted away and said, 'I've got to post a letter, will you come with me?' So I went, and when we went outside he said, 'I haven't got a letter to post, I only wanted to get you out'. Well that was the sort of start of it.

"Well, then he said to me what I thought he said was, 'Will you come flying with me one day?' I was a very travelsick person, so I though oh, I can put that off, so I said 'Alright', and he said, 'Right. I'll fetch you at 3 p.m.' I said, 'What do you mean?' and he said, 'On Sunday'. I said, 'I didn't say I would come on Sunday, I said one day', 'Oh no', he said, 'Sunday'. So I said, 'Oh well, alright if it doesn't rain'. So, I went back to the others and said 'Pray for rain on Sunday at 3 will you, because I don't want to go up in an aeroplane in the slightest'.

"Anyway, he came, and we went off to Stag Lane; evidently he was a private pilot and he had his licence, and of course it was this little open Gypsy Moth, and we put on these helmets and things, and I thought 'Well, I'll risk it'. I suppose I was so scared of being sick in front of a strange young man. So we came down and he said, 'Will we have tea here?' And I said, 'No, I don't want to stay here anymore'. So, we went off to a hotel in Radlett for tea, and I think that started it. Then we started going out and that's how I met him"

Alan Blumlein was 29, and Doreen Lane 24. Their courtship would last another two and half years before they married on 22 April 1933, with J. B. Kaye as best man. Before that, Doreen would be forewarned about some of Alan's 'peculiarities'. "There was a joke amongst some of his friends, they used to call it 'Blumlein-itis' or 'First Class Mind'. It seems that he didn't want to know anyone who didn't have a first class mind. And, some friends of his, before I married him, said to me, 'Now Doreen, you're going to come up against Blumlein-itis'. I said 'What do you mean?' They said, 'Alan won't have anything to do with anyone who hasn't got a first class mind'; and I did come up against it, but that was him you see, and it was, at times, very awkward, because at times he was unintentionally very rude to some people; he didn't seem to be able to get his brain down to their level". J. B. Kaye, however, was in no doubt that Doreen was the right woman for Alan Blumlein: "I was delighted when I met Doreen because she was a character, and she had a sense of humour, very charming and I thought 'Yes, this girl will be able to keep Blumlein under a sufficient degree of domestic control that will avoid any serious rifts'. I thought to myself there will be a few brick-ends flying about the kitchen, but that's all part of life's rich pageant"

The next Patent that Blumlein applied for was No. 363,627, 'Improvements in or relating to Apparatus for the inter-conversion of Electrical and Mechanical Energy such as Used in Sound Recording and Reproducing Apparatus' (applied for on Friday, 12 September 1930). While the title is rather long winded, it actually refers to a very simple method for exchanging the materials used for damping the various vibrations that occur in devices such as the Columbia recorder with substitutes that would be more efficient. It was therefore, a method of alleviating this phenomenon by modifying the frequency response of the apparatus by substituting the damping material, or inserting suitable material, to reduce the effect of resonance on the whole structure.

In vibratory devices such as recording and reproduction systems, it had been customary to introduce damping in the form of frictional or elastic forces imposed by a material connected with the moving system. In this way, the vibration of the moving system when oscillating (in the manner intended) would be suitably controlled. Blumlein called this the 'authorised mode' of damping. It was also possible therefore that unwanted movements such as vibrations within the moving system itself could be present (caused by inherent flexibility in the supports of the system). Blumlein called these the 'unauthorised modes' of damping.

Where damping was introduced into a system (whether the damping force was electric or electromagnetic), it would quite usually happen that these forces would be completely ineffective in suppressing the unauthorised modes. Therefore, the whole rigid structure of the device might have a resonant frequency in the working range at which the lack of damping would cause a resonance of quite undesirably large amplitudes. Worse still, these could occur from quite small forces acting upon it.

Blumlein explains it thus: "The invention described in this Patent relates to devices employed for the inter-conversion of electrical and mechanical energy, and is particularly directed at devices in which parts of the apparatus tend to vibrate at varying frequencies such as sound recording and reproducing systems. Where a damping material is introduced to alleviate the effects of vibration in an authorised mode, it does not follow that this material will have the corresponding effect on vibrations of the unauthorised mode. In fact, as is usually the case, this is what occurs. The object of the invention is to introduce a means whereby the difficulties of damping the various vibrations caused by resonant frequencies, are overcome and sufficiently suppressed so that unwanted amplitudes due to resonation do not occur.

"The invention consisted of a pivoted electric coil, the resonant pivotal oscillations of which are damped electro-magnetically. It was designed to replace the damping methods previously used to offset the effects of vibration, such as cork, rubber compounds and other like materials. The electromagnetic damping method herein, proved sufficient to damp almost all of the oscillations that occur due to the resonant frequency of the assembly"

By the end of 1930, Alan Blumlein and Herbert Holman were tackling the question of the mounting of the cutting head assembly and the consequences that their initial inventions were having upon it. This Patent, No. 368,336 was applied for on Monday, 1 December 1930.

Prior to the Columbia recorder much of the recording that had been carried out was done with the method known as Lateral cutting. This allowed the cutting head to pass through the wax blank thereby cutting a groove whilst being accentuated in a lateral plane by the amplitude of the sound being recorded. The net result of this was that the head would cut the groove deeper and, naturally, there was a reciprocal effect for low volume passages causing the cut of the groove to become shallow. The lateral principle had been used (despite this effect) little changed, since the 1880's, the result of which caused a great deal of unwanted noise and was quite inefficient as a sound recording system.

Blumlein and Holman now proposed a method which was intended to allow the cutter to float in a vertical plane so that imperfections in the waxes would not cause the groove depth to vary to such a degree, but rather only from the effect of the amplitude of the sound being recorded. In so doing, their invention would not only lay the foundation for the record industry that was to follow (right up until the Compact Disc era of recent years), but also the stereophonic sound system that by now Blumlein must surely have been contemplating.

Figure 3.10 - Detail from Patent No. 368,336 (1930) showing of the moving coil cutting head assembly

Figure 3.11 - Detail from Patent No. 368,336 (1930) showing the mounting of the moving coil cutting head from above

Patent No. 368,336, 'Improvements in and relating to the Mounting of Pivoted Apparatus, such as Electrical Sound recording Devices', relates to "The mounting of parts of an apparatus (having motion about a fixed axis), used with electric devices employed for recording sound on wax discs.

"In such a device, the recording stylus, in addition to being subjected to lateral vibratory motion, whereby the sound record is made, is usually mounted so that it may incorporate some vertical movement to accommodate for any irregularities in the wax surface, and thereby maintain a uniform depth of cut. The inertia of the apparatus in response to this vertical movement naturally has a considerable effect on the accuracy and sensitivity of the recording. One of the objects of the invention in this Patent was to overcome previously encountered inertia difficulties and thereby improve the apparatus and to extend the working frequency range to which it will respond"

It consisted of a method of pivoting parts of the apparatus in such a way that the effective forces tending to move about the pivotal axis were reduced. This would also require a method of assembly for the 'floating parts' of the cutter in such a way that the effective inertia at the stylus point was reduced to a minimum. All of this was to be based around a 'U' shaped electromagnet on the arms of which were mounted bulky energising windings. Between the poles of the electromagnet would be the pivoted moving coil on which the recording apparatus, stylus assembly (cutting head) would be mounted.The moving coil would in this way form a secondary winding in the actuating circuit. Being mounted thus in a magnetic field of great strength it would oscillate in a manner suitable for the cutting of a record.

"The moving coil assembly is mounted in a manner whereby it oscillates in an ostensibly vertical axis (this was to allow for any unevenness of the disc surface), however, the movement of the whole device is desired about a horizontal axis. Because of the angular relationship of the electromagnetic circuit to the moving coil assembly, this is in actual fact possible, and the resulting operation is a freely moving, suspended cutting assembly, contained within the control of the electromagnetic field, able to move in both axes.

"In order that the recorder may produce a uniform depth of cut (in an uneven surface of a wax blank), it is necessary that a small increase of depth of cut should produce as large an acceleration as possible of the recorder upwards, or vice versa, so as to restore the correct depths as quickly as possible. This condition is best realised when a line drawn from the sapphire (of the cutting head) to the axis of the floating assembly subtends an angle of 45° degrees to the wax surface. If however a very uneven blank is being used, the forward movement of the sapphire will cause a change in speed. Under these conditions, it is advantageous to reduce this angle to less than 45° degrees, in order that this forward movement for a given vertical movement (into the blank) may be reduced. It was found that angles between 30° degrees and 50° degrees were the most advantageous"

What Blumlein and Holman have described here is a method of cutting a blank record using the assembly they had already Patented, i.e. the Columbia recorder, but having modified the cutting head assembly so that it is suspended in a magnetic field that will allow free movement of the head in the horizontal and vertical axis. In this way they could determine the optimum cutting angle for the assembly to the disc surface, and the angles subtended by the sides of the groove to the vertical, which turned out be around 45° degrees. As time went by, this system was to become known as the '45/45 degree cutting method', and it eventually won over the 'Hill and Dale' system because the lateral movement produced a mono output, the vertical being the 'S' component, thus mono compatibility. Later, it was discovered that mono pickups could be used for binaural recordings provided their vertical compliance was good enough not to plane the groove flat.

Such was the nature of their invention that Blumlein had proposed it would be best suited to a material other than wax, and suggested that cellulose acetate be used instead as the material for the master disc. He also mentions that the cutting tip should be made from sapphire rather than the steel needles which were commonplace at the time; of course this too was eventually taken up as the de-facto mastering material for record cutting. The 45/45 degree cutting system would thereafter be used in record manufacture, almost unchanged from the date of this Patent, to the present day.

Microphones & More

Somewhere in late 1929, just as the basic construction of the MC1A recorder was being completed, Blumlein had the idea that the project for which Shoenberg had employed him had far greater potential than that which he was currently engaged in. Why not, he reasoned, continue the process to design an entire recording system having all the elements of an electro-magnetically operated cutting and recording system, rather than just construct the recorder? If the recordings that were being made were of a better quality to start with, surely it would improve the overall final quality of the finished records pressed from the masters cut on the Columbia recorder.

As early as May 1929, Blumlein and Holman were engaged in conceptual designs for a high frequency microphone that they had worked on in parallel with the Columbia cutter until the end of that year. They appear to have started the work as an investigation into the effect of a diaphragm on a coil, but stopped this to concentrate on the cutter first. By the time they returned to it, another year would have gone by. As we know, Shoenberg had already been given the task of developing a high frequency microphone and it was to this that Blumlein now turned his attention.

Concentrating on the completion of the MC1B first, Blumlein decided to apply the principle of the moving-coil to a pressure microphone as soon as the job was finished, as he felt certain that it was equally important to have a licence-free microphone to pick up the sound. As soon as the Columbia recorder was ready and had been tested and approved, work on the high frequency microphone began again where it had been left at the end of 1929. This was critical to the concept of a complete Columbia system as, in the absence of a Columbia high frequency microphone, the only alternative was the Western condenser transmitter (CT) microphones that they had been using and the use of these would still require royalty payments.

The calculations on a moving coil microphone were carried out in the spring of 1930 and seemed to indicate that a permanent magnet design would have inadequate field strength. The original intention had been to design an electromagnetic microphone which could be powered by car batteries. In fact such a design was tried using Austin Seven batteries from several cars in the car park, but these proved inadequate. Never the less, Blumlein decided to go ahead with the project and had the design for a diaphragm and its surroundings ready by August 1930. The spacing between the centre and outer poles was set by a well-fitting ring of cadmium bronze of low resistance which also helped to damp the moving coil. The diaphragm itself, constructed by Holman, needed to be both lightweight and stiff. This was made from a piece of balsa wood which had been cut into a very thin layer, impregnated with celluloid, with thin sheets of shallow concave aluminium foil waxed onto each side.

