Application Date: April 14, 1935. No. 12370/35.
Complete Specification Left: April 3, 1936.
Complete Specification Accepted: Oct. 26, 1936.
Improvements in or relating to the Transmission of Impulses of Different Kinds over a Common Channel, for example in Television and like Systems
I, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-
The present invention relates to the transmission of impulses of different kinds over a common channel, for example in television and like systems. The invention I for example applicable to systems of the kind in which synchronising impulses are transmitted in the line and frame intervals, that is to say the intervals between the scanning of successive lines and frames of a picture, and in which the line frequency impulses are arranged to persist throughout the framing intervals. The last named feature is usually necessary in order to ensure that the oscillation generator which is held in step at the receiver by the line impulses shall not fall out of step during the frame intervals.
According to the present invention, there is provided means for transmitting two different kinds of spaced electrical impulses along a common channel, said means comprising a switching arrangement whereby either kind of impulse can be fed to the channel, wherein the switching arrangement is such that the changeover from one kind of impulse to the other is automatically prevented from taking place during the occurrence of any of said impulses.
According to a further feature of the present invention in a system of the kind referred to, there are provided a source of line frequency impulses of relatively short duration, a source of line frequency impulses of relatively long duration, a source of frame frequency impulses and means for utilising the frame frequency impulses to control a switch in such a manner that during the frame impulses the line impulse of longer duration are fed to a transmission channel whilst at other times the line impulses of shorter duration are fed to the channel. Means can hen be provided for rendering the switch unresponsive to the frame impulses during a line impulse of longer duration so that the switch is only operated by the frame impulses during an interval between successive line impulses of longer duration.
The invention will be described as applied to a television system but it will be understood that in its broadest aspect it is not limited to television and like systems.
For the sake of simplicity the line impulses of shorter duration will be referred to henceforth simply as the line impulses, the line impulses of longer duration being referred to as the broad pulses.
The television system to be described by way of example is one employing interlaced scanning in which the picture to be transmitted is completely scanned in two traversals thereof, the path followed by the scanning in one traversal interlacing with that followed in the next traversal. The pictures scanned in each traversal may be either the same picture or successive frames of a motion picture film.
At the transmitter there are generated master oscillations at twice line frequency. By means of suitable frequency dividers, which may take the form of multi-vibrators, there are produced impulses at line frequency and impulses at frame frequency.
The line frequency pulses are fed to the input of a multi-vibrator which generates therefrom line impulses of substantially rectangular wave form and of duration equal t about 10% of line period. Thus the controlling line frequency pulses are fed through a condenser to the suppressor grid of the first of two pentodes having their anodes and control grids cross-connected though condensers and having grid leaks connected between their control grids and the common cathodes. Current is fed to the anode circuits of the pentodes through suitable resistances. The time constant of the condenser connecting the anode of the first pentode and the grid of the second and the resistance of the leak connected to the second control grid is made long compared with the period of the controlling pulses. In one practical example the condenser is of 0.01 microfarads and the resistance of 2 megohms. The other cross-connection condenser may be of 50 micro-microfarads capacity and the grid leak of the first pentode may be variable and of about megohm. Adjustment of this variable resistance serves to adjust the duration of the pulses generated.
The first pentode is normally conducting and the control impulses, making the suppressor grid more negative, render the pentode insulating. The positive pulse thus transmitted to the control grid of the second pentode renders it conducting and a negative pulse is applied to the control grid of the first pentode. When the charge on the condenser between the grid of the first pentode and the anode of the second pentode has leaked off sufficiently the first pentode will again conduct and the cycle will have been completed.
Line impulses (of duration equal to about one tenth of a line period) from the control grid of the second pentode are fed to the fourth grid (counting from the cathode) of the second of a pair of heptode valves. A second multi-vibrator is provided similar to that just described but adjusted to generate impulses at twice line frequency under the control of pulses from the master generator. These impulses are arranged to have a duration substantially equal to 35% of a line period and they are taken from the control grid of the second pentode to the fourth grid of the first of the pair of heptodes. The relative phase of the control impulse applied to the two multi-vibrators is adjusted (by suitable phase adjusting means) so that in the impulses fed to the heptodes the commencement of a line impulse coincides substantially exactly with the commencement of one of the broad pulses.
