US2295023A - Television system - Google Patents

Description

SeptYS, 1942. w. A. BEATTY EI'AL 2,295,023

TELEVISION SYSTEM Filed March 26, 1940' 4 Sheets-Sheet l Fig. 1.

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TELEVISION SYSTEM Filed March 26, 1940 4 Sheets-Sheet 3 Flg 4 Fig.8;

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TELEVISION SYSTEM Filed March 26, 1940 4 Sheets-Sheet 4 'Fig. 9

//7Venton' Patented Sept. 8, i942 Chatterjea, Sidcup, Kent, England, assignors' to International Standard Electric Corporation,

New York, N. Y.

Application March 26, 1940, Serial No. 325,954. v In Great Britain April 3, 1939 Claims. (Cl. PIS-5.2)

This invention relates to methods of utilising synchronising pulses in television systems in such a manner that information or intelligence other than the normal frame and line synchronising information is conveyed in the various trains of line and/or frame synchronising pulses, the said additional information being inserted into the trains of pulses in such a manner that the normal frame and line synchronising information is still available. Such additional information can, for instance, relate to the D. C. component of a television transmission; alternatively, discrimination between one frame and another can be achieved in colour or stereoscopic television, or the system can be adapted for conveying information relating to interlaced scanning, or to the expansion or compression of the volume range of a sound signal accompanying a vision signal.

The system can be applied to television transmissions in which vision intelligence is radiated either as a positive or as a negative signal from a definite carrier level representing black, synchronising signals for the said vision intelligence being radiated as amplitude modulations of the carrier in the blacker than black carrier levels. However, for the purpose of explanation, modifications of the present British Broadcasting Corporation television system will be described. Some details of various embodiments of the proposal will now be given.

The invention and embodiments thereof will be described with reference to the accompanying drawings in which:

Fig. 1 shows a curve illustrating portions of a vision signal and frame pulse train as at present radiated by the B. B. C.;

Figs. 3, 4, 8 and show modifications of the pulse trains shown in Fig. 1;

Figs. 2 and 5 show diagrams illustrating carrier levels under different conditions;

Fig. 6 illustrates a type of target plate suitable for generating one type of time modulated pulse;

Fig. 7 is a circuit arrangement of a D. C. component restoration embodiment of the proposal; Fig. 9 shows a special pulse utilised for the purpose of controlling the insertion of time modulated pulses.

The desirability of retaining or restoring the D. C. component in television transmission and reception is well known. The usual methods of restoring the D. C. component are either that a fixed and frequently repeated peak value of radiated signal, such as a definite maximum or a definite minimum is observed and utilised in stored, or that the average value of the vision or like signal is observed and utilised so that the D. C. component is restored.

When restoring the D. C. component by observation of a fixed maximum or fixed minimum value of-signal some losses are inevitable as it is a necessary condition for this method of operation that some current should fiow through a unidirectionally conducting device and a drop of applied voltage thus occurs."

Partial D. C. restoration can also be achieved by observing and utilising the average value of picture signals. For this purpose, owing to the fact thatthe energy control of a television picture transmission can be considered as being made up of two energy components one of which (i. e. all signals up to black out level) is fixed,

and the other (i. e. all signals above black out level) variable, it is not possible to utilise the energy content of either one of these compo nents to control reproducing apparatus which accepts both the components. When reconstituting a picture on a cathode ray tube, the signal component having a variable energy content only is utilised, and although both the variable and fixed energy content signals are passed to the cathode ray tube control circuits, matters are usually so arranged that a reverse bias cuts off that portion of the signal having a fixed energy content; therefore observation of the average value of a vision signal cannot give correct D. C. restoration when using this method of picture reconstitution.

In order to provide a more definite means of D. C. restoration it is now proposed that the characteristics of the transmission be varied in such a manner that some or all of the signals having a fixed peak value, as for instance line and frame synchronising signals, or lines rising to black carrier level in the frame pulse series, be time modulated in such a, manner that the energy content of this time modulated portion of the combined vision and synchronising signal is a function of average brightness, identical to or practically identical to the energy content function of average brightness of the combined vision and synchronising signal.