The edge of the diaphragm was compressed (it had no balsa wood at this point) and at first the flat edge which had been thinned by etching was simply clamped. This however, caused the edge to wrinkle and gave the diaphragm inadequate compliance. Blumlein decided that the two ridges, which were formed between the clamp and the diaphragm, were too deep, and affected the acoustic impedance of air passages behind the diaphragm. These had been designed by 'ham' Clark, but Blumlein re-designed them, this time with four ridges, which seemed to solve the problem. The coil was made of anodised aluminium and backed onto an aluminium former (which had a slit in it to eliminate eddy currents). This was riveted onto the diaphragm.

The grille for the microphone and the air cavity between it and the diaphragm, and the cavity behind the diaphragm were also designed by Blumlein, who was fully aware that Western had a Patent for a moving coil microphone, meant that air in this cavity could not be used for damping. Instead, Blumlein included a calculated amount of cotton wool to prevent cavity resonance. This had been determined by measurements carried out by Ivan L. Turnbull. Damping of the bass resonance was designed once again to be carried out by loading the microphone from its amplifier so that the input impedance, set by a series resonant circuit which also did most of the equalisation, created the damping.

At this point, Blumlein decided that a feature of all the microphones would be that they should be interchangeable and have the same resonant frequency of 500Hz. This meant that careful control of the diaphragm mass and the stiffness of the edge would have to be maintained, and this work, coupled with the other tests, took much of the remainder of the summer of 1930.

The first prototype microphone that Blumlein was happy with was the result of over twenty attempts at constructing the coil and the surrounding cavity. Blumlein was rarely satisfied unless something was absolutely correct in his estimation and, this particular instance, is a good demonstration of how he would labour away at something regardless of time and energy expended until it was right. Curiously it was also initially called the 'MC1A', which had been the name of a prototype of the cutter. The microphone was named the MC1A because by this time the recorder was called the MC1B. In time however, to avoid confusion, the microphone was called the 'HB1A', named after 'Holman & Blumlein'.

The first HB1A microphone to be completed was received by Blumlein on 4 November 1930 and was tried the next day 5 November, though it gave rather disappointing results. The problem seemed to be that the coil was not moving freely in the gap. Refinements were made to a second prototype with the centre pole-piece being reduced by 0.01 inch, which gave some improvement, though impedance tests with a loaded diaphragm indicated that it was not moving as a true piston, but pivoting about a point near the edge. Blumlein concluded that this had arisen because the tension in the diaphragm edge was not uniform and so a screw tensioning arrangement was added, which became a permanent feature of the design and used to adjust the resonance of the diaphragm to 500Hz. At the same time, despite Turnbull's original calculations, Blumlein decided to double the amount of cotton wool in the cavity behind the diaphragm. The specification for the second model of the microphone was now considered significantly different from the first and was thus called the HB1B and sent away for construction.

At around this time the 'HB's began to acquire other names; nicknames, to which many people, including Blumlein's wife-to-be Doreen, would always refer. They were called 'HB's, or "Hells-Bells", which was a favourite phrase of Alan's when things didn't quite work out for him, usually followed by "Buckets of blood!" Sometimes, 'HB' took on the guise of "Hot and Bothered", because Doreen thought that Alan always became so when he was frustrated, mostly with himself.

With the changes made to the original HB1A-1 (done while the HB1B was being constructed), a series of extensive tests of the motional impedance were carried out through November and into December, to enable the parameters of the equaliser for the 'HB's to be calculated. These tests usually had good results, but it was very time consuming and Blumlein must have become a bit disillusioned with the time it was all taking because he designed a new equaliser with the components arranged in a series of switchable banks, which was then used in yet more tests. Finally, Blumlein was satisfied that the prototype microphone and the new recorder were ready for the first tests. These were carried out on Monday, 8 December 1930.

The first recording made was of a Mr. Sparks playing the piano (reference No. RWTT535/1), followed on the next day by the Van Philips Dance Band (RWTT536). There are no notes of Blumlein's or anybody else's comments, however a frequency run dated 19 December shows very good, smooth and even characteristics. There were further voice only recording tests (RWTT549/1 & RWTT549/2), carried out that day using both the HB and CT microphones for comparison. These were repeated on 23 December (RWTT554/1 & RWTT554/2). The same day (Tuesday, 23 December 1930), Blumlein wrote down a detailed description of the HB microphone and its special features that would enable the draft specification to be drawn up (eventually Patent No. 369,063).

Yet more tests were carried out in the New Year; on 6 January 1931, The Greenings Band was recorded (curiously, the serial numbers RWTT/1 & RWTT/2 seem to have been used, but it unlikely that these refer to the first and second ever recording sessions). Using a bass 'depreciator' (which we would know today as an attenuator) of 0.7dB at 100Hz and 2.3dB at 50Hz, and a high frequency boost of 0.7dB at 500Hz and 2dB at 5kHz. The results of all these tests were noted by Blumlein and incorporated into the first production model of the HB1B microphone and its associated equaliser, which was ready by the end of January. The new microphone had the two new features which Blumlein felt sure eliminated the cause of much of the trouble in the earlier tests; the increased clearance for the moving coil, and a stretchable diaphragm which had involved increasing the size of the diaphragm itself. The profile of the inner surface of the stretching ring governed the size and shape of the cavity outside the edge of the diaphragm, and this would cause a great deal of consternation in the next few weeks during a series tests that were carried out to determine the effect of this.

Often two or three different profiles a day were designed as Blumlein tried to get the optimum shape, the work often going on until late in the evening. It was probably now that the phrases 'Hells Bells' and 'Hot and Bothered', began to come to the fore. In the end, 18 profiles were made with the final design ready in late February 1931. In early March a new problem occurred; there was dissipation in the field-coil which warmed up the microphone chassis and altered the bass resonant frequency. This problem was traced to the alteration in the mass of air vibrating and it was quickly solved by changing the bass resonance setting slightly.

On 20 March, further recording tests were carried out comparing the Western CT microphone with the new HB1A-1 and the HB1B with various 'brightening' adjustments being made. By 29 March, when three discs were cut (RWTT679/1, RTWW679/2 & RTWW679/3), each using the HB1B, the brightening effect of +4dB at 5kHz was found to be "...most acceptable", and in May, further HB1B microphones were received from construction and tested. In this form, the microphone was to be used in EMI recording studios, by the BBC at The London Television Station, Alexandra Palace for many years to come. Curiously the BBC did not choose the HB microphone for radio work preferring the BBC-designed Type A ribbon microphone. Perhaps this was partly due to cost, the 'Type A' being 'inexpensive' at £9-0s-0d, while the moving coil HB1B cost £40-0s-0d. There were later variants all of which were used by the BBC for television work. The HB2, HB3 and HB4 microphones all derived from experiments with the field strength, the final design being the HB4C. Blumlein's notes on microphones end on 16 November 1933, when he made an entry for the flux measurement 'No. 82' of an HB1B microphone, though additional work was carried on (for many years) by Clark, Turnbull, Holman and others.

The company now applied for the specification for the microphone which lists Blumlein and Holman as inventors: Patent No. 369,063, on Wednesday, 13 May 1931, as a moving coil microphone which had a rigid diaphragm suspended in an electromagnetic field. The invention consisted of an electro-acoustic device having a vibrating system comprising of a substantially rigid, piston-like, diaphragm connected at its outer edge to a flexible air-sealing surround.

"The diaphragm is composed of a light wood such as balsa covered on either or both sides by a thin plate of aluminium (or other light metal or alloy). The main resonance of the device is controlled by electromagnetic damping with an electromagnetic coil adapted to move within a magnetic field. The microphone works by the movement of sound waves impinging upon the piston-like diaphragm. In order for this to work the diaphragm should be rigid so that it is free from any resonant frequency due its own flexure, however, the diaphragm should be of sufficiently low mass that it will respond well to the high frequencies acting upon it. Part of the object of this invention was to overcome the conflicting requirements of high rigidity and low mass, this objective would be met if the diaphragm was made of a three ply construction, consisting of two thin layers of metal (such as aluminium), enclosing a centre of light wood (such as balsa).

"If air is permitted to pass freely from the front to the back of such a diaphragm, the response of the device at all low frequencies will be impaired due to the diaphragm receiving almost equal vibrations of sound pressure on both its front and back faces. Therefore it is necessary to provide a closed cavity (or baffle), containing or shielding the air behind the diaphragm and the sides of the air cavity.

"Such a seal in a microphone cannot take the form of a frictional packing on account of the delicate nature of the movements of the diaphragm, therefore a thin elastic surround is provided between the diaphragm and the sides of the cavity. The diaphragm then forms a rigid piston, closing the mouth of the cavity, the edge of the diaphragm being sealed by a thin surround which stops the direct access of air to the cavity, but nevertheless permits the diaphragm to move. In this manner, the surround, which forms the air seal, may conveniently support the diaphragm also.

"At the back of the diaphragm, within the cavity, is an attached coil which can move freely in a magnetic field provided by an electromagnet or permanent magnet. The coil serves to convert the mechanical movement of the diaphragm into electrical impulses, and may also be used for the provision of damping for the resonance of the mass of the diaphragm and its elastic constraints"

Figure 3.12 - Detail from Patent No. 369,063 (1931) a cross-section of the protoype HB1A microphone

The HB1B microphone became the eventual product of this Patent and it was considered so good it immediately became the preferred choice of the newly formed EMI, and soon after HB1B's were being sent to the BBC. Fairly soon after, EMI were manufacturing them to be sent to studios all over the country. Engineers loved it, and the recordings produced from these microphones, especially classical recordings, were the best obtained up to that point in time. The microphone was hugely successful and would be used for many years to come. Originally known as the Blumlein microphone, in later years his name was dropped from the product, but was retained in the microphone technique by which most audio engineers will have heard of him.

By September 1931, the senior management of the new EMI company had finally realised the worth of their engineering and electronics prodigy and had met to consider a special bonus for Blumlein, Herbert Holman and 'ham' Clark for all their work on the Columbia recording system. They decided to award him the princely sum of £200 for his efforts, not an inconsiderable sum of money at the time, yet seemingly very small compared to the vast amount that the new recording system was now saving them in royalties to Western Electric and Bell Labs, and the royalties it would earn from other companies using it to cut records.

Never the less, typically gracious for their consideration, Blumlein wrote to his employers on 13 September 1931, thanking them:

"From: - A. D. BLUMLEIN 67 Earls Court Sq. S. W. 5.

13th Sept 1931

J. Gray Esq.

Columbia Graphophone Co. Ltd.

Dear Sir,

I am writing to you to thank the company for the bonus which I have just received in connection with the new recording system. I appreciate that, that the face value of the bonus is considerably enhanced by the depressed times in which it is made, and I am therefore very grateful for it. May I also thank the company for having given bonuses to Messrs Holman and Clark, whose very good work, I was hitherto afraid was not fully appreciated. My fears on this score, I find unfounded.

Thanking you Sir, Yours faithfully."

Binaural Sound

One day in 1931, Alan Blumlein took Doreen to the cinema and said to her during the film: "Do you realise the sound only comes from one person?" Doreen, by her own admission, was not a technical person and so replied to him, "Oh does it?" and he said, "Yes. And I've got a way to make it follow the person"

Alan Blumlein had just tried to describe his first thoughts about the system he would always call 'Binaural Sound', but which we have come to know better as Stereophonic or Stereo Sound. Blumlein explained to Doreen that, if she could imagine being blind, and sitting in the cinema, she would be able to point out exactly where the person was on the screen with his system. This, of course, was what he was trying to achieve, not this 'terrible effect' where the sound comes from one side of the screen when the actor was at the other side.

In truth Blumlein had been considering binaural sound for some time before the conversation at the cinema, probably conceiving the idea somewhere between March 1931 when the HMV/Columbia merger was taking place, and 1 November 1931, when the joint research laboratories were set up. While most people were seemingly content with monophonic recordings, or single channel sound, Alan Blumlein typically, had already decided that sound recording should try as best as possible to match human hearing. As humans have two ears, and the properties of binaural hearing had been known for some time, it seemed obvious to him that recordings should also have at least two-channels of sound, if not more.

