476,935

PATENT SPECIFICATION

Application Date: May 15, 1396. No. 13795/36.

Complete Specification Left: April 26, 1937.

Complete Specification Accepted: Dec. 15, 1937.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Television Systems

I, ALAN DOWER BLUMLEIN, a British subject, of 32, Audley Road, Ealing, London, W.5, Middlesex, do hereby declare the nature of this invention to be as follows:-

This invention relates to television systems and has particular reference to methods of obtaining automatic gain or volume control. When television signals are to be transmitted to or from moving objects such as aeroplanes, a rapid gain control effect is required.

According to the present invention in a method of obtaining automatic gain control in a television system, the carrier wave is modulated by picture signals, line and frame synchronising signals any be separate signals from which gain controlling potentials may be derived, said line synchronising signals modulating the carrier wave in the blacker than black sense and being succeeded immediately by grid separate signals which modulate the carrier wave in the whiter than white sense. The frame synchronising signals may modulate the carrier wave in the whiter than white sense and may be interrupted by the line synchronising signals. The gain control potentials may be derived from the separate signals by a radio frequency rectifier separate from the picture signal rectifier or a peak rectifier may be used fed with the modulation signals after demodulation.

In order that the nature of the invention may be more clearly understood a method of obtaining automatic gain control in accordance therewith will now be more fully described with reference to the accompanying diagram which shows an alternating current wave form explaining the invention. The wave form shown is intended for use with a transmission system in which the signals are transmitted without the direct current component being present but it will be understood that a wave form according to the invention may be used in a system in which the direct current component is present.

Referring to the diagram, the point 1 represents zero, the point 2 represent the mean value of the picture signal current, while 3 represents the peak or maximum of modulating signals. The amplitudes of the picture signals are limited to a range within the zero and peak points so that maximum white in the picture does not reach the peak value of the modulating signals and black does not reach zero value. The peak points are thus values representing whiter than white signals and the zero points represent blacker than black signals. The diagram shows several lines 4 each having a duration of 80 micro-seconds, together with line synchronising signals 5 obtained by reducing the carrier wave to substantially zero for 10 micro-seconds, and a frame synchronising signal 6 extending over ten complete line periods between the points A and B. As will be seen, the line synchronising signals 5 extend in the blacker than black direction and the frame synchronising signal 6 extends in the whiter than white direction.

Immediately following the line synchronising signals is an impulse 7 which extends in the whiter than white direction and is equal in duration to a line synchronising signal, that is a period of 10 micro-seconds. These impulses are used to provide gain controlling potentials at a receiver and for this purpose they may be rectified by a rectifier separate form the picture signal demodulator, or after demodulation a picture signal rectifier may serve to yield gain controlling potentials proportional to the height of the signals 7 and also the frame synchronising signals 6 which, as will be seen from the diagram are interrupted by the line synchronising signals continuing during a frame synchronising signal. The rectification is effected in such a manner as to provide negative potentials which can be used directly for controlling the gain of high frequency amplifying valves in the usual way. The time constant of the couplings between the peak receiver and the grids of the controlled high frequency valves is arranged to be longer than the duration of a line but may be shorter than a frame and as the frame period may equal one twentyfifth of a second it will be seen that a very rapid automatic gain control effect is obtainable.

The wave form may be generated by mixing in with the picture signals short and long large amplitude pulses both in the same sense as picture white and then limiting their amplitude so as to give a rectangular pulse wave form. In a later part of the transmitter large amplitude pulses at line frequency are mixed in the opposite sense relative to picture white and sufficiently earlier in time to form when limited, the rectangular blacker then black line synchronising pulses.

These latter pulses may be arranged to overlap in time the former pulses and are made of sufficient amplitude to annul part of them so that there is a clean transition from one pulse to the other. For instance, the pulses in the white direction may be made 19 micro-seconds in width and be delayed 1 micro-second with respect to the beginning of the pulse in the black direction. If the latter pulse is 10 micro-seconds in width the wave form shown in the diagram will be produced.

In order that picture interference may not occur it is necessary at the receiver to black out or suppress some of the pulses which occur in the white direction. If as in the case described the transmission is sent with a high value of carrier modulation representing white, the automatic gain control peaks must be blacked out to prevent them appearing as white in the picture. Again if a low value of carrier modulation represents white in the picture the line synchronising signals must be blacked out. Such "blackening out" may be effected in the manner set forth in the specification of the present Applicant’s Patent Application No. 25498/34.

Dated this 15th day of May, 1936.

F. W. CACKETT,

Chartered Patent Agent.