Referring to Fig. 1, there are shown bracketed, at A, -a train of vision signals and line synchronising pulses; at B, a train of frame synchronising pulses; and at C, a train of pulses of .9 of a line duration rising from zero carrier to black level of 30% carrier. A frame of 202 lines comprises 188 lines as shown at A, 4 lines such a manner that the D. C. component is reas shown at B, and 10 lines as shown at C.

Therefore the average level of the carrier varies The line scanning periods shown at A can be considered as commencing at the points H, when the carrier falls from 30% to zero for-a duration of .1 of a line scanning period, rising at point i2 to 30% carrier, at which black level the carrier 5 is maintained for a further 5% of a line scanning period This 15% of the line scanning period is known as the synchronising and following black out period. Following this period vision signals are radiated for .845 of a line scanning period,

after which the carrier is maintained at 30% level for .005 of a line, thus completing the full line scanning period.

The 4 line scanning periods shown at B, comprise 8 frame pulses of .4 of a line period, when the carrier falls from 30% level to zero, each pulse being followed by a period of .1 of a line period during which the carrier rises to 30% level.

The 10 line scanning periods shown at C comprise pulses of .1 of a line period during which the carrier falls from 30% to zero, followed by .9 of a line period during which the carrier is maintained at 30% level.

On analysing a complete frame period it will be seen that there are 188.5 lines during which vision signals can be radiated, and 14 lines during which the signals do not rise above level.

If a more detailed analysis of the various carrier levels is made, it can be shown that out of the total 202.5 line scanning periods in a frame, the 30 carrier can be variable above 30% of carrier level for a time equal to only 160 line periods, and 9 that the carrier rises toi30% for a time equal to 20 line periods, while zero level is maintained for a time equal to 22.5 line periods.

Referring to Fig. 2, there are shown three block diagram of which H indicates the carrier value when an all white picture is being transmitted, and J when an all black picture is being transmitted. The diagrams are drawn approximately to scale, the various carrier levels being indicated along the ordinates and the line scanning periods" along the abscissa. If the carrier levels over the frame period as indicated by diagram H, Fig. 2, are integrated it will be seen that the average level is 83% of maximum carrier, indicatedby dotted line LiLi while integration of carrier levels shown in diagram J gives an average carrier level of 27% indicated by line InLs. by 3.1 to 1 from an all white picture to an all black picture. Diagram K shows in a similar manner the carrier level LsLa for a grey picture. On examining the various diagrams, it will be 5 seen that the double hatched areas D below the 30% ordinate are identical under all conditions of picture brightness, while the single hatched areas are variable. It will also be seen that I the dotted area F indicating the period when the 6 carrier falls to zero is fixed under all conditions of brightness.

It is now proposed that in order to provide means of controlling D. C. levels for the combined I signal, the area F shall also be made variable, 5 and that this variation shall be such that when considering this area as a positive pulse, or a train of positive pulses, the D. C. level of such train of pulses will vary in the same manner as I the D. C. level of the whole signal.

One method of varying the area F is to time modulate the line synchronising pulses in such a manner that these pulses when integrated with the frame pulses shown at B, Fig. 1, give an area I F which is variable in the desired manner. A

, responding to intelligence.

method of performing this time modulation will now be described.

Referring to Fig. 3, there is shown a vision and synchronising signal similar to that shown in Fig. 1 except that the line synchronising pulses are .025 of a line scanning period followed by aninterval of .125 of a line scanning period,

thus giving as before a total time of .15 of a line scanning period for line synchronising and following black out interval. The leading edges I3 of the line pulse coincide with the leading edges ll of the line pulses shown in Fig. l. and since line synchronising is usually taken from the leading edges of the pulses, identical line synchronising conditions are maintained, this portion of the frame period is shown bracketed as A1. The portions of the frame signal bracketed at B1 is identical with that shown at B, Fig. l.