Once again, he was somewhat surprised that nobody else had come up with the idea, just as with the application of electro-magnetic damping to the recording system for Columbia, but seeing as nobody had, Blumlein had chosen to take on the task himself. It is true that Bell Labs at around this time had been working on a system where arrays of microphones were being fed into arrays of loudspeakers, but Blumlein's idea was fixed in his mind. As human hearing was based on two ears, so his sound theory should be based on the binaural characteristics of sound reaching those ears. Just how far into the project to complete the Columbia recording system Blumlein was at the time he decided that binaural sound was the goal, is now uncertain. Again, typical of the man, he kept very few notes on his work, usually explaining the ideas in his head on a blackboard, or the back of a scrap of paper to his colleagues, rather than write them out long-hand as he solved the problems he set himself.

Years later, during a radio interview, Philip B. Vanderlyn, who had been a young research engineer at EMI working under Blumlein and who would go on to write a paper with Henry Clark and G. F. Dutton for the Institution of Electrical Engineers on stereophonic recording derived entirely from Blumlein's work, explained how many key moments in Blumlein's train of thought had been lost this way: "Every day he would come round the research laboratories and talk with everyone. Going back through his papers, I would have expected to find a lot of theory, thoughts and calculations. But they didn't emerge and I often wondered where the devil they were. I would have expected some sort of master plan. But the core of his work was missing"

What is certain is that somewhere in the late summer of 1931, Blumlein began to explain to Shoenberg his idea for a totally new concept of recording sound, something which he felt sure would revolutionise the flat, lifeless recordings then being made. While Shoenberg was naturally excited that his star engineer had yet more potentially lucrative ideas, it was probably natural for Shoenberg to be more than a little cautious. After all, they had only just completed and Patented the Columbia recording system. Wasn't that an entirely new and wonderful system for making recordings? Now, here was Blumlein with another idea again, proposing dramatic new ideas before the current one had even had a chance to be unveiled.

In a series of surviving hand-written notes dated 25 September 1931, Blumlein heads the page "Binaural Speech Trials", though there is no direct evidence that any such trials were carried out at this time. Blumlein calculates the value of an element he calls 'K', which is the ability to modify the signal of the microphones to recreate the phase difference at the listeners ears. The path difference from the two loudspeakers can give sufficient phase difference when they are summed in the li s t e isteners ears, if the difference in amplitude of the signals is then correctly modified. This Blumlein does by deriving the 'sum' and 'difference' signals, the difference signal being fed through p/2 using a current fed capacitor which also reduces the signal by 6dB per octave (6dB/8ve). This new difference signal is then added to and subtracted from the sum signal to form two new signals to the loudspeakers. Blumlein called the process 'Shuffling' (which we take today to have a different meaning to that which Blumlein originally uses here).

Figure 3.13 - Blumlein's hand-written notes from the Binaural speech trails in which the principle element 'K' is first mentioned

Many years later, Shoenberg would admit, as would many of Blumlein's colleagues, that the idea of binaural sound was in fact so far ahead of its time, that he and the others simply could not get their heads around the concept. It was something that lived in the mind of Blumlein and was, to all intents and purposes, misunderstood by many of those around him. This did not deter Blumlein from working on the project as we now know, nor did it deter Shoenberg from encouraging him in his efforts. It is however, a little sad to think that throughout the remainder of 1931, when the binaural sound Patent was finally published, probably very few of his friends and colleagues, even the engineers and scientific ones capable of understanding most of the technological advances of the time, were unaware of just how incredible and far sighted the idea had been. They, like EMI, would have to wait another twenty-five years or more to see the fruits of Blumlein's mind ripen with the advent of stereophonic sound recordings.

Patent No. 394,325, "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems", was first applied for on Monday, 14 December 1931. It comprises 22 pages of outline, with 11 supporting diagrams and their legends. Quite how Blumlein intended to apply his work to a practical design we shall probably never know as many of the applications that were tried were rushed; work on the television system that EMI had committed itself to increasingly took over the research laboratories. The principle for stereophonic sound however is clearly laid out with subtle and tantalising yet definite hints at cinematographic use as well as a multi-channel audio system beyond the specifications of this one.

Blumlein was an immensely modest man, quite often not fully aware of his own genius. However he must have had some feeling for the enormity of this work as he begins the Patent by explaining (in some detail) how the human hearing system works and how this relates to the methods he was trying to apply within his 'Binaural Sound' system as he called it. Much of the original Patent has been reproduced here, with the references to the diagrams (figures), (which are reproduced alongside), which Blumlein used to outline his invention.

"The invention relates to the transmission, recording and reproduction of sound, being particularly directed to systems for recording and reproducing speech, music and other sound effects especially when associated with picture effects as in talking motion pictures. The fundamental object of the invention is to provide a sound recording and reproduction system whereby a true directional impression may be conveyed to a listener, thus improving the illusion that the sound is coming from the artist or other sound source as presented to the eyes. In order to fully appreciate the physical basis of the invention the stages of its development as well as the known and established facts concerning the physical relations between sound sources, and the human ears will be briefly summarised.

"Human ability to determine direction from which a sound arrives is due to binaural hearing. The brain is able to detect differences between sounds received by the two ears from the same source and is thus able to determine angular direction. This function is well known. With two microphones correctly spaced and with the two channels entirely separate, it is also known that this directional effect can also be obtained for example in a studio; but if the channels are not kept separate (for example, by replacing the headphones by two loudspeakers) the effect is largely lost."The invention contemplates controlling the sound, emitted for example by loudspeakers, in such a way that the directional effect is retained.

"While the operation of the ears in determining the direction of a sound source is not yet fully understood, it is fairly widely established that the main factors having effect are phase differences and intensity differences between the sounds reaching the two ears; the influence which each of these has being dependent upon the frequency of the sound being emitted.

"Broadly, the invention consists of a system of controlling the intensities of sound to be (or being) emitted by a plurity of loudspeakers or similar sound sources in a suitably placed relationship to the listener. This is done in order that the listener's ears will note low frequency phase differences and high frequency intensity differences suitable for conveying to the brain the desired sense of direction of the sound origin. In other words, the direction from which the sound arrives at the microphones determines the characteristics (and more especially the intensities) of the sounds emitted by the loudspeakers in such a way as to provide this directional sensation.

"Furthermore, the invention consists of a sound transmission system wherein the sound received by two or more microphones, (with low frequency differences in phase of sound pressure at the microphone) is reproduced as difference in volume at the loudspeakers. Two microphones should be spaced with their axes of maximum sensitivity so directed relative to one another (this is 'shuffling' as we know it now) and to the sound source that, the relative loudness of loudspeakers which reproduce the impulses is controlled by the direction from which the sound reaches the microphones.

"The invention also consists of a mechanical system where the above references two channels of sound impulses are recorded in the same groove of a record, and in combination with the means of transmitting the same impulses by radio telegraphy, as well as photographic recording or transmission and/or reproduction of pictures"

Figure 3.14 - Detail from Patent No. 394,325 (1931)- Fig.1

"For the purpose of demonstration, the sounds recorded and reproduced may be received from a source by two pressure microphones (a1 and a2), mounted on opposite sides of a baffle (b) which serves to provide the high frequency intensity differences at the microphones in the same way as the human head operates on the ears.

"The outputs from the two microphones are after separate amplification by similar amplifiers (b1 and b2), taken to suitably arranged circuits (c) comprising transformers (see Figures 3 and 4) or bridge networks circuits, which convert the two primary channels (called the summation and difference channels). These are arranged so that the current flowing into the summation channel will represent half the sum, or the mean of the current flowing in the two original channels; while the current flowing into the difference channel will represent half the difference of the currents in the original two channels.

Figure 3.15 - Detail from Patent No. 394,325 (1931) - Fig.2

"Two velocity microphones (n and o) are placed with their axis perpendicular to one another and each at an axis of 45° to the direction of the centre of the screen and the direction of the source of the sound. It can be seen that movement of the sound source (a) laterally to a position (p) removed from the centre of the field, will result in the sound waves striking microphone o at a more acute angle than they strike microphone n, and differences in the microphone outputs will result.

"The microphones are sufficiently close together to render phase differences of the incident sound negligible, and the output amplitudes therefore differ approximately proportionally to the obliquity of the incident sound. They may therefore be amplified similarly, and supplied directly to the loudspeakers to which they will give the correct amplitude differences for the for the desired directional effect, provided the relationship between the various dimensions of the recording and reproduction layouts are correct"

Figure 3.16 - Detail from Patent No. 394,325 (1931) - Fig.3

Figure 3.17 - Detail from Patent No. 394,325 (1931) - Fig.4

"Figure 3 shows a convenient transformer arrangement (for Figure 1), where the input currents from amplifiers b1 and b2 are separately fed to two primary windings, one on each of two transformers. The secondary winding of each transformer provides a sum or difference output current on account of the senses in which the primary coils are wound as shown. Figure 4 shows a diagrammatic representation of a sum and difference transformer similar to that of Figure 3".

Figure 3.18 - Detail from Patent No. 394,325 (1931) - Fig.5

Figure 3.19 - Detail from Patent No. 394,325 (1931) - Fig.6

"Figure 5 and Figure 6 represent the portion of the circuits indicated by c in Figure 1 circuits for sum and difference arrangement outputs, modified in order to obtain the desired sound effects. Assuming the original currents differ in phase only, the current in the difference channel will be p/2 difference in phase from the current in the summation channel. This difference in current is passed through two resistance's d and e in series, between which a condenser f forms a shunt arm. The voltage across the condenser will be in phase with that in the summation channel. By passing a current in the summation channel through a plain resistive attenuator network composed of a series of resistance's (g and h) and a shunt resistance (i), a voltage is obtained which remains in phase with the voltage across the condenser f in the difference channel.

"These two voltages are then combined and re-separated by a sum and difference process (such as previously adopted) to produce two final channels. The voltage in the first channel will be sum of these voltages, and the voltage in the second channel will be the difference between these voltages. Since these voltages were in phase, the two final channels will be in phase, but will differ in magnitude. By choosing the value of the shunt resistance i in the summation channel and the shunt condenser f in the difference channel, for a given frequency any degree of amplitude difference in the final channels can be obtained for a given phase difference in the original channels.

"For higher frequencies it is not necessary to convert phase shifts into amplitude differences, but simply to reproduce amplitude differences. The shunt condenser f in the difference circuit is therefore built out with a resistance k, whose value is substantially equal to that of resistance i, in which case the amplitude differences for high frequencies are passed without modification.

"Where the microphones employed have a velocity type that an edge-on microphone gives, an output proportional to the obliquity of the source is desired. Figure 6 represents a suitable arrangement for this form where the shunt condenser f and resistance k are in series, and the shunt resistance i is replaced by shunt resistance's l and m which form artificial attenuators. By altering the relative attenuation in l and m, the intensity differences in the two lines corresponding to the given obliquity of sound is controlled"

Figure 3.20 - Detail from Patent No. 394,325 (1931) - Fig.7

"There is a simple method by which modifications for increase or decrease of differences between channels may be effected if no conversion of phase differences into amplitude differences is required. Figure 7 is a diagrammatic demonstration of this, which can prove particularly useful for the operation of more than two loudspeakers. If the transmission is effected in the form of two channels r and s, of similar phase but different amplitudes, an alteration of these amplitude differences may be effected by connecting one wire of each channel r and s together at t, and connecting a choke (see box) u between the other two wires of the two channels.