The anodes of the heptodes are connected though a condenser to an output channel and through a resistance to a source of anode current. the cathodes of the heptodes are connected together. The second, third and fifth grids of the heptodes are connected together and through a resistance to a point upon a voltage source which is positive relative to the cathodes.
It is possible to arranged, by feeding frame frequency impulses of suitable sense and polarity to the first grids of the heptodes, that during the framing impulses the first grid of the second heptode is so negative that the line impulse fed to the fourth grid (in a positive sense) are ineffective in producing an output signal and the first grid of the first heptode is at such a potential that the broad pulses applied to its fourth grid are effective in the output circuit. It is further possible to arrange that these conditions are reversed when the frame impulse is not present. The signals transmitted to the output channel thus compress line impulses excepting during the frame intervals when broad pulses are substituted for the line impulses.
It is found however that it is difficult to arrange that the frame pulses operate the switch (constituted by the potentials on the first grids of the heptodes) always during an interval between broad pulses as is clearly necessary. It is therefore preferred to proceed as follows:-
Controlling impulses of frame frequency obtained as will be explained hereinafter, are fed to the suppressor grid of a third multi-vibrator arranged to oscillate at frame frequency but arranged to operate in the same general manner as the other two multi-vibrators above described. This multi-vibrator is set to produce frame frequency impulses of substantially rectangular form and of duration equal to about three of four line periods for example. The values of the condenser and leak associated with the grid of the second pentode of this multi-vibrator in one example are 1 microfarad and 2 megohms respectively. The control grid of the second pentode is connected to the first grid of the first of the pair of heptodes and the anode of the same pentode is connected through a condenser to the fist grid of the second heptode. This latter grid is connected to the cathodes of the heptode though a grid leak.
Instead of applying the frame frequency impulses to this multi-vibrator directly they are applied in the form of positive pulses through a condenser to the first grid of an auxiliary heptode, this grid being connected to the cathode of the heptode through a leak. To the fourth grid of the auxiliary heptode are applied broad pulses in the negative sense. These may be derived from the control grid of the first pentode of the broad pulse multi-vibrator. The second, third and fifth grid of the auxiliary heptode are connected together and through a resistance and a large condenser in parallel to the cathode.
The apparatus then operates as follows:-
The pair of heptodes are so arranged that when the frame frequency multi-vibrator is inoperative (with the first pentode conducting) the second heptode of the pair has current flowing in its anode circuit and line impulses applied to its fourth grid are effective in sending line impulses into the channel associated with the anodes of these heptodes. The first heptode is under these conditions inoperative on account of a high negative bias on its first grid.
When a frame impulse arrives on the first grid of the auxiliary heptode, which is normally insulating, this valve is arranged to remain insulating if a negative broad pulse is at that time active on the fourth grid. As soon as the broad pulse ceases the auxiliary heptode conducts and the negative pulse thus applied to the suppressor grid of the first pentode of the frame multi-vibrator triggers off this multi-vibrator which acts as a switch and applies a positive pulse to the first heptode of the pair and a negative pulse to the second heptode of the pair. The latter therefore insulates, thus stopping the transmission of line impulses to the transmission channel and the first heptode becomes operative and broad pulses are sent to the channel. This condition is maintained until the frame pulse ceases, the duration of the pulse being determined by the adjustment of the frame multi-vibrator.
Thus it is arranged that if a frame impulse arrives during a broad pulse, the transmission of broad pulses does not commence until the next following broad pulse arrives whilst if the frame impulse arrives during an interval between broad pulses (which is always also an interval between line impulses) the next broad pulse is transmitted.
The transmission of broad pulses at twice line frequency to constitute the frame synchronising signal enables the oscillation generator responsive to the line impulses to be maintained throughout the frame intervals and further allows of interlacing without lack of symmetry of the signals.
In one example scanning in 405 lines per two frames, the frame frequency is 50 cycles per second the master (and broad pulse) frequency is 20,250 cycles per second and the line frequency is 10,125 cycles per second.
Dates this 24th day of April, 1935.
REDDIE & GROSE,
Agents for the Applicant
6, Bream’s Buildings, London, E.C.4.