COMPLETE SPECIFICATION

Improvements in or relating to Television Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, Middlesex, do hereby declare the nature of this invention and n what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

This invention relates to television systems and has particular reference to methods of obtaining automatic gain or volume control.

When television signals are to be transmitted to or from moving objects such as aeroplanes, a gain control effect which will act rapidly is required, and while the invention has particular application to aircraft television systems and also to other light-weight equipment, it may also be employed in a general way, in wireless or line systems employing, for example, co-axial conductors.

It has previously been proposed for television purposes to modulate a carrier wave with picture signals and with line synchronising signals which modulate the carrier wave in the whiter-than-white sense and in the Specification of Patent No. 436,809 a wave form is shown in which a synchronising signal is constituted by a series of impulses the first of which modulates the carrier wave both in the directions of picture white and picture black. It has further been proposed to utilise synchronising signals at a receiver for the purpose of providing an amplifier gain control effect.

The object of the present invention is to provide a method of producing a television wave form from which automatic gain control potentials may be obtained and receiver circuits responsive to such a wave form.

According to the present invention in a television transmitting and receiving system the wave form sent out by the transmitter is a carrier wave modulated by picture signals and includes sets of both whiter-than-white and blacker-than-black signal pulses which occur at pre-determined intervals and the receiver is provided with an automatic gain control system which is designed to be responsive to the difference between the amplitude of said signal pulses, said receiver being also provided with a synchronising system which is designed to be responsive ton one set of said signal pulses. The particular wave form sent out by a transmitter in a system according to the invention is a carrier wave modulated by picture signals an by line and frame synchronising signals, the line synchronising signals being constituted by said blacker-than-black signal pulses each of which forms a portion of a double pulse, the other portion of which is constituted by one of said whiter-than-white signal pulses.

In a particular transmitting system for use in a system according to the invention the carrier wave sent out is modulated by said whiter-than-white and blacker-than-black signal pulses and also by frame synchronising signals which modulate the carrier wave in the whiter-than-white sense, said frame synchronising signals being interrupted by the line synchronising signals in the blacker-than-black sense whereby automatic gain control potentials and line synchronising signals can also be obtained during the occurrence of the frame synchronising signals. The whiter-than-white and blacker-than-black signal pulses are substantially equal in duration and form double pulses which are substantially rectangular in shape.

A circuit arrangement for producing a wave form in accordance with the invention may include a valve to which picture signals interrupted at the line and frame frequencies are applied, said valve having its output combined with a second valve, to which a set of pulses at the line frequency are applied, the combined output being passed to a third valve, the output from which is combined with the output from a fourth valve supplied with a second set of pulses at the line frequency, but advanced with respect to said first set of pulses.

A particular form of television receiver adapted to be responsive to a carrier wave modulated by a wave form produced in accordance with the invention, includes high or intermediate frequency amplifying stages, a picture signal rectifier and a rectifier which provides gain controlling potentials for said high or intermediate frequency amplifying stages, said gain controlling potentials being derived from one portion o the double pulses in the wave form. Part of the output from one of the high or intermediate frequency amplifying stages, said gain controlling potentials being derived from one portion of the double pulses in the wave form. Part of the output from one of the high or intermediate frequency amplifying stages is fed to a further amplifying stage, and another part is fed to a circuit arrangement including the rectifier which provides automatic gain control potentials, the output of said further amplifying stage being maintained constant by applying to it automatic gain control potentials of selected value.

In order that the invention may be more clearly understood and readily carried into effect, a method of obtaining automatic gin control in accordance therewith will now be more fully described with reference to the accompanying drawings in which:-

Fig. 1 shows a wave form including picture signals, line and frame synchronising signals, and additional signal pulses used for providing automatic gain control,

Fig. 2 shows a circuit arrangement for producing the wave form shown in Fig. 1,

Figs. 3 and 4 show respectively sets of signal pulses which are applied to the circuit of Fig. 2 for the production of the wave form shown in Fig. 1,

Fig. 5 shows a form of receiving circuit suitable for use with the wave form shown in Fig. 1, and Fig. 6 is a circuit arrangement illustrating detail in the operation of a receiver.

Referring to Fig. 1 of the drawings, the point 1 represents zero, the point 2 represents the mean value of picture signal current, while 3 represents the peak of maximum of modulating signals. The amplitudes of the picture signals are limited to a range within the zero and peak points so that maximum white in the picture does not reach the peak value of the modulating signals and black does not reach zero value. The peak points are thus values representing white than white signals and the zero points represent blacker-than-black signals. The diagram shows several lines 4 each having a duration of 80 micro-seconds together with line synchronising signals 5 obtained by reducing the carrier wave to substantially zero for 10 micro-seconds, and frame synchronising signal 6 extending over ten complete line periods between the points A and B. As will be seen, the line synchronising signals 5 extend in the blacker-than-black direction and the frame synchronising signal 6 extends in the whiter-than-white direction.