"The outgoing channels v and w, whose difference is to be a modification of the original difference, are connected by one wire to the common point t of the original channels, and by their other wires to tappings along the choke u. If the differences are to be increased the tappings at which the output channels are connected lie outside the tappings to which the input channels are connected, so that the choke operates in effect as an auto-transformer amplifying the difference voltages. Similarly, for a reduction of differences, the output channels are tapped intermediately between the two input channels. This arrangement works well with a number of loudspeakers for binaural reproduction"

Figure 3.21 - Detail from Patent No. 394,325 (1931) - Fig.8

Figure 3.22 - Detail from Patent No. 394,325 (1931) - Fig.9

Figure 3.23 - Detail from Patent No. 394,325 (1931) - Fig.10

Figure 3.24 - Detail from Patent No. 394,325 (1931) - Fig.11

"Each of these diagrams demonstrate a recorder assembly whereby both channels may be cut by a single tool on the same groove (with several variations), resulting in a recording at 45° to the wax (or other) surface giving the sum and difference as the effective lateral and hill and dale amplitudes. A light stylus is pulled into two directions at right angles to one another and each at the preferred 45° angle to the surface.

"In Figure 8 for example, 1 and 2 represent the driving elements of two recorders normally adapted for cutting lateral-cut records. These in turn drive 3 and 4 (drive arms), the ends of which are connected by ligaments 5 and 6 to the end of a reed 7. The reed carries a cutting sapphire 8 which makes the groove in the record surface.

"Movements of the recording arms 3 and 4 produce movements in the end of the reed 7. Thus, currents in movement 1 will cause the reed 7 to move along an axis approximately 45° to the vertical, rising from left to right. Similarly currents in movement 2 will produce movement of the reed 7 in an axis at right angles to the former axis, while currents in both movements will of course result in vertical movement of the reed"

Having outlined the invention within the Patent, Blumlein then when on to describe how he felt binaural recording would be, "especially applicable to talking pictures, but not limited to such use. It may be employed in recording sound quite independently of any picture effects and in this connection (as well as when used in cinematographic work) it seems probable that the binaural effect will be found to improve the acoustic properties of recording studios, and to save any drastic acoustic treatment thereof while providing much more realistic and satisfactory records for reproduction". Blumlein went on to conclude, "In general, the invention is applicable, in all cases, where it is desired to give directional effects to emitted sound"

This quite enormous piece of work, comprising as it does some 70 claims, was so far ahead of its time that again very few people (if any outside the immediate clique of engineers at EMI), understood its implications. Blumlein had been officially transferred (internally) from Columbia to EMI on 1 November 1931, when the joint research laboratories had been set-up, and the construction of the binaural system continued throughout 1932, with experiments conducted in earnest soon after, starting in early 1933. Though initially the experiments were made using poor pressure operated microphones, with omni-directional polar patterns and so gave limited results, later, when Blumlein and Holman's own microphones were used, the results became noticeably better.

Blumlein had calculated that the audio signals which would be received by a listener sitting (preferably) or standing in an off-centre position relative to a sound source, would arrive at different times at the ears. By placing two matched (identical) microphones eight inches apart (the average distance between a person's ears), to simulate a human listener, these signals could be processed in terms of phase before applying them to a spaced pair of loudspeakers. Blumlein assumed that phase was the only difference that needed to be calculated; in fact with the microphones as closely spaced as he had them, he was probably correct as the path lengths of the audio signals were so close for their attenuation to be nearly identical.

Figure 3.25 - The twin ribbon binaural microphone side elevation (Courtesy of EMI)

Figure 3.26 - The twin ribbon binaural microphone capsule detail (Courtesy of EMI)

Because he was concerned about strange phase anomalies at high frequencies that had not been investigated and understood at that time, Blumlein confined his analysis of the response of the human ear to frequencies below 750Hz. All that was needed now was a series of tests at different frequencies, plotting the response of the two microphones to build up a model of how human hearing 'works'. This process was called 'shuffling' by Blumlein, and he could simulate this human response through his circuit, which in turn he called a 'shuffler circuit'. Once completed, the model could be used to calculate the phase and amplitude differences from the sound reaching each of the microphones separately and carry these signals through to the loudspeakers. What he hoped would then be reproduced, was an audible representation of the sum and difference of the phase and amplitude of the sound, which we as humans detect as the perception of the directional origination of the sound.

Though we now know that Blumlein had a firm grasp of the principles being employed within his invention, he obviously realised that not everybody would understand as plainly as he did exactly what it was that was being demonstrated. Consequently he wrote an exhaustive and very descriptive report for Shoenberg and the senior management at EMI, hoping to explain to them just what 'Binaural Sound' was (as he had by now christened his work). Unfortunately, EMI still did not fully grasp the importance of the work, and while it had no compunction in allowing Blumlein to experiment at will, the work was shelved after his death in 1942. Blumlein continued with the binaural experiments right up to and just after his transfer to the high definition television system, which was soon to occupy most of the engineers and scientists at EMI. He wrote an extensive report with one of the Recording Research Section engineers, Jim Castle, entitled 'Recording Research - Hill and Dale Overloading' on 20 May 1932, in which they wrote: "...the results of some tests which were made in order to check a theory of harmonic production in Hill and Dale records due to the finite size of the reproducing ball". They concluded that: "...it would appear that the general theory and formulae (calculated in the report) for harmonic and intermodulation tone production are correct. Secondly, the Hill and Dale system as known to us is not satisfactory for levels as high as +3dB at frequencies above 3,000 c.p.s. (cycles per second), at 33 rpm (curiously, 33 rpm. Not thirty-three and a third rpm). The fact that much higher levels than this can be tracked at middle frequencies would appear to have little advantage since scratch at these frequencies is no real trouble"

Figure 3.27 - Binaural recording head detail (Courtesy of EMI)

Figure 3.28 - Binaural recording head detail clearly showing how excess wax was removed by suction pipe (Courtesy of EMI)

On 4 July 1932, Blumlein wrote out a detailed document in which the binaural reproduction system was described in full. It would seem that this eighteen page, hand-written account, served as a fore-runner to a much simpler document that Blumlein would write three weeks later. In the later eight-page memorandum, written to Shoenberg on 21 July 1932, Blumlein tries to describe his binaural ideas in non-mathematical language, the main reason for which seems to be to persuade Shoenberg to sanction the forthcoming experimental programme that was being planned. Perhaps not fully grasping the depth of the system, Shoenberg none the less, possibly as a result of this memorandum, gave the go-ahead for the construction work to begin on the microphones, shuffling circuits, wax cutter and pickup. Blumlein writes, "This memorandum describes briefly the general troubles experienced with single channel reproduction, the difficulties experienced with the more obvious binaural systems, and the proposed method of overcoming these difficulties"

Throughout 1932, EMI was busy checking and testing any other recording/reproduction system that appeared on the horizon to see if it was better than that designed by Blumlein or, more likely, it infringed the Blumlein Patents in some way. One such system was the Siemens Halske (Recording) System, which had been supplied to Carl Lindstrom in Berlin in August 1932, evaluated in a report by Blumlein to Shoenberg on 26 September 1932. Blumlein writes: "The Siemens gear was well built and typically German in lay-out. I formed an opinion that the system would probably give a second-rate commercial product with good loud 'gramophone tone' of a rather metallic and confused type. I cannot see that the Siemens system infringes any serious Patents that I know of. The microphone, though similar in appearance to the W. E. Co.'s, probably avoids their air damping Patents. The amplifier H.T. is fed through a filter having both inductive and resistive series elements. The H.T. supply however, is not rectified AC. The recorder is a moving iron-type resonance device being apparently damped by the sheet rubber used to seal off the movement. Unless all frequencies below, say, 500 c.p.s. are heavily depreciated the recording must be 'dirty', in that bass notes will modulate superimposed high frequencies. The recorder employs no strikingly novel feature".It would seem that EMI did not have much to worry about from this system.

It had also become obvious to Blumlein by this time that he, Holman and Clark needed additional engineers in order to complete all the experimental and development work that was required, and this led to a curious set of circumstances which culminated in the employment of Maurice Geoffrey Harker. One evening Blumlein came down into the car park at the side of the Research and Development building to discover that his car had a puncture. At the same time Arthur Cooper, who worked in the Patents department came down to his car, saw that Blumlein had a puncture and proceeded to help him fix it. During the process Blumlein told Cooper that he was looking for new additions to the team and that if Cooper could think of anyone then he should let Blumlein know. As it happened, Cooper knew of Maurice Harker, then fresh out of college who had just received a first class honours degree in heavy electrical engineering (coincidentally the same degree that Blumlein had). Cooper gave Blumlein Harker's telephone number and the next day Blumlein called him and asked him to attend an interview on following afternoon, Thursday, 29 September 1932.

"At my interview, Blumlein asked me all sorts of questions, and I told him how I'd built crystal radios and later a single valve radio set", Maurice Harker recalled, "Some time into the interview he asked me if I knew what a 'Decibel' was? [The Decibel had replaced the TU (Transmission Unit) introduced by Bell in 1929] (Well), I'd taken all the regular scientific journals of the day so I had a reasonable idea, but not an accurate one, so I said 'Yes, it is the smallest degree of change in the amplitude of sound that a human ear can detect'. Well, this is quite inaccurate, but accurate in a sort of way (you see), and I suppose Blumlein was reasonably satisfied with this answer, because the following morning (the Friday), he telephoned me at my parents' house in Pinner, near Watford where I was living, and told me that I was engaged and could I start the following Monday, 3 October 1932. And of course, I did; though I now had the problem of how I should get from Pinner to Hayes. (So), that weekend, I scoured the area for a suitable second-hand car and finally purchased a four-year old Austin Seven for £25 with money I had borrowed from my parents. That little car served me, my colleagues and EMI for many years to come"

Figure 3.29 - Maurice Geoffrey Harker from a photograph in his pilot's licence May 1939 (Courtesy of Maurice Harker)

With the addition of Maurice Harker and more engineers to arrive in the months that followed, Blumlein now felt ready to commence work on the binaural experiments. The first of these took place on 8 January 1933; the work being recorded in a file by Blumlein which is headed 'Polar Curves of two W.E. C.T.'s for Binaural Work'. Using a pair of calibrated Western Electric condenser microphones (C.T.'s - Condenser Transmitters), spaced at about 7 inches apart a series of test were carried out to plot polar patterns. Blumlein perhaps should have used his own moving coil microphones for this experiment rather than of those manufactured by Western Electric, but EMI were re-equipping their laboratories at the time. This meant that there would have been a shortage of EMI products and, having come from ST&C, a subsidiary of Western, Blumlein knew that these WE microphones, amplifiers and wax cutters, now all surplus to requirements, would be adequate.

On 13 February 1933, Blumlein designed a shuffler circuit, for which his hand-written notes still exist, with the main phase-shifting element now including a bridging inductance, which reduced insertion loss. He designed the low phase-shift transformers the next day and, starting on 20 February conducted tests of the microphones, circuits and the shuffler, so that by March 1933 a small team of EMI engineers were gathered together for a series of listening tests. There were still problems associated with the system, for example the 78 r.p.m. shellac records being used for the experiments did not lend themselves that well to the subtleties we now associate with stereo sound properties. In addition, the microphones themselves introduced considerable cross talk, which distorted and degraded the signals that were reproduced.

Nonetheless, the method did work and on 12 July 1933, the first calibration of the recorder took place, with the bandwidth reported as being around 4kHz. Eventually Blumlein and Holman would design and construct four binaural gramophone pickups, the earliest surviving example of which is called the 'PU3A'. It was of the moving armature type (which is surprisingly massive), and was first tested in late 1933. It still survives to this day in the EMI archives. The second pickup, the 'PU2', tested in June 1933, did not survive, while the third example, the 'PU4A', had an improved design using two coils, though it is not mentioned as being tested until January 1934. It too can be found in the EMI archives. Of the 'PU1', nothing is thought to have been constructed; it may well have been the designation for the design drawing only.