Improvements in and relating to the Transmission of Impulses of Different Kinds over a Common Channel, for example in Television and like Systems
I, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-
The present invention relates to the transmission of impulses of different kinds over a common channel, and is particularly but not exclusively concerned with television and like systems.
The invention is applicable, for example, to television systems of the kind in which synchronising impulses of different kinds are transmitted in the line and frame intervals respectively, that is to say, in the intervals between the scanning so successive lines and frames of an object of which an image is to be transmitted. A known system of this kind, in which the present invention may be employed with advantage, is that described in British Specification No. 425,220, in which the synchronising signals transmitted are of unidirectional, impulsive character, leading edges at the line scanning frequency being arranged to persist throughout the framing intervals, in order to ensure that, at the receiver, the scanning oscillation generator which is held in step by the line impulses shall not fall out of step during the frame intervals.
The present invention accordingly provides apparatus for the transmission of spaced electrical impulse of different kinds, from either of two separate sources respectively, into common output circuit, in which there are provided switching means arranged, when in one condition to permit the transmission of impulses from one of said sources to said output circuit of impulse from the other of said sources, characterised in that means are provided for automatically preventing the change-over of said switching means from one of said conditions to the other from taking place except during the spacing periods between the impulses from at least one of said sources.
The invention further provides apparatus according to the preceding paragraph, particularly for television and like systems, wherein said two sources are arranged to generate respectively line frequency synchronising impulses of relatively short duration, and auxiliary synchronising impulses of a longer duration, said auxiliary impulses providing leading edges which recur in phase with and at the same frequency as said line frequency impulses, and wherein means are provided for generator switching impulses at fame frequency and for utilising said switching pulses to control the changeover of said switching means, the arrangement being such that auxiliary impulses are transmitted to said output circuit during the framing intervals and that line frequency impulses are transmitted at other times.
The invention will now be described by way of example with reference to the accompanying diagrammatic drawing, which illustrates the application of the invention to a television transmission system. In the drawing,
Fig. 1 shows a part of a television transmitter, and
Fig. 2 is an explanatory diagram to which reference will be made in the course of the description of Fig. 1.
The television transmitter to be described by way of example is one employing interlace scanning in which the object of which an image is to be transmitted is completely scanned in two traversals thereof, the path followed by the scanning in one traversal interlacing with that followed in the next traversal. The pictures canned in each traversal may be either the same picture or successive frames of a motion picture film. For convenience, the frequency at which traversals of the object take place will be referred to as the frame frequency.
Referring to Fig. 1, master oscillations of a frequency equal to twice the line frequency are generated by means of a master oscillator 3; assuming, for example, that 405 lines are scanned in each of two successive frame scans, and that 50 frames are scanned per second, the frequency of the oscillations generated by the master generator is arranged to be 20,250 cycles per second. The master oscillations may take the form shown in Fig. 2a, for example, and may be generated in any known or suitable manner. Master oscillations from generator 3 are fed to frequency-dividers 4 and 5, the outputs of which comprise respectively oscillations at line frequency and impulses of approximately rectangular form recurring at the frame frequency. The frequency-dividers 4 and 5 conveniently take the form of multi-vibrators of any known or suitable kind, and the impulses produced by frequency-divider 5 are preferably arranged to have a duration of the order of a line period.
Line frequency oscillations from frequency-divider 4 are fed to a multivibrator 6, which is arranged to generator line synchronising impulses of substantially rectangular wave form, and of a duration equal to about 1/10th of a line period; the impulses generator by multivibrator 6 are illustrated in Fig. 2b.
Oscillations from master generator 3 are fed directly to a further multivibrator 7, which is arranged to generate synchronising impulses at twice the line frequency, as illustrated in Fig. 2c, each of these impulses having a duration substantially equal to 40% of a line period.
Frame frequency impulses are fed from frequency-divider 5 to another multivibrator 8 through a heptode valve 19, the function of which will be described later and which will at this stage be assumed, for purposes of explanation, to be replaced by a direct connection; multi-vibrator 8 is arranged to generate a set of frame impulses, of the form shown in Fig. 2d, the frame impulses recurring at the rate of 50 per second, in the example under consideration, and each having a duration between 2 and 3 line periods. The frame pulses serve, in a manner which will be more fully described hereinafter, as switching pulses.