Immediately following the line synchronising signals is an impulse 7 which extends in the whiter-than-white direction and is equal in duration to a line synchronising signal, that is a period of 10 micro-seconds. These impulses are used to provide gain controlling potentials at a receiver in the manner to be described with reference to Fig. 5 of the drawings. The gain controlling potentials obtained are proportional to the height of the signals 7 and also to the frame synchronising signals 6 which, as will be seen from Fig. 1, are interrupted by the line synchronising signals continuing during a frame synchronising signal.

The wave form shown in Fig. 1 may be generated by the circuit arrangement shown in Fig. 2 of the drawings. Incoming picture signals are applied to the valve 8 through a coupling condenser 9, the picture signals being free of the pulses 5, 6 and 7 shown in Fig. 1. The intervals between the signals representing successive lines and successive frames may contain spurious signals such as are produced for example by an electrical scanning tube these spurious signals being suppressed by the introduction of the pulses 5, 6 and 7. Signal pulses 10 of the form shown in fig. 3 are applied to the grid of a valve 11 through a coupling condenser 12. These signals are mixed in an anode resistance 13 common to the valves 8 and 11 and are passed to a valve 14 through a coupling condenser 15, the valve 14 having a comparatively high resistance 16 in its cathode lead. The effect of the insertion of the resistance 16 is to give the characteristic curve of valve 14 a long linear portion with comparatively little curvature at the region of anode current cut-off. The valve 14 is so biassed as to cut off the pulses added by valve 11 and to produce flat tops on the pulses 7 shown in Fig. 1. The line frequency pulses 10 shown in Fig. 3 are of slightly longer duration than those obtained in the final wave form, and they occur towards the end of the interval between lines. A further valve 17 serves to mix with the output of valve 14 pulses 18 of the form shown in Fig. 4. The pulses 18 are applied to the grid of valve 17 through a coupling condenser 18a, the outputs of valves 14 and 17 being combined in a common anode resistance 19. The pulses 18 occur at line frequency only and end slightly after the beginning of the pulses 10 shown in Fig. 3. The over-lapping of the pulses 10 and 18 assist in producing a clean transition from one pulse to the other in the final wave form.

The mixed output from valves 14 and 17 is passed through a coupling condenser 20 to the grid of a valve 21 which also has a comparatively high resistance 22 in its cathode lead. The valve 21 tends to cut off the tops of the pulses 18 shown in Fig. 4, thus giving in its anode circuit a wave form of substantially the shape shown in Fig. 1. The beginning and end of the pulses 5 are produced by the pulses 18 shown in Fig. 4, while the pulses 7 are formed by the pulses 10 shown in Fig. 3, with the exception of the portions neutralised by the over-lapping portions of the pulses 18.

The resistances 16 and 22 in the cathode leads of valves 14 and 21 respectively, serve to depreciate the gain of these valves so that for a small current the characteristic is sensibly linear and a sharp cut-off is obtained at the bottom of the characteristic curves.

The long pulse 10f shown in Fig. 3 produces the flat top frame synchronising signal 6 shown in Fig. 1, this long frame pulse being divided up by the pulses 18 in Fig. 4, to produce the waveform shown in Fig. 1.

The waveform shown in Fig. 1 is used to modulate a carrier wave generated by transmitting equipment not shown in the drawings. The method of modulation is preferably such that the level 1 in Fig. 1 represents zero carrier and the level 3 represents carrier maximum. The inverse method of modulation may be adopted and again it is not necessary that the peaks 1 and 3 should represent zero or maximum carrier respectively, but they may represent intermediate values.

Fig. 5 shows diagrammatically the portions of a television receiver adapted to utilise the automatic gain control effect provided by the wave form shown in Fig. 1. Five stages of a receiver are represented by the block diagrams 22, 23, 24, 25 and 26. The output from the stage 25 is passed to an automatic gain control rectifying arrangement contained within the dotted rectangle 27. The signals fed from the stage 25 are rectified by a diode 28 in conjunction with condensers 29 and 30 of small value, constituting with an inductance 31, a radio frequency filter. The time constants of the condensers 29 and 30 with reference to the diode anode resistance is such as to pass the full frequency range of the signals or a sufficient frequency range to reproduce the pulses 5 and 7. The direct current biassing arrangements are not shown in the drawing as it is well known, for example, that a low frequency return must be provided for the lead connected stage 25 with the diode 28.