That August, Blumlein took a vacation in Cornwall staying with Doreen's family near St. Ives. Maurice Harker had explained to Blumlein that he too was taking a vacation in the West Country with his wife-to-be and a caravan, and that they would stop off here and there along the way for a night or two. "And Blumlein said, 'Well, if you are nearby you should stop off and see us in Cornwall', to which I thanked him and, as I had met and knew Doreen Blumlein, and knew the offer to be genuine, I said that we would. As it happened, when we did arrive at Doreen's family house, Blumlein was away in Portsmouth apparently looking over a submarine. Despite this being his vacation, he was still willing to take any opportunity to extend EMI's interests, you see, and at that time we may well have been looking at the early possibilities of what would become 'Sonar'. Anyhow, he returned the next day and we all went for an afternoon's excursion and decided to go swimming. Well, as you probably know, Blumlein was quite a swimmer, far better than me, and we had a most enjoyable afternoon swimming, after which he came and looked over this caravan which I was towing with this little Austin Seven, and that seemed to fascinate him also"

By 30 August 1933, having returned from his holiday, Blumlein decided that the circuit for the binaural system could be extended to include the wax cutter, which had first been applied to the series of tests carried out in July. This circuit allowed switching to a recorder or to a pickup and included an amplifier with a 26dB gain and variable loss pad in its output. Originally intended for a ribbon microphone, this was added into the difference channel before feeding into the shuffler evidently because Blumlein had discovered the signal in the difference channel was less than expected. Somewhere around 9 December 1933, the wax cutter was tried out for the first time with two commercial recordings being played one into each channel, one vertical, the other lateral. Blumlein reported that "good separation", was achieved, and he judged that binaural recording was finally practicable.

When Prince Edward, the Prince of Wales (later King Edward VIII), visited Hayes in 1934, it was this recording that was demonstrated by Blumlein to him, along with a live demonstration from a suspended microphone in a room on the floor above. This was being operated by two junior lab assistants who were walking in front of it, and the sound was being fed to the loudspeakers in the room below in which the VIPs were sitting. Apparently the Prince had listened intently as Blumlein explaining how binaural worked to him, but had not understood the principle that well. During the demonstration, Prince Edward listened to the recording and then the live demonstration, and then exclaimed, "Ah, it's moving!" He had finally understood. Maurice Harker recalled that the Prince then requested to see the microphone that was being used. This, however, posed a problem as the room in which the two lab assistants were walking and talking in front of the microphone had not been on the scheduled tour and was in fact just a bare room with two very junior (and now bored), lab assistants in it. Never the less, the Prince was shown to the elevator to ascend one floor. Meanwhile, Blumlein bolted up the stairs as fast as he could to warn these two young lab assistants (who, by this time, were apparently lounging on the floor), that they were about to be graced with a visit from the Prince of Wales and their future King!

On 14 December 1933, the first six 10-inch wax masters were cut using the binaural system with four more cut the following day, these have subsequently become known as the 'walking and talking' test recordings, and vary in length from 3 minutes 30 seconds up to 10 minutes and 20 seconds. Most of the binaural equipment had been placed in Room 106 which adjoined the small auditorium at EMI. This auditorium was used primarily by the Company Amateur Theatrical Group. There was a small raised stage with curtains either side and seating space for around 100 people, behind the stage curtains was a cinema screen and the auditorium was often used for the showing of movies. All around the room, the walls were lined with maple-wood which gave a rather resonant ambience and tended to create reverberal problems which would later cause Clark and Holman many problems as they struggled to perfect the microphones. Behind the curtains, which were draped along the side walls, were large metal racks on which loudspeakers had been installed by Geoffrey F. Dutton (who was mostly responsible for the experimental loudspeaker design at EMI). These could be reached by means of a gantry, which led to a narrow corridor behind the speaker boxes themselves. The first experiments carried out in the auditorium were rudimentary to say the least with Blumlein, Maurice Harker, Alfred Westlake and Frank Runcorn Trott (who was always known as 'Felix'), experimenting by walking past and around the stationary microphones as they talked and held conversations with each other. In one, Blumlein asks Felix Trott to turn the impedance down as he walks around from left to right, and then turn it up again as he moves back again. He then asks how the time if doing as the cutter was running out of wax.

Figure 3.30 - Frank Runcorn Trott, always known as 'Felix' (Courtesy of EMI)

Figure 3.31 - Alfred L. Westlake shown here inspecting cut master discs (Courtesy of EMI)

Years later, Felix Trott recalled those earliest experiments: "Walking and talking was done in a large room known as the auditorium, looking rather like a completely empty cinema auditorium except the floor wasn't sloped, it was flat. And it was laid with hard Canadian Maple, so that it made good loud, bang, bang, bang noises if you walked flat-footed round. So you had the two sorts of things to locate on: the voice, which of course had a whole lot of different noise patterns, and Blumlein's shoes going firmly down on this hard Maple floor"

A progress report of 16 December remarks that the recordings made have a "definite binaural effect". A complete list of the recordings was drawn up for the files at EMI on 19 December 1933, and designated the tests as Nos. 5757, and 5758. In Test No. 5757, Thursday, 14 December 1933, Cut No. 1 has Blumlein, Westlake, Trott and Harker talking in the auditorium with Blumlein and Westlake in front. Cut No. 2 is as the first but with a different arrangement, now Blumlein is now at the back. Cut No. 3 has all of them walking and talking in a muddle and changing positions. Cut No. 4 has them talking in turn, in pairs, more slowly but with muddled change-overs. Cut No. 5 is described as 'Heavy Shuffle', with the difference on each channel reduced by 3dB. The people arrangement is as Cut No. 4. The final cut for Thursday, Cut No. 6, is again listed as 'Heavy Shuffle', with the difference in each channel reduced again by 3dB, and now with Blumlein talking alone in the auditorium. It is now that he speaks to Felix Trott about the time remaining. A channel plotting is given for recordings one to five as 'A Channel = 18, B Channel = 21', and the plotting for recording six as 'A Channel = 19, B Channel = 22'.

Test session No. 5758, the following day, Friday, 15 December 1933, lists the four recordings made that day. Cut No. 1 has Blumlein only walking and talking in the auditorium. It is again given as 'Heavy Shuffle', with the Difference channel as '10,000 W and 70 W'. Plotting levels were 'A = 18, B = 21'. Cut No.2 has the same arrangement as Cut No. 1, but with a 'Light Shuffle 10,000 W, 70 W and 40 W. Plotting levels were 'A = 18, B = 21'. Cut No. 3 has Westlake, Trott, Harker and Turnbull talking in turn with all the shuffles as the previous cut and the plottings also the same. In the final cut that Friday, the auditorium arrangements were as for Cut No. 3, but the order of shuffling was changed from 'Heavy 40 W, 10,000 W and 70 W', to 'Light 70 W and 40 W', and back to 'Heavy 40 W'. The plottings for the channels were 'A = 19, B = 21'.

Figure 3.32 - The Audiotorium at EMI where many of the binaural recording tests were made (Courtesy of EMI)

The recordings gave a fair account of each man's position in the room as the cuttings were made and the results, which were played to Shoenberg, were sufficiently good to warrant the apparatus being taken to the then fairly new EMI recording studios in Abbey Road. The Abbey Road studios in St. Johns Wood (which was actually No. 3 Abbey Road), had been purchased as a private house by The Gramophone Company, for the then huge sum of £16,500 on 3 December 1929. During the next two years, EMI would spend over £100,000 converting the property into recording studios. It became part of EMI following the merger in April 1931, and was officially opened on 12 November 1931.

At Abbey Road, a complete binaural record cutting system was installed by Ivan Turnbull, Ham Clark and their assistant in the then largest room, Studio No. 1 (this is now called 'Studio 2'. It is the same room later used by The Beatles, and is often referred to as 'The Beatles Studio'). The system was used initially on Thursday, 11 January 1934, to record two cuts of Ray Noble's Dance Band, with the microphones placed approximately 45 feet distant from the musicians. The two recordings, which were given the internal numbers TT.1557-1, and TT.1557-2, were a result of plottings from 'A (lateral setting) = 18, B (hill and dale setting) = 18', for the first cut, and 'A = 24, B = 24', for the second, with the amplifiers set +6 and +6 for the first and 0 and +6 for the second. Both cuts were using 'heavy shuffling', with '6dB brightening (5000~sharp)'.

The next day, Friday, 12 January 1934, three pianos were placed converging towards a standard HB microphone with the outer pianos making an angle of approximately 60° degrees with each other. Six records were cut with various microphone plottings recorded: Cut No. 1 (Test No. 5769-4), had the binaural microphones approximately 12 feet from the tip of each piano and the recording was noticed as too heavy, plotting 'A = 26, B = 22'. Cut No. 2 (Test No. 5769-1), was with the microphones positioned as in Cut No. 1, but the plotting was reduced to 'A = 22, B = 18'. Cut No. 3 was a repeat of Cut No. 2 but with a plotting of 'A = 22, B = 20'. Cuts No. 1, No. 2 and No. 3 were of The Hungarian Rhapsody by Brahms. For Cut No. 4, the binaural microphones were advanced 6 feet nearer the pianos, which placed them approximately 18 inches behind the HB microphone. The plotting is given as 'A = 18, B =16. Cut No. 5 had the microphones restored to their original positions i.e. as they had been for Cuts 1, 2 and 3, but with a plotting of 'A = 22, B = 20'. Both Cut No. 4 and Cut No. 5 are of Ride of the Valkyries. Finally, Cut No. 6 was made after the main sessions had finished and consisted of snippets of music from a rehearsal. The microphones were withdrawn to approximately 25 feet from the ends of the pianos and the plottings were given as 'A = 26, B = 24'.

A progress report of these recordings was drawn up on 13 January, by Turnbull in which he states that: "Binaural gives opening out on music, but the effect is more marked for speech". The report was circulated on 16 January to those who had been present. It had been decided, based on these two days recordings, to go ahead and try something a little more ambitious. Therefore on Friday, 19 January 1934, nine sides were cut of a rehearsal of Mozart's Symphony No. 41 'The Jupiter', with the London Philharmonic Orchestra, conducted by Sir Thomas Beecham. This must have been a special thrill for Blumlein as he was a great admirer of Beecham and had a comprehensive set of records of his work which he and Doreen would sit and listen to for hours at a time at home in the evenings. Felix Trott and Maurice Harker both recalled working with the great man who left little doubt who was in charge of proceedings: "We would make sure that Sir Thomas had everything he required…" Felix Trott recounted, "…down to the fresh bottle of 'good' whisky in his dressing room, which quite often would be replaced with a second, fresh one, in the afternoon!"

Figure 3.33 - Sir Thomas Beecham (at the back) inspecting The Columbia Recording System at Abbey Road in 1933

Turnbull's progress report of the Beecham recordings, which was drawn up on 20 January, and circulated on 22 January 1934, states of the binaural effect, that there had been, "...solidarity, but less effective than on speech". The report goes on to list the following details: "All subs. cut with heavy shuffling and 6dB sharp boost at 5,000 c.p.s. Amplifier plotting = +6 and 0'. Cut No. 1 had the microphones placed approximately 13 feet high and approximately 13 feet from the nearest first violin. The plotting is given as 'A = 24, B = 22'. Cut No. 2 was exactly as Cut No. 1 but is of part two of the Symphony. For Cut No. 3, the microphones were moved approximately six feet further back and one foot higher. The plottings remained the same. For Cut No. 4, the positions remained but the plottings changed to 'A = 26, B = 24'.

Recording No. 5 saw the microphones moved approximately 7 feet further back i.e. approximately 25 feet from the nearest first violin. This was for part three of the Symphony and the plottings stayed the same Cut No. 6 was as Cut No. 5, but the plotting changed to 'A = 28, B = 26. Cut No. 7 was as Cut No. 5 but again the plotting changed, this time to 'A = 30, B = 28'. Cuts No. 8 and No. 9 were of part four of the Symphony with No. 8 having a plotting of 'A = 26, B = 24', and No. 9 having 'A = 28, B = 26'.