. while the portion bracketed at C1 is similar to that shown at C, Fig. 1, except that the line pulses have been shortened to .025 of a line scanning period; this duration of the line pulse being that used when an all black picture is being trans-- mitted..

When an all white signal is being transmitted, the line synchronising pulses have been time modulated in such a manner that they now have a duration of .105 line scanning periods, being followed by an interval of .045 line scanning periods during which the carrier level is maintained at 30% level. These pulses are illustrated in Fig. 4' which is similar to Fig. 3, except that the line pulses have been lengthened The leading edges 44 of the pulses shown in Fig. 4 coinciding with the leading edges ii of the pulses in Fig. 3.

Fig. 5 diagrams L, M, P show average carrier levels with the time modulated line synchronising pulse, for all white, all black, and grey pic-' tures respectively. These diagrams follow the same general arrangement as the diagrams H,

J, K, Fig. 2. On integrating the carrier levels shown in diagram L, it can be shown that the average carrier level is 82.5% of maximum, while the average carrier levelfor the condition shown in diagram M is 28.75% of maximum.

, It can thus be seen that with the time modulated line pulse the D. C. level varies by 2.9 to 1 for signals varying from all white to all black. It can also be seen thatthe area F representing the period when the carrier falls to zero varies in the same proportion i. e. 2.9 to 1 for the same by brightness conditions. The area F varies linearly w. A. Beatty, Ser. No. 313,041, filed January 9.

1940, corresponding to British Patent No. 523,575 and which as stated in said application may be a part of the assembly of an Electron discharge device such as shown in British Patent No. 519,653 accepted April 2, 1940, there are described methods of generating rectangular pulses of constant amplitude but having a duration which is a time function of the amplitude of a signal cor- Curve I, Fig. 1, of that application shows rectangular pulses RL having their leading edges fixed and their trailpulses can be derived in a similar-fashion to that shown in the above application,-the intelligence in .this case being the average carrier level of the combined vision and synchronising-signal.

, .point It. The output from the valve I5 is such Referring to Fig. 6 of the presentapplication there is shown a target plate QRST of a shape suitable for the generation of line synchronising pulses varying in duration from .025-to .105 of a line scanning period. The side QR of the target is used to determine the leading e es of the synchronising pulses, such edges occurring at equal time intervals. The variable trailing edges of the pulses are determined by the side S. T. Pulses of minimum duration with a D. C. level of 28.75% are obtained by scanning along the line QS, while pulses of maximum duration with a D. C. level of 82.5% are obtained by scanning along the line RT. The side RT is 4.2 times the length of QS. The shape of the plate is determined by assuming small increases in brightness with corresponding increases in the area F. Fig. 5, and integrating the averagecarrier level for each brightness condition. Graphically plotting the variations of F minus a fixed amount representing the 8 frame synchronising pulses during the period 3132, against average carrier level, gives a diagram corresponding to the plate shape shown.

that a positive signal appears at the anode l1,-

whlle as is well known the same signal reversed in phase appears at the cathode It. The combined vision and synchronising signal is applied via condenser I9 to the control grid 20 of the cathode ray tube 2|, the D. C. component being lost by this method of connection. The valve 22, and associated network serve to restore the D. C. component the manner of operation being as follows.

It can be seen that the new type of synchronising, pulse is not obtained at the expense of other information or desirable conditions. The leading edges of the pulses occur at equal time intervals, the said leading edges being utilised for the synchronising of line scanning generators in'receivers, while the duration of the pulse plus following black out period, remains constant thus providing an adequate fly-back interval for the back stroke of the signals generated by line scanning generators.

It can be seen that if a television signal incorporating the new type of pulse be fed to an output valve in a video amplifier, the valve having a bias resistor of such a value that, when the vision signals appear as positive signals at the anode, the synchronising sig als in known manner appear as positive signals at the cathode; the average level of the synchronising pulses integrated over 9. frame period is identical with the average level of the combined signal taken from the anode.