Line frequency synchronising impulses from multivibrator 6 are fed to the fourth grid (proceeding from the cathode) of a heptode valve 9 through a condenser 10 associated with a leak resistance 11, and broader pulses from multivibrator 7 are fed to the corresponding grid of a second similar heptode 12 though a condenser 13 having associated therewith a leak resistance 14. The anodes of heptodes 9 and 12 are connected together, and through a resistance 15 to the positive terminal of a source (not shown) of anode current, the negative terminal of which is earthed. The output of the apparatus is taken from the anodes of the heptodes through a condenser 16. The second, third and fifth grids of each of the two heptodes are connected together and to a point at a suitable positive potential is the anode current source, and the two valves are suitably biased by means of a resistance 17, associated with a by-pass condenser 18, connected in their common cathode circuit.
It will be appreciated that by feeding switching impulses of frame frequency (such as those generated by multivibrator 8) to the first grids of heptodes 9 and 12 in opposite polarity and at suitable amplitudes, it can be arranged that the heptode 9 is operative to feed line impulses to the output circuit, through condenser 16, at all times except during the framing intervals, while heptode 12 is operative to feed broader pulses to the output circuit only during the framing intervals. Such an arrangement, which forms, in part, the subject matter of co-pending Application No. 8299/36 (Serial No. 448,097), is found to have the following disadvantage: it is difficult to arrange in practice that the heptode 12, which is that which passes the broader pulses to the output circuit, is always switched on during an interval between successive broader pulses, so that there may appear in the output circuit mutilated broader pulses which have less than their normal duration. As will be clear to those versed in the art, if such a mutilation is permitted to take place, the scanning operation taking place at the receiver may be upset, and correct interlacing may be lost.
The frame-frequency impulses from frequency-divider 5 are accordingly not fed directly to the multivibrator 8, but are fed in the positive sense, through condenser 22, to the first grid of the heptode 19, the anode of which is connected though resistance 20 to the positive terminal of the anode current source and through condenser 21 to the input of multivibrator 8. The broader pulses from multivibrator 7 are fed in the negative sense to the fourth grid of heptode 19 through condenser 23, and heptode 19 is so biased, by means of biasing resistance 24, that no anode current flows therein except in the intervals between the pulses applied to its fourth grid. That is to say, the broader pulses from multivibrator 7 drive heptode 19 beyond cut off, and if a frame-frequency impulse from frequency-divider 5 arrives at the first grip of heptode 19 while a broader pulse from multivibrator 7 is operative upon the fourth grid thereof, the frame –frequency impulse does not become operative until the broad pulse disappears. The pulse set up at the anode of heptode 19 due to the frame-frequency impulse is then sufficient to trigger off multivibrator 8. The operation of heptodes 9 and 12 at other times that during intervals between broader synchronising impulses is thus avoided.
Fig. 2e illustrates the synchronising signal set up in the output circuit in the neighbourhood of a framing interval.
It will be appreciated that since 405 lines are scanned per two frames, and since the frame frequency impulses from frequency-divider 5 recur regularly at the rate of 50 per second, successive frame synchronising signals, constituted by groups of the broader pulses of Fig. 2c, will commence in different phases with respect to the line synchronising signals. Thus the first broad pulse of the frame signal following that shown in Fig. 2e will commence half a line period after the leading edge of the preceding line pulse.
The provision of broad pulses at twice line frequency during the frame intervals not only enables the line scanning generator at the receiver to be maintained in synchronism during those intervals, but also provides frame signals which are capable of producing very accurate interlacing.
The invention is not limited to television systems, but is applicable also to many telegraph systems in which there are transmitted impulses, or groups of impulses, which are of different kinds; the invention may also find application in systems of picture telegraphy in which a predetermined brightness value in the picture to be transmitted is represented by one or another of a limited number of kinds of spaced electrical impulses of fixed but different amplitudes or durations. Many modifications of the invention within the scope of the appended Claims will occur to those versed in the art.
Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-
Dated this 3rd day of April, 1936.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1936.