The results were judged by Blumlein and his colleagues from 'not bad' to 'marginal', and the discs remain, to this day, in the vast and complete archives of EMI at Hayes, where a record of every cut and pressing ever made was logged and kept. From the ensuing tests, carried out over three months from February to May 1934, many of the stereo microphone techniques were devised, which are still commonly used to this day by recording engineers all over the world. The musical trials were hurriedly completed, as Blumlein was eager to get on with main aim of the work, which was to perfect binaural film. It had also been some time since Blumlein had begun simultaneously working on the development of the high definition television system, among which were some of the pioneer engineers from the original sound-to-film processes of the late 1920s. It is a testimony to the engineers working with Blumlein that, while he may not always have been present for these experiments, and was not directly responsible for all the microphone techniques, his colleagues applied the name of 'Blumlein' to these techniques regardless. Therefore, his name has remained in use to this day, in most cases. Indeed it is usually through 'the Blumlein stereo microphone technique', that most audio engineers first hear his name.

Some time later, on 7 February 1935, Blumlein would apply for an additional Patent, No. 456,444, "Improvements in and relating to Electrical Sound Transmission Systems", in which he outlined many of the microphone positioning techniques that had resulted from the use of the binaural system. The Patent explains how, with their various outputs mixed, microphones can be used in a manner of ways including the 'A-B' microphone technique ('A' is always left, 'B' is always right. Incidentally, the Germans call this method the 'X' and 'Y' technique. Current usage of A-B and X-Y is that A-B is 'spaced' and X-Y is 'coincident'), which despite having the same initials as Alan Blumlein's name, probably refers more to the fact that he calls two such microphones 'A' and 'B' in his Patent, rather than after himself. There is also mention of the method that we now call 'M and S' microphone (Mid & Side). Blumlein knew this method as S-M microphone, S = Sum, and M = Difference, 'M' = ½(A+B), and 'S' = ½(A-B).

Binaural Film Experiments

In Patent No. 394,325, Blumlein describes the possibility of using his binaural system for adding an extra sound track to film, with the existing sound track in the normal position carrying the 'sum' signal, and an additional track carrying the 'difference'. This was done to ensure that a binaural soundtrack could be played in an unmodified cinema theatre. The job of getting this to work was given to Cecil Oswald Browne, who had worked on the original HMV mono sound-to-film system in 1928-29 which, while technically quite successful had, for commercial reasons been shelved after only two films had been shot. Both HMV and Columbia continued thereafter to produce 16-inch pressings used for sound in many movie theatres.

By 1933, Browne, like many of the staff at Hayes was deeply engaged in the development of television. Blumlein had explained binaural however to an intrigued Browne in 1933, and the two of them resolved that a binaural film process could be constructed with a series of new projectors, amplifiers and shufflers. Many of these were eventually designed and made by Holman, Turnbull, Harker, Clark, (and their assistants), under the daily supervision of Blumlein.

Browne himself designed and constructed the 35mm film recorder and had to invent two totally new galvanometers in order to fit the two recorded tracks sufficiently close together, instead of the one that the then standard monophonic film systems were using. It was decided that variable area recording would be used instead of the so-called 'squeeze' tracks. The soundtrack itself comprised two 'unilateral' variable area soundtracks (one edge modulated), recorded side by side in approximately the same space that a monophonic soundtrack of the time would have taken up. For playback, a double cathode photocell was needed to read the sum and difference tracks, and the task of designing and constructing this was given to William F. Tedhams. He had in fact developed many of the photocells for the earlier work and was also busy working on television and the development of cathode ray tubes.

Figure 3.34 - Cecil Oswald Browne (Courtesy of EMI)

Figure 3.35 - Philip B Vanderlyn (Courtesy of EMI)

It is worth pointing out at this stage just how radical the project that was being undertaken must have seemed, coming as it did just a matter of a few years after the very first 'talkies' had appeared in cinemas. Talking pictures had originally been dismissed as a novelty when they first appeared in 1927, though their popularity with the cinema-going public ensured their success and quick adoption. Now Blumlein was proposing that it was possible to record stereo films before the first stereo recordings had even been released.

Browne completed the camera/recorder in early 1935 (which had a fixed focal length lens), and the other equipment, the amplifiers, shufflers and projectors were ready by early May 1935. The first test on the twin recorder had been carried out in April 1935, with a designed upper resonance of around 9kHz, but there were subsidiary resonances at lower frequencies which needed to be damped by immersing the movement in oil. This was not very effective at first because the amount of power to be dissipated heated the oil and actually reduced its viscosity.

Castor oil was tried and, while it gave sufficient damping (the galvanometer was flat to 1kHz with a gentle rise of 4dB to 9kHz), the oil proved insufficiently transparent to allow enough light through to expose the film fully. By the end of May 1935 however, a better optical system had been developed with new mirrors which worked much better. At around this time the reproducer sound head was tested and found at 8kHz to be about 12dB below that which it should be, most of which was attributed to the slit and preamplifier.

By June 1935, the first series of tests on the binaural film system were ready to be carried out. Although there was some frequency modulation, probably because of vibration, the major problem was a noticeable degree of noise because the squeeze track had not yet been incorporated, and the channels were unbalanced. This gave rise to a degree of overloading, which was considered a bearable evil until the problem could be solved. A movie camera was borrowed as a temporary solution, probably a left-over relic from Browne's work in 1928/29, and this was used to determine how much the binaural system would override reverberation problems in making talking movies (a big problem in the early films).

It was now that a series of logistical problems began to occur which hindered the further development of the system for a while. Firstly, the amplifiers needed to be borrowed for several days to contribute to a major demonstration of the Company's television system given at Abbey Road. Secondly, several members of the staff had to be found to design the sound system for the Alexandra Palace television station contract, which obviously took priority, and this held things up further. This all meant that it was not until the middle of June before the first actual binaural filming could take place and, even then, a suitable location had to be found. It is for this reason that the first films were actually taken outside in early July with the 'Trains at Hayes Station', 'Throwing Stones' and 'Fire Engines' films all being shot in this first twelve days of that month.

It was decided that the auditorium at Hayes should be used once again for the inside filming. It was 40 feet wide and had acoustics similar to those of a cinema, certainly more than any of the labs. It had also been used in March 1935 for the first loudspeaker tests of the binaural audio system. Though it had been the cause of some reverberal problems for Clark and Holman (still busy perfecting the latest versions of the HB microphone), the room provided the best option for these tests and it was equipped with both microphone types, crystal (which Holman had originally been working on before Blumlein's arrival at Columbia. This had a glass diaphragm) and velocity microphones. A 14 foot-wide set was hastily constructed; a suspended curtain on the stage area, and 16kW of lighting rigged. Most of the recordings were to be made with the velocity microphones which had given better results in film conditions and, during the first week of July a few test films were made without sound.

At first, it would seem that Blumlein did not have any specific plan in mind for the binaural film tests, but instead, circumstances forced him outside while the auditorium was prepared. The first of these outside films has become known as the 'Throwing Stones' film, and is actually a silent test (which could have been for any number of reasons, including the possibility that the audio tracks were out of synchronisation). The silent reel, which is 103 feet in length and lasts for just 1 minute and 6 seconds, has a sequence showing Maurice Harker, Herbert Holman, Felix Trott, Albert Westlake, and Philip Vanderlyn, outside the EMI research building throwing stones. There is no specific date recorded for when this filming took place, but it is reasonable to assume that it was shot in the first twelve days of July 1935.

The first of the binaural sound reels is the one which has become known as 'Trains at Hayes Station'. It is 487 feet, lasts for 5 minutes 11 seconds, and shows various steam trains arriving and departing from Hayes station. Again, there is no date for the filming, but it too would have taken place in early July. Blumlein's team (Blumlein was not present himself for the filming) took the binaural sound system and camera to an empty office building which overlooked Hayes railway station and started to film trains as they arrived and departed. The scenes are photographed from a high vantage point to give the camera as wide a view of the station as possible including the surrounding tracks. In the distance, among the various buildings and sheds which have smoke (and presumably steam) coming from them, it is clearly possible to read the sign 'Nestlé' on the roof of one building just beyond the station platform.

Figure 3.36 - Still image from the binauaral film 'Trains at Hayes Station', July 1935 (Courtesy of EMI)

 

Figure 3.37

 

Figure 3.38

As the 'Trains' film progresses, it is obvious that several microphone positions are being tried out, with frequent alterations of both microphone location and type. It is very probable that they were trying to demonstrate just how distant they could achieve the effect of the binaural sound and not surprisingly this film has some of the best stereo effect recordings of all. At one point, a fast passing train is captured on film and the soundtrack clearly distorts as the microphones (which on this occasion were probably ribbon-velocity types), suffers from being totally outside the limits of their magnetic field. This would have been due to the sheer amount of sound pressure level being placed upon them.

Once the auditorium was completed and readied, the binaural film equipment was moved inside for further tests and the next reel, which is almost certainly the first shot inside, does have a date on it, Friday, 12 July 1935. It runs for 116 feet, a total of 1 minute 15 seconds, and it shows Westlake, Vanderlyn and Trott playing tricks with a short pole which they hold and then try to climb over it without removing their hands from the ends. This footage thus became known as the 'stick-trick' film. The sequence opens with Turnbull holding a clapperboard, which looks to be made of two pieces of wood approximately two feet long. The clapperboard system was probably needed to synchronise the sound to the film.

There is real element of comedy to the footage as Westlake hands Vanderlyn the 'stick' and then proceeds to contort himself and climb through it. Felix Trott then appears from the right wearing a hat and his white laboratory coat. Trott wants to have a go at the stick trick, so he takes off his coat while Vanderlyn, standing on the left, takes off his hat for him. "Oh no", says Trott, "I can't do it without my hat on", and takes it back from Vanderlyn who had placed on his own head. Trott then tries, without much success at first, to perform the stick trick, though he does eventually almost get there. Just as Trott has almost completed the trick a voice from off-stage, which sounds like Blumlein, but could also be Turnbull, says, "OK. Stop" and the film ends there.

Figure 3.39 - Binaural film 'Stick Trick', Vanderlyn on the left, Trott with the stick, Westlake on the right (Courtesy of EMI)

Figure 3.40 - Still image from the binaural film 'Stick Trick', Trott halfway through the trick (Courtesy of EMI)

The contrast of the soundtrack on the film itself is very low (determined by how opaque the dark parts on the film are relative to the transparent area of the film). The soundtrack therefore suffers from not having a simultaneous development process to the pictures in the chemical bath (this did not come about until some years later when colour development was understood). The level of modulation on the film is also quite low and these two factors combined led to very noisy reproduction when played back.

Four days later on Tuesday, 16 July 1935, two further tests were filmed, the first of which is a section of footage 115 feet long, lasting for 1 minute and 14 seconds, and again shows Westlake, Vanderlyn and Trott this time talking among themselves. Following this is a second section of the reel, which is just 35 feet long. It is this section which has become known as the 'Walking and Talking' film. This short section is particularly fascinating as Blumlein himself takes part in the test; it is believed to be the only known instance of Alan Blumlein captured on film. It is also one of the few occasions where his voice is recorded (apart from the short sequence of talking with Felix Trott on the binaural recording of 14 December 1933).

The sequence opens with a rather bored-looking, white-coated Felix Trott, standing on the stage with the large drape hanging down behind him, holding the clapper-board. By this time the microphone had been improved so that it could be mounted closer to the camera and it could record sound from the whole of the stage. Felix Trott, looking directly at the camera, then 'claps' the pieces of wood together and walks off-stage to the right saying nothing. From the left appears Herbert Holman, wearing a dark suit, striding five paces to cross the stage to the right saying 'one-two-three-four-five', as he goes, but without ever once deviating from his gaze ahead to look at the camera.

Then Blumlein appears, looking directly at the camera. He is wearing a lighter, grey suit, the jacket of which he is attempting to button as he too strides across the stage from left to right, covering the area in front of the camera in six steps, and counting 'one-two-three-four-five-six'. It is just possible to hear him say 'seven' as he disappears off stage having successfully buttoned the inside button of his jacket by this time. Blumlein's voice is clear, concise and determined sounding. He has a quite definite London accent and, compared to his colleagues, sounds quite a bit louder - perhaps even a little droney.