Owing to the fact that the average level of the synchronising pulses depends upon their duration, this average level is not affected by any amplitude distortion which may occur in video aniplifiers either at the transmitter or the receiver, and it can be seen that once the pulse duration has been determined by the average level of an early transmitter stage, the synchronising pulses can be readily utilised either at the transmitter or receiver to control or reinsert D. C. levels, when these have been partially lost due to amplitude distortion of the amplifying stages, or wholly lost by employing resistance-capacitycoupled amplifying stages in a video amplifier;

The manner of working the proposal can .be more readily understood by considering the following practical example.

Referring to Fig. 7, an output valve 15 receives a combined vision and synchronising signal, the latter being time modulated and being applied to the control grid IS. The D. C. compo- The reversed phase signals appearing at the cathode it are fed to any known type of amplitude filter shown as a block diagram A. F. such as are at present used for the purpose of separating synchronising pulses from a combined vision and synchronising signal, and giving synchronising pulses of constant amplitude under all conditions of average carrier level. In addition to performing the usual function of supplying suitable'pulses for time base synchronising, the amplitude filter AF can be utilised to supply time modulated pulses of constant amplitude to the valve 22, via the condenser 24 for the purpose of restoring the D. C. component, which has been lost by the manner of utilising the output of the time constant which is longer than a line scannlng period and shorter than the lowest cut of! frequency of any previous amplifier stage which may be for exampl 200 cycles per second. The battery 26 supplies current to the potentiometer 21, which is so adjusted in conjunction with the resistors 28 and 29 having common connectors with the anode 30 of the valve 25 and the oathode 3l of the cathode ray tube 2|, that when an all black picture is being transmitted the screen of the cathode ray tube just ceases to be illuminated. As soon as vision signals are received, the screen is illuminated in a known manner, but at the same time the synchronising'pulses appearing on the control grid 32 of the valve 22 have a longer duration and owing to the time constant of the circuit 23, 24 there is arise in voltage across the resistor 25 making the control grid 32 of the valve 22 more positive. This in turn has the sheet of making the anode 30 of the valve 22 less positive, and therefore cathode il of the cathode ray tube more negative relative to the control grid 20 of the cathode ray tube. It can be seen therefore that any increase pacity coupling of the output amplifier stage.

By suitably adjusting the values of resistors 25, 28 and 29, a faithful restoration of the D. C. component can be achieved.

If the resistor 33 in the grid control circuit of thecathode ray tube is of a large value compared to the combined parallel impedances 'of the valve 22 and the resistor 28, then the D. C. component can be restored without effective change of grid input load to the cathode ray tube, thus ofiering an advantage over previously proposed systems of D. C. restoration.

It can also be seen that with this method of D. C. restoration, an alternating current circuit or amplifier can be utilised for the D. C. restoring function, thus avoiding the necessity of retaining the D. C. component in the amplifier nent of the signal has been retained up to the 7 stages.

The above circuit is given by way of example only and it should now be obvious to those skilled in the art that there are many other circuit arrangements capable of restoring the D. C. component with the aid of the time-modulated pulse black out line periods following atrain of frame pulses in such a manner that the pulses shown bracketed at C, Fig.'1, vary in their duration in such a manner that the total of the periods of time in a frame puls during which the carrier is maintained at level.is a function of the average carrier level for the combined vision and synchronising pulse.

Referring to Fig. 8, there is shown a portion of a vision signal and frame pulse period identical to that shown in Fig. 1, except that the pulses 34 have been time modulated in accordance with average carrier level in a manner similar to that previously described. The time modulation is carried out in such a manner that the trailing edge 35 of the pulses 34 occur at equal time intervals, thus maintaining line synchronising,

'while the integrated duration of the eight fixed duration pulses 36 and the ten variable duration pulses 34 is a linear function of the average carrier level of the combined vision and synchronising signal. The method of determining and producing such pulses should now be obvious from the previous example. A method of producing variabl duration pulses having their trailing edges occurring at equal time intervals is described in the above-mentioned application No. 313,041, and similar means can be employed in this instance. There are various well-known methods of suppressing one pulse train and inserting another pulse train into a television sig-- nal and train of pulses, and any of these known methods can be employed for the purpose of inserting the pulses of variable duration into the frame pulse period. For instance, a long pulse covering a period of ten line-scanning periods as shown in Fig. 9, can be utilised to make operative or disable circuits controlling the generation or insertion of the different types of puls in such a manner that variable duration pulses 34 are.

only inserted during the correct period.