The rectified signals obtained from the diode 28 are passed to an amplifying valve 33, and from thence, through a coupling condenser 34 to diode valves 35 and 36 arranged to operate in push-pull fashion. The voltage set up across the diode anode resistance 37 will then be proportional to the double amplitude of the rectified wave provided by the diode 28. This double amplitude peak is in effect the distance between the peaks of successive rectified pulses 5 and 7 shown in Fig. 1. The arrangement is one which is capable of effecting a direct current re-establishment by the condenser 34 and diode 35, together with a peak rectification by the diode 36 co-operating with the resistance 37 and its shunt condenser 34 and the anode resistance 39 of valve 33 in series with the resistance of either of the diodes 35 and 36, is arranged to be longer than the duration of any pulse including the duration of the part of pulse 6 between any consecutive breaking pulses 5. In other words, this time constant should be longer than a line period. The time constant of resistance 37 and condenser 38 should be several times longer, and is the controlling time constant for the speed of the automatic gain control effect. The voltage across the resistance 37 is fed back through lead 40 to one or more high or intermediate frequency stages 22, 23, 24 and 25, and also to the stage 26 which is will be noticed follows the stage 25, from which the signals for providing the gain control effect are tapped out. Gain controlling potentials may thus be applied to one or more of the stages mentioned.

The control of the stage 26 is so adjusted that for average reception conditions the output of this stage is substantially constant. For example, a change of input of the order of 40 dbs. may produce change of rectified voltage at the diode 28 of from one volt to two volts. This may produce a change of rectified voltage across the resistance 37 of from ten to twenty volts, and this change of voltage applied to the controlled stage 26 must alter its gain by 2:1 so as to maintain substantially constant output. The output from the stage 26 is applied to a rectifier 41 and a filter 42 shown within the dotted rectangle 43, the rectified and substantially constant output signals being taken from a terminal 44. The adjustment of the stage 26 to convert the variable signals from the stage 25 into substantially constant signals my be made by means of a potentiometer not shown in the drawing which controls the fraction of the voltage across the resistance 37 applied to the control grid of the amplifier in stage 26.

In the case of a given sent of reception conditions, manual control of additional negative or positive bias potentials supplied to any of the stages 22 to 26 may be provided, so that the output from rectifier 28 may be kept within certain range for which the correction by the stage 26 is most satisfactory. The correction as shown in stage 26, which, as previously pointed out, operates beyond the point from which the signals providing the automatic gain control effect are tapped out, may be omitted, or the correction may be applied in one or more low frequency or high frequency stages, or both.

A rapidly acting gain control effect is obtained by the arrangement described, because the frequency of the recurrent pulses may be as high as 10000 per second. The quickness of operation of the automatic gain control effect is controlled by the time constant of the resistance 37 and its associated condense 38, which may, for example, be of the order of 10 lines. Thus, the time constant of the automatic gain control will be one hundredth of a second, which is very much shorter than is possible in normal sound channel automatic gain control systems.

In order that the picture interference may not occur, it is necessary at the receiver to black out or suppress some of the pulses which occur in the white direction. If, as in the case described, the transmission is sent with a high value of carrier modulation representing white, the automatic gain control peaks must be blacked out to prevent them appearing as white in the picture. Such blacking out may be effected in the manner set forth in the Specification of Patent No. 446,663, the circuit shown in Fig. 2 of the drawings accompanying that Specification being suitable for the wave form shown in Fig. 1 of the drawings accompanying the present Specification. Again, if a low value of carrier modulation represents white in the picture, the line synchronising signals must be blacked out.

An alternative form of circuit arrangement for effecting blacking out of pulses is shown in Fig. 6 of the drawings accompanying this Specification. In this Fig. the signals are applied to delay network represented by the block 45, and a tapping is taken from the input to the delay network to a limiter valve 46 which only passes amplitudes in excess of the peak white amplitude. The output from the limiter valve 46 is passed to a further delay network represented by the block 47, the time constant of this delay network being double that of the network 45. Signals are taken from the input and output of the delay network 47 and passed to the control grids of two valves 48 and 49 respectively, the anode currents of these valves being combined in a common load resistance 50. The voltage across the load resistance 50 is mixed with the aid of further amplifying valves 51 and 52 with vision signals obtained from the output end of the delay network 45. The mixed output from the valves 48 and 49 is fed through a condenser 53 to the control grid of valve 51, and the vision signals from the delay network 45 are fed to the control grid of valve 52 through a coupling condenser 54. The valves 51 and 52 again have a common load resistance 55, the mixed output being passed to a cathode ray tube connected to the terminal 56. The mixing of the signals in the valves 51 and 52 is in such a sense that the signals obtained from the delay network 47 serve to reduce to black the vision signals arriving from the delay network 45. The arrangement will in effect produce black vision signals in place of white from just before the white pulse period to just after the white pulse period, the amount of overlap being the delay of the first delay network 45. Any difference to delay for the direct vision path from the network 45 to the valve 52 and the blacking out path, including the delay network 45 produced by the inevitable stray capacities and inductances of the amplifying valves, may be corrected by altering the delay of the network 45 while maintaining the delay of the network 47 constant. The high frequency cut-off of the delay network 45 must be sufficiently high to ensure that the vision signals will be transmitted without distortion, but it is not necessary for the delay network 47 to be of the same quality. A period of overlap of the order of half micro-second is sufficient to ensure complete suppression of the white signals.