Figure 3.41 - Still image from 'Walking & Talking' with Blumlein entering on the left and Holman to the right (Courtesy of EMI)

Next comes Westlake, who is wearing a long white coat over a dark suit and looks as if he is fixing his tie just as he walks onto the stage. His voice is distinctly quieter than Blumlein as he too chants 'one-two-three-four-five-six', also glancing at the camera. Westlake is followed by the tall, thin figure of Philip Vanderlyn who appears, looking directly at the camera, wearing light grey trousers and a white shirt (which looks as if it has no tie), with his sleeve rolled up. Vanderlyn is the clearest of the speakers as he walks quite precisely across the stage counting to 'six'. Following Vanderlyn and appearing before he has reached 'five', is Ivan Turnbull, who appears some time before Vanderlyn has exited the stage to the right. Turnbull speaks rather quietly as he strolls quickly across the stage also wearing a white shirt, sleeves rolled up (this time definitely with a tie). Turnbull also only gets as far as 'five', when Felix Trott appears and walks across the stage, not really looking at the camera, but getting to quite a distinct 'seven'.

The sequence then begins again with Holman appearing for a second time, crossing again without looking at the camera, counting to 'six'. Blumlein now also appears for a second time, looking directly at the camera again, his jacket securely buttoned as he walks to a point about one third of the way across the stage. He only counts as far as 'two' however, before the audio on the film stops. Blumlein continues to walk across the stage counting in silence until he reaches 'five', by which time Westlake has appeared again following him. It is here that the film runs out. This entire sequence lasts for just over 24 seconds.

Blumlein decided to use the HB1Bs for the tests, with an additional pair of high frequency microphones as the existing pressure microphones available would be too large and cumbersome to be spaced sufficiently close enough together. The high frequency microphones were of the ribbon type, which are very delicate to work with and these particular ones each had two ribbons inside, with an absorber behind the ribbon to turn it from velocity to pressure sensitive operation. At first, these high frequency microphones gave enormous trouble to the team, failing to give either a decent frequency response or any degree of sensitivity. Henry Clark decided to apply Blumlein's velocity calculations to the two new microphones and discovered that, with a few alterations, they too could be made to work, which they did eventually after a series of configuration tests.

Following the 'Walking and Talking' film, several other tests were carried out in July 1935. There is an additional piece of footage, 117 feet long, running for 1 minute 16 seconds, which has the EMI fire engine (there was a small Company fire department at Hayes) driving across a field which was just around the corner from the factory building. Though the 'Fire Engine' film is not dated, it is entirely possible that this was filmed after 16 July 1935. Pictures taken by Maurice Harker on the day clearly show that it was a bright summer's day presenting ideal lighting conditions.

 

Figure 3.42

 

Figure 3.43

 

Figure 3.44

The fire engine drove back and forth across the field past the binaural microphones which were placed on stands and protected from the wind by square shields. They were driven by two Austin Seven batteries which had been borrowed from two cars in the EMI car park for the afternoon. As the filming progressed, Blumlein left his office to join Vanderlyn, Trott, Turnbull and Harker to see how everything was progressing. By pure coincidence, just as he strode across the field to the camera position, Maurice Harker took a photograph. This photograph remained unknown in a photo album kept by Harker untouched for over sixty years.

 

Figure 3.45

The outdoor binaural films were considered to be something of a success with perhaps 'Trains at Hayes Station' being the 'best' known of the results. Once back at EMI, the unused (at that time) top floor of the R&D building was converted into a makeshift laboratory where tests could be made with both the microphones and the camera. Any alterations that were necessary could then be made while the auditorium was being prepared for the next and most ambitious of the binaural films that would take place that summer of 1935.

 

Figure 3.46

Somewhere around the time that the fire engine film was being taken, acoustic damping was installed in the auditorium to help with the reverberation problems that had been encountered and, on Friday, 26 July 1935. Blumlein had decided to use the indoor stage again for a short film which became known as 'Move the Orchestra'. It has also become known as the 'Playlet' and is, in actual fact, two short versions of the same scene, where an several members of the EMI Amateur Theatrical Group produce a 'typical' pub scene, with people attempting to order drinks from an incompetent waiter.

The camera was placed front and centre of a 20 foot-wide set, and does not move. The scene was lit with 30kW of lighting and the binaural sound reproduction system, despite the earlier problems with filming indoors seemingly rectified, was still proving problematic. The scene opens in a small café or bar with a woman and a young boy sitting on the left. The manager shows in a young lady who asks for a quiet table and is shown to a seat on the right. She then asks the manager to send the waiter. At this point, the music begins. A band is supposedly playing out of shot and off to the right (the music, which can be plainly heard to the right, was produced by a gramophone to the right behind the draped curtain). The lady, on hearing the music, winces and complains: "I said I wanted a quiet table, this is much too near the band. Can't you do something about it?" (she asks). "I'm afraid not Madam. You see, the band is very popular with the rest of my clients", the manager replies in a French accent. "Well can't you blow it up or move it or something?" she says. "Oh Madam. I could not blow it up!" replies the astonished manager, "I could move it"

Figure 3.47 - Still image from 'Move The Orchestra', the playlet filmed on 12 July 1935 (Courtesy of EMI)

He then claps his hands, tells the band to move and, magically, the sound moves to the left-hand side of the shot, appeasing the customer. What had happened? Two juniors had been recruited to carry the gramophone from one side of the set to the other, behind the backdrop, from the right hand side of the scene, where the lady had complained, to the left side as a waiter waves the 'band' across. The scene then continues when a gentleman joins the lady at her table, the waiter finally arrives and they order two "Gin & It's". The waiter then proceeds to drop everything he carries, first off to the right and then, after bringing the drinks and being paid (1/6d), he disappears off to the left and more crashing sounds are heard.

Blumlein, despite the obvious humour intended, was actually trying to demonstrate the practicality of associating sound location with visual representation of the source of the sound itself. It was, and indeed still, is an integral part of film production, where the 'soundscape' is very often backing-up the visual image. The 'Playlet' was filmed twice, once at 396 feet, lasting for 4 minutes 13 seconds, and again for 336 feet, lasting 3 minutes 33 seconds. The first take was reviewed and the sound level considered too low, and so it was raised for the second take, though it has to be said it hardly improves.

The following day, Saturday, 27 July 1935, the film was taken to Humphries' Laboratories for processing. The edited, finished version of 'Move the Orchestra' was completed by early September 1935, just when work on the binaural film project was abruptly halted, by which time Blumlein in his 'weekly reports', had considered the binaural sound to be "...fairly satisfactory technically". In August, the squeeze track had been added and a noise reduction circuit included as well as new galvanometers with mechanical adjustment and, by mid-September, improvements were also made to both the crystal and shuffled velocity microphones. In addition, EMI installed a purpose-made cine-screen to give 50% more picture brightness. The tests were halted however, probably because of the pressing need to complete the high definition television system first. Shoenberg, always a realist, had concluded that it was better to learn to walk before trying to run, and so binaural audio and film tests were shelved for the time being and Blumlein's talents, and those of the staff working with him, were re-directed to other tasks.

The results of the binaural film tests however, all of which were filmed on the volatile nitrate stock, are a fascinating insight into the world of Blumlein, EMI and his colleagues at the time. There are seven reels in all and they still lie in the EMI archives in their original tins (though now, thankfully, due to the unstable chemical nature of the nitrate film, they have been transferred to acetate-based stock and various forms of digital media). This needed to be done because nitrate is highly volatile, prone to degradation and eventually self-combustion. Like much of Blumlein's work, the material was to end up in the vaults of EMI, where it lay for over 50 years untouched. In 1982, and only after determined and constant persistence from a number of concerned individuals, EMI undertook to engage Norman W. Green, of the Independent Television Companies Association, to transfer the now delicate film to a modern acetate-based safety stock. It was from this which video copies could be made and eventually EMI also transferred the footage to Laser Disc. Alan Blumlein's binaural films were shown in public for the first time at a theatrical presentation in February 1988, and at the Audio Engineering Society 'Sound with Pictures' Symposium in May 1988. They are, despite their historical significance, still not available on video for the general public to see, though the now safe acetate-based copies and the Laser Disc once again reside in the EMI archives.

As for binaural sound recordings, suffice to say that it was not until 1958, some 16 years after Alan Blumlein had died and, 27 years after it had first been Patented, that 'binaural' made its next appearance as 'stereo' on long playing records. By this time, unfortunately for EMI, the Patents had long since expired. After the war, because of the nature of the commercial losses that had befallen so many manufacturers, the British Patent Office ruled that Patents registered pre-war could be extended for an additional few years. This allowed EMI to extend Alan Blumlein's Binaural Sound Patent, which had expired in 1947, for another five years until 1952.

However, they still did not apply the material within the Patent to any practical project that they were researching at the time. When the Patent finally expired on 13 December 1952, EMI had not used Blumlein's material based on binaural sound for a project of any kind since the mid-1930s.

Figure 3.48 - Arthur Haddy, who was in charge of the Decca stereophonic sound experiments in the 1950s (Courtesy of Eric White)

In 1955, Decca, one of EMI's great rivals, largely unaware of Blumlein's work some 24 years earlier, tried to Patent a dual-channel sound system which they called 'Stereophonic Sound'. Almost at once, Decca's engineers realised that they were covering ground which Alan Blumlein had explored more than twenty years earlier. Arthur Haddy, who was in charge of Decca's technical department at the time, thus became aware that many of the problems that they had already been considering for the last five years, had all been worked on by Blumlein in 1931. Despite much effort on the part of the research team at Decca, every possible alternative to their system had already been considered and contained within Alan Blumlein's specification No. 394,325; Decca conceded defeat. The exercise did however, at last bring to light the possibilities of stereophonic sound and EMI, finally realising what they had on their hands, began experimenting again with the system in April 1955. At Abbey Road a series of tests were made by Sir Malcolm Sargent and, a few months later, stereo tape recordings were being released for the first time. Eventually this led to the first stereo long playing records appearing in 1958.

What Alan Blumlein had described to his wife Doreen in that cinema one day in 1931, was eventually to prove to be one of the most significant developments in audio engineering of the twentieth century.

Moving On Again

By the time the last of Blumlein's eleven audio Patents was being applied for, his attention had long since been diverted to the all-out effort being made by everybody at EMI to perfect a high definition television system and succeed in providing Britain with a practical public television service. It is a measure of the man however, that even during so frantic a period as that which pervaded the EMI laboratories during 1934 and 1935, that Blumlein still had the time, energy and will to improve upon aspects of the Columbia recorder (which, incidentally was known at Columbia as 'The Bacon Slicer').

In 1932, he had re-designed the cutter itself, with the new version designated the MC4. This had rubber-mounting blocks to aid suspension instead of the steel springs which the earlier version had used. In this way the mass of the moving elements and drive power were considerably reduced. This was improved upon further in the MC4A, which was capable of recording frequencies up to 11kHz, primarily because of the lower moving mass. Then finally, another version, the MC4B, was introduced with rubber interposed in the path of the stylus. This was matched to a power amplifier, a PX25, with an output impedance capable of handling the new load on the cutter and it was this version that eventually became part of the commercial system. Several of these were installed at Abbey Road with one remaining in constant use, unaltered, until 1948.

Blumlein's Patent, No. 417,718, 'Improvements in or relating to Vibratory Devices such, for example as are used in the Electrical Recording or Reproduction of Sound', was applied for on Tuesday, 7 March 1933. It was an improvement to the vibratory properties of the recording system outlined in the earlier Patent No. 350,998, which, together with Patent No. 350,954, constituted the Columbia recorder. Though the original work had been co-written with Herbert Holman in 1929/30, the improvements outlined here were apparently all Blumlein's work. This would appear to be evidence of his known trait for returning to various pieces of his work from time to time, where he would ponder over various aspects and then invariably come up with some improvement. In this Patent, 16 additional claims are made, along with several re-designed drawings to illustrate the changes.