In order to utilise the pulse system given in this example, it is necessary to have an auxiliary receiver which functions only during the frame pulse period. Such receivers are already known, having been used for observing the am plitude of framepulse signals for the purpose of achieving automatic gain control of video amplifiers; If a similar type of receiver is utilised for the observation of the energy content of a train of frame pulses having the characteristics specified, then the output from this receiver can be utilised for the purpose of restoring the D. C.

' component when this has been lost due to am-- pllfication through an alternating current amplifier. In 'view of the details given in the previous example it is not considered necessary to give a particular circuit arrangement for this latter proposal, as many possible arrangements should be obvious to those skilled in the art. The time constants of the D. C. controlling circuits instance, a mean value of vision signal pulse duration can be made to correspond with a definite degree of amplification control at the sound transmitter input, compression of signals above a required volume being accompanied by a corresponding lengthening of the duration of the pulses, while expansion of the signals is accompanied by a shortening of the pulse duration. As already described, the variable duration pulses can be separated from the combined vision and synchronising pulse signal, and in this case the output of the pulse discriminating receiver can be utilised for the purpose of controlling the gain of the sound receiver accompanying the vision receiver, compression at the transmitter being counteracted by automatic expansion at the receiver, while expansion at the transmitter is automatically counteracted by compression at the receiver. Any of the wellknown gain control circuits can be adapted for the purpose of carrying out this proposal.

The above proposals refer to methods of utilising continuously variable time modulated pulses; examples utilising pulses time modulated in steps will now be given.

In Brit'sh Patent Specification No.' 443,896 reference is made to colour television, stereoscopic television, and "interlaced scanning, and the necessity of obtaining cyclical changes in a definite manner is discussed, one method suggested being that of changing the duration of synchronising signals. serting such variable duration synchronising signals has not been disclosed, and it is now proposed that such variable duration signals are characterised in that the edge of such signals used for synchronising purposes occur at equal time intervals, while the other edge not used for synchronising occurs at varying time intervals, such variations occurring in steps in a definite cyclical order.

For example, in a three colour television system it can be arranged that every third frame alternate frames can be obtained in certain proposed methods of stereoscopic television, such methods and the necessity for the discrimination being discussed in the above mentioned pat-' ent specification.

must necessarily be somewhat different to those previously given, as the integrated signal is obtained only once every frame, however, as the A variable duration pulse system as described can also be utilised in such a manner that a television sequential scanning system can be changed into an interlaced scanning system at the receiver, or an existing interlaced scanning system can have the interlacing factor multiplied at the receiver. For instance, the existing A definite manner of in- 405 line television system transmitted by the B. B. C. can have the frame synchronising pulses modified in such a manner that existing synchronising methods are not interfered with and at the same time specially adapted time bases would be capable of giving in receivers line spacing equivalent to 1215 lines, i. e. three times as fine a line spacing as is possible, with a correctly interlaced 405 line picture.

One manner in which the above proposal can be put into practice is as follows. Referring to Fig. a, there is shown a train of 6 pulses similar to those shown at A, Fig. 1. As is known, these pulses are used for synchronising the frame scanning time base generator. There has also been proposed a method of obtaining very definit-e synchronising of frame-scanning time-base generators. Briefly, this method depends upon the deriving of very short pulses from the leading or trailing edges of the frame and line pulses, and during the frame pulse period utilising one of these derived pulses to synchronise the framescanning time-base generator. synchronising by a derived pulse occurring at definite intervals is achieved by additively combining the derived pulses with the frame pulses from which said derived pulses have been derived, one group of signals being passed over a transmission channel with a time delay, while the other group of signals is passed over a transmission channel having a smaller or no delay.