While alternating current coupling is shown in the circuit of Fig. 2, it will be understood that direct current coupling may be employed if the vision signals are required to contain the direct current component, provided such component is fed in at the input to valve 8 and direct couplings are provided throughout between the valve 8 and the modulator.

Having now particularly described and ascertained the nature of may said invention and in what manner the same is to be performed I declare that what I claim is:-

  1. A television transmitting and receiving system wherein the wave form sent out by the transmitter is a carrier wave modulated by picture signals and includes sets of both whiter-than-white and blacker-than-black signal pulses which occur at predetermined intervals and wherein the receiver is provided with an automatic gain control system which is designed to be responsive to the difference between the amplitude of said signal pulses, said receiver being also provided with a synchronising system which is designed to be responsive to one set of said signal pulses.
  2. A television system according to claim 1 wherein the wave form sent out by the transmitter is a carrier wave modulated by picture signals and by line and frame synchronising signals, the line synchronising signals being constituted by said blacker-than-black signal pulses each of which forms a portion of which is constituted by one of said whiter-than-white signal pulses.
  3. A television transmitting system adapted for use in the system according to claim 1 or 2, wherein the carrier wave sent out is modulated by said whiter-than-white and blacker-than-black signal pulses and also be frame synchronising signals which modulate the carrier wave in the whiter-than-white sense, said frame synchronising signals being interrupted by the line synchronising signals in the blacker-than black sense whereby automatic gain control potentials and line synchronising signals can also be obtained during the occurrence of the frame synchronising signals.
  4. A television system according to claim 2 wherein the whiter-than-white and blacker-than-black signal pulses are substantially equal in duration and form double pulses which are substantially rectangular in shape.
  5. A circuit arrangement producing a wave form in a television system according to claim 1 including valve to which picture signals are applied, said valve having its output combined with that of a second valve to which a set of pulses at the line frequency are applied, the combined output being passed to a third valve, the output from which is combined with the output from a fourth valve supplied with a second set of pulses at the line frequency, but advanced with respect to the first set of pulses.
  6. A television receiver adapted to be responsive to a wave form sent out by a transmitter operating according to any one of claims 1 to 4 and including high or intermediate frequency amplifying stages, a picture signal rectifier, an automatic gain control circuit and a line synchronising circuit, the automatic gain control circuit being designed to be responsive to the difference between the amplitude of said signal pulses and the synchronising circuit designed to be responsive to one set of said signal pulses.
  7. A television receiver according to claim 6, wherein part of the output from one of said high or intermediate frequency amplifying stages is fed to a further amplifying stage, and another part is fed to said automatic gain control circuit, the output of said further amplifying stage being maintained substantially constant by applying to it automatic gain control potentials of selected value.
  8. A television receiver according to claim 6, wherein part of the output from one of said high or intermediate frequency amplifying stages if fed to a further amplifying stage or directly to a picture signal rectifier, and another part is fed to said automatic gain control circuit which includes a resistance/capacity combination having a time constant equal to several line periods.
  9. A television receiver according to claim 6, 7 or 8, wherein a rectifier included in said gain control circuit is coupled through a resistance/capacity combination having a time constant longer than a line period to a pair of diode valves arranged to provide an output voltage proportional to the total amplitude of the double pulse synchronising signals.
  10. A television receiver according to claim 9, wherein the provision of the capacity in said resistance, capacity combination prevents the transmission of direct current to said diode and wherein said capacity in conjunction with the first of said pair of diode valves operates to restore the direct current.
  11. A circuit arrangement for producing an alternating current wave form substantially as described with reference to Fig. 2 of the accompanying drawings.
  12. A television receiver including an automatic gain controlling circuit arrangement substantially as described with reference to Fig. 5 of the accompanying drawings.

Dated this 26th day of April 1937.

F. W. CACKETT,

Chartered Patent Agent.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1938.