"The Patent describes a vibratory device incorporating a moveable armature which is mounted so that it can oscillate about a substantially fixed axis, and for practical reasons this armature has a slight freedom of movement in a direction normal to this axis. It had been found that is certain sound recording devices it was necessary to make the torsion compliance of the armature mounting about its axis of vibration, fifty times or more the lateral compliance. The invention therefore outlines a torsion bar, made for example of steel, wherein the stylus bar is pivoted about a point, with sufficiently high torsion compliance so that the strain does not make it fragile to the point where it would easily break. This was to be done by a means of damping of the armature movement, which is mounted on a resilient bearing, the motion of which in the direction at right angles to the longitudinal axis of the shaft subjecting the resilient bearing to compression. A form of the armature support is also noted within the Patent, outlining the use of short rubber tubes for vibration support. The compliance of these rubber supports, in respect of the rotary motion of the armature about its axis, would provide an improved form of mounting for the moveable element in the recording device"

Alan Blumlein's penultimate audio Patent during this period deals with the subject of the problems that were being encountered with low and very low frequencies, when two pressure microphones are arranged in order to achieve a binaural recording effect. The relationship between the microphones became ambiguous if the distance between them exceeded half the wavelength of the sound waves in question which was obviously unsatisfactory especially for mid and high frequencies of the range. The answer had been touched upon in his Binaural Sound Patent No. 394,325, but not really elaborated upon.

Placement of two microphones closer to each other did produce the desired results, but led to phase cancellation difference problems at the microphone outputs. While, theoretically, this did not present a major problem, it did mean that the microphones themselves and any related circuits had to be designed so that they were very sensitive to small phase changes. This in itself required the use of large amplification and transformers designed to introduce accurately controlled phase changes at low frequencies. The introduction therefore, of great amplification could lead in turn to the introduction of unwanted noise.

The main objective of Patent No. 429,022, 'Improvements in and relating to Sound-Transmission, Sound-Recording and Sound Reproducing Systems', applied for on Monday, 23 October 1933, was to overcome these difficulties. "The impulses, picked-up by a plurity of microphones, are reproduced by a plurity of loudspeakers, thereby conveying to the listener the sense of localisation of the sound source". This was outlined further with a series of diagrams illustrating how the Patent should be applied.

"The Patent describes a shuffling network, whereby phase differences between the outputs of microphones, in a spaced relationship designed for the picking up of sound to convey to the listener a localisation impression of the sound source, are converted into differences of loudness between loudspeaker outputs. The diagram clearly shows two pairs of pressure microphones 1 and 2, 3 and 4. These are arranged symmetrically about the centre line or axis of the sound field; one pair, 1 and 2 being widely spaced from one another, and the other pair, 3 and 4 being closely spaced. Separate shuffling networks 5 and 6 are provided one for each pair of microphones.

Figure 3.49 - Detail from Patent No. 429,054 (1934) showing relative microphone positions 'S1' and 'S2' with shuffling circuit

"From this diagram, and the explanation of networks outlined in Patent No. 394,325 it can be seen that microphone pair 3 and 4 are suitable for dealing with middle and high frequencies, but not entirely satisfactory as regards low frequencies; while the wider spaced microphones 1 and 2, although convenient for low frequencies introduce ambiguity in the directional impression which they provide with regard to medium and high frequencies. The shuffling networks therefore are arranged in order to deal with the specific frequency tendencies associated with the relevant pair of microphones. The voltages from the shuffling networks transformed to a common impedance (R) are fed to two loudspeakers which are common to the two networks.

"Two pairs of leads (11 and 12) are shown to connect each shuffling network to each loudspeaker. The pair of leads from the high frequency shuffling network (6) i.e. the network associated with the microphones that are closely spaced, are in each case shunted by an inductance (9 and 10) the value of which is determined from the equation below. The pair of leads from the low frequency shuffling network (5) i.e. the network associated with the microphones that are widely spaced, are in each case shunted by a condenser (7 and 8). Because the leads to each loudspeaker are connected to the outer terminals of the appropriate inductance and condenser, these remain in series as a shunt across the loudspeaker.

"The condenser 7 and inductance 9 are in series with one another as a shunt across the leads 11, while condenser 8 and inductance 10 in series, form another shunt across the leads 12. The relative values of the inductance's and capacities employed are determined from the following equation:

Equation 3.02

L represents the value of the inductance, C the value of the capacity, and R the value of the impedance"

Blumlein went on to discuss the possibility of inserting various filter types and several alternative types of microphone, as well as to the microphone spacing itself. In essence what is being outlined here is a form of binaural recording using two microphones. Years after, this would be known as the 'Blumlein stereo microphone technique', and is still used to this day with little or no alteration necessary.

Finally, the last of Alan Blumlein's eleven, definitively 'audio' Patents, is No. 429,054, 'Improvements in and relating to Sound-Transmission, Sound-Recording and Sound Reproducing Systems', applied for on Saturday, 10 February 1934. Once again, he had returned to an older theme, that of binaural sound, and had come up with some modifications and improvements. In some respects, it can be viewed as an answer to an article that had appeared in The Journal of the Acoustical Society of America, Volume V, October 1933. In the article, an arrangement of two microphones is described, one described as a 'velocity' microphone and the other a 'pressure gradient' microphone. It is possible that a 'pressure gradient' microphone had been modified with a damped 'backpipe' to convert it to omni, but the term 'pressure gradient' had stuck (today, a velocity and a pressure microphone type would be considered one and the same thing. Here however, they seem to refer to a directionally sensitive and a non-directionally sensitive microphone) had been used together to form a single microphone element. Together, they had gathered sound waves from a source in order to convey to the listener an impression of binaural sound.

Alan Blumlein had in fact already discussed this possibility within Patent No. 394,325, but it was obvious, and evidently essential to clarify his meaning within a separate and defined Patent, hence No. 429,054. While it is highly unlikely that the article in the American journal was intended to infringe upon any potential device from the Blumlein Patent, it is also possible that there had been a misinterpretation of the use of the different microphones. Specifically, the use of a velocity and a pressure gradient microphone pairing, in order to develop a new Patent outside of that being lodged by EMI. Regardless of the reasons for the American article, and it should be pointed out that many American companies were at the time also developing recording devices similar to that which Blumlein and Holman had Patented for Columbia. It was therefore felt necessary that EMI and Blumlein clarify their position with regard to this specific use of a stereo microphone technique with a definitive Patent and two diagrams illustrating the point.

"The invention and outlines a series of processes in which various arrangements of microphone types are used to convey to a listener a binaural impression of sound. More specifically, it relates to the use of a velocity microphone used in conjunction with a pressure microphone, which together represent a single piece of apparatus. In this instance the two microphones can be brought much closer together without the previous phase problems that had been encountered in Patent No. 394,325 and described at length in Patent No. 429,022. The outputs of the microphones were fed to two channels that in turn fed the loudspeakers. One channel received and transmitted the sum of the microphone outputs, while the other receives their difference. These outputs are in phase, but their relative magnitudes depend upon the direction of the sound arriving at the microphones, and these can in turn be made suitable for binaural sound by arranging them to be insensitive to sound arriving from a central position"

The two diagrams that were used to demonstrate how Blumlein intended this to work explained the detail further: "In figure 1 a directionally sensitive, velocity (or strip) microphone (a) is placed in close proximity to a non-directionally sensitive pressure gradient microphone (b). These are in turn connected to a network (c), the outputs of which pass through leads d and e to two loudspeakers. The strip of microphone a is arranged along the axis of the sound field and is clearly insensitive to sounds from a sound source S1, but has a maximum sensitivity to the sounds emanating from the source S2.

"The sensitivity of microphone b is substantially independent of the sound source. The leads d and e transmit respectively half the sum and half the difference of the microphone outputs, and with the sound source S1 the two outputs in the leads are the same, whereas with the sound source in position S2 it is the microphone outputs that are the same. The impulses in leads d and e are therefore a maximum value as well as a zero value, which will in turn provide full sound in one loudspeaker, and substantially none in the other, thus giving the correct impression of a laterally displaced sound source.

"Figure 2 shows an alternative arrangement where microphones of a similar impedance, as in the case of two velocity sensitive microphones, can be used. The two microphones are represented by a and b respectively, with the terminal of one connected to one terminal of the other. The remaining terminals of the microphones are connected to the opposite ends of the primary winding (f) of a transformer.

"The primary winding of a second transformer (g) is connected between the common microphone terminals and the mid-point of the first transformer primary (f). In this way the output of the secondary of one transformer will consist of the sum of the microphone impulses, while the output of the secondary winding of the other transformer will consist of the difference of the microphone impulses"

Essentially, what Blumlein was saying, was with careful choice of microphones one being directionally sensitive of course, and suitable transformer ratios in order to obtain the desired ratio of the microphone outputs to the loudspeakers, the sound impulses could be controlled to provide the binaural effect aimed at by the system. In so doing, the relative amplitudes of the sound impulses in the two channels could accord with the direction from which the sounds reached the microphones. It succeeded in getting the message across, and no further questions such as those raised The Journal of the Acoustical Society of America were published in his lifetime.

Postscript

As happened often in Alan Blumlein's life, his attention would be drawn away for various reasons from the specific task at hand, and moved towards another subject. This might be a subject which, at first, would seem to be totally unrelated. This would happen on two further occasions during the period when his audio Patents were being applied for, in which he returned to the subject of telephony. In 1932, while working on his binaural sound system, Blumlein applied for Patent No. 402,483, 'Improvements in Electric Coils'. In this specification, he provided a means for a small and easily constructed inductance or loading coil to be constructed. It was designed to give a small value of effective resistance for a given inductance at telephone frequencies, but only produced a very small magnetic field, and was therefore not liable to induction from external electric fields.

In order to achieve this, Blumlein encircled the windings of the coil with a magnetised circuit comprising a forward path through the coil to a point such that the line of flux turned outwards at 45° degrees towards the coil axis. In this way, the forward path of reluctance was high compared to the return path reluctance for lines of force passing far from the windings. Arrangements of the outer magnetic circuits and the air gap spaces between them were made in order to ensure that the ratio of reluctances of the forward to return paths was higher for lines far removed from the windings than for lines closely adjacent to them. This ensured that at low frequencies, the proportion of the total magneto-motive force used to drive the flux on the forwards path of the outer magnetic circuits was much greater than the proportion used to drive the flux through the forward path of the main magnetic circuit.

It was an effective shielded loading-coil for telephone circuits. The question is, why did Blumlein suddenly, three years after he had last worked on telephone circuits return to the subject? The answer probably lies in the subject matter which we associate him working with in June of 1932, the binaural system. Blumlein had, by this time, completed the many experiments with the Columbia recording/cutting system, which of course had an electro-magnetic damping system. It was very likely this and his growing understanding of electromagnetic and magneto-motive forces, which inspired his return to telephony and an application which led to yet another Patent specification being filed in his name.

Yet another example of a Blumlein Patent which at first sight seems to be totally out of line with the subject he was working on at the time, was applied for on 7 March 1933, Patent No. 419,284. This is a method of removing the screening from an iron or iron-alloy core of an inductance element in order to reduce the loss of frequencies of the order of 200 kHz or more. This was due to self-capacitance of the windings of the element and the generation of eddy currents in the core.

These eddy currents in the core were produced because the resistance was not as high as that of a good radio frequency dielectric material and were compounded by the self-capacitance of the windings; the losses were therefore comparatively high. Blumlein's answer to this was to provide a radio frequency circuit having an inductance coil in which capacitive losses were reduced. He did this by making the core windings of a magnetic material in so fine a state of subdivision that the coil could be employed without any undue loss of frequencies because an electrostatic shield was effectively disposed of between the core and the windings. This Patent once again demonstrated how amazingly flexible Blumlein's mind could be, as it is only marginally associated with the main topic on which he was working at the time. His colleagues would remark repeatedly on his ability to adapt trains of thought that he had to any subject and still produce an invention practically out of nothing.

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