A modification of the method outlined above can now be used for the purpose of increasing the interlacing factor of a received picture. If matters are so arranged that the synchronising blocking circuit is operative up to the commencement of the sixth pulse and if synchronising is obtained by utilising the next short pulse derived from the trailing edge of the sixth pulse, then synchronising is dependent upon the duration of the sixth pulse. It is now proposed that the final pulse in a train of frame pulses be made of variable duration, as shown in Figs. 10b and 100. In 10b, the pulse is shortened by /6 of a line scanning period, and in 100 by V of a line scanning period, the leading edges of the pulses occurring at equal time intervals while the trailing edges are variable. Owing to the fact that frame synchronising is taken from the trailing edge of the sixth pulse and since this trailing edge has three .equally spaced variable times of occurrence, then the interlacing factor is increased by three, that is to say the original 202.5 lines per frame is effectively interlaced, so as to give a line spacing of 1215 lines per picture. This method of reconstituting a picture has advantages for large screen television where the presence of scanning lar to those already described can be used, except that by means of suitable frequency dividing arrangements such as multi-vibrators or mechanical discs controlled by phonic motors, the variable duration pulses are inserted in their correct order. Such frequency dividing methods are well known in the television art.

It can be seen that more than one type of variable pulse can be used in the same transmission, for instance, time-modulated line-synchronising pulses giving D. C. light control can be used at the same time as time-modulated black-out line periods in a frame pulse period, give colour synchronisation in a colour television system. When a dual variable-pulse system is used it is necessary to compensate the variations of one pulse in accordance with the change in the variations in the other pulse. This can be readily achieved by utilising the average value of one pulse series to modify in a suitable manner the deflection of the beam across the target used for generating the other variable, duration pulses.

What is claimed is:

1. In a picture transmission system wherein picture signals and synchronising impulses are both transmitted and wherein there are periodic intervals of zero transmission between transmission of said picture signals, means for setting up picture signal voltages, means for generating and superimposing on said signal voltages a train of substantially rectangular pulses of time durations concurrent with and greater than said picture signals, said rectangular pulses being of the same polarity as said picture signals, said rectangular pulses each having an edge equally spaced in time from a corresponding edge of a succeeding pulse for synchronization purposes, means for deriving a control voltage to produce varying spacing of the other, each of said pulses in accordance with said variation, relating to intelligence to be transmitted, means for varying the duration of transmission of said synchronising impulses in accordance with said control voltage means including an amplitude filter for separating out said pulses, and means for deriving a regulating voltage from said separated pulses.

lines is normally objectionable to persons close-- to the screen.

Receivers which are not specially adapted to utilise the sixth frame pulse of variable duration would have their frame time bases synchronised in any well known manner, thus achieving the normal 405 line picture. It will be seen that normal line synchronising is maintained through the various frame pulse sequences.

In order to insert the variable duration pulses, relating to these latter proposals, into the frame' pulse signals in correct sequence, methods simi- 2. System according to claim 1, said control voltage being a function of the average signal level of the picture signal voltages.

3. System according to claim 1, said control voltage being a function of the average signal level of the picture signal voltages, the system including a, receiver and brightness regulating means in said receiver controlled by said regulating voltage.

4. System according to claim 1 wherein said picture signal voltages correspond to different colours over successive periods and wherein said control voltage and hence the pulse duration is determined by the colour to which the picture signals correspond.

5. System according to claim 1 including means for setting up in successive periods signal voltages corresponding to alternate eye views for stereoscopic transmission, wherein said control voltage and hence the pulse duration defines alternate eye viewing periods.

WILLIAM ARNOLD IBEATTY.

PRAFULLA KUMAR CHATTERJEA.

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