US2227050A

US2227050A - Television transmitting or receiving system

Info

Publication number: US2227050A
Application number: US190192A
Authority: US
Inventors: White Eric Lawrence Casling; Ratsey Osbert Linn
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1937-02-18
Filing date: 1938-02-12
Publication date: 1940-12-31
Anticipated expiration: 1957-12-31

Description

Dec. 1940- E. L. 0. WHITE ETAL I 2,227,050

TELEVISION TRANSMITTING OR RECEIVING SYSTEM Filed Feb. 12, 1958 \I K I J INVENTORS E.L. C. WH/TE BY 0.1..RATSEY A' 'r RNEY Patented Dec. 31, 1940 TELEVISION TRANSMITTING R RECEIVING SYSTEM i i Eric Lawrence Casling White, Hillingdon, and 0sbert Linn Ratsey, Havant, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application February 12, 1938, Serial No. 190,192

In Great Britain February 18, 1937 4- Claims. (01. 178-71) This invention relates mainly to television transmitting or receiving systems in which syn-j chronizing signals are employed which have an amplitude outside the amplitude of the picture 5 signals, but is also applicable to any system which employs signals comprising two portions, one portion having amplitudes greater than a certain datum value and the other portion having amplitudes less than said datum value.

In television transmitting systems the signals transmitted are composed of picture signalsinterspersed with synchronising signals, the synchronising signals being usually in the blackerthan-bl-ac direction. For various reasons it may be desirable to adjust the amplitude of the picture signals without altering the amplitude of,

the synchronising signals. If, for example, the picture signals and the synchronising signals are fed through a potentiometer, it will be found that on adjustment of the potentiometer to decrease the amplitude of the picture signals,-the amplitude of the synchronising signals will likewise be reduced.

The necessity for adjusting the amplitude of the picture signals relatively to the synchronising signals may arise in the case where television signals aretransmitted to a receiver from which the received signals after amplification are retransmitted. In this case it may be necessary to adjust the ratio of i the picture signals to the synchronising. signals before re-transmission, either to correct for misadjustment of the originally transmitted ratio or for attenuation 35 of the synchronising signals due to, non-linear amplification in the transmitter so. that the ratio can be adjusted to that required for the main transmitter.

In addition, it is frequently necessary during television transmissions to change from one scanning device to another scanning device or from one transmitter to another transmitter. This changeover must be accomplished by fading out the signals from one scanner or transmitter 45 and then fading in the signals from another scannerror transmitter. It is not feasible to mix two sets of signals in varying proportion, as in sound faders, since the two sets of synchronising signals may not be in phase. In the case where difierent synchronising signals are employed in the scannersor transmitters, the fading-out of one set of signals and the fading-in of another set of signals cannot, in the normal way, be effected, since with a reduction in the amplitudeof the picture signals a reduction in amplitude of the synchronising signals also occurs, and hence,

at a certain instant during. thechangeover the receiver will not be fed with synchronising signals.

It is therefore the chief object of the present invention to provide an improved transmitting or receiving system whereby adjustment of the amplitude of picture signals or like signals can be effected without substantially reducing the amplitude of synchronising or like signals.

According to the main feature of the present invention there is provided a circuit arrangement for use with television signals comprising two portions, i. e. picture signals and synchronising signals, or for use with similar signals, one

portion having amplitudes greater than a datum means for varying the amplitude of the picture.

or the like signals, and wherein in order to permit the amplitude of the picture or the like sig nals to be varied in amplitude without substantially varying the amplitude of the synchronising.

or thelike" signals, the unilaterally conducting device is arranged to permit the synchronising or the like signals to be substantially unaffected by said amplitude varying means. v

In one embodiment of the present invention there is. provided a circuit arrangement for we with television or the like signals comprising a potentiometer device in series with a unilaterally conducting device, to which both portions of the signals are fed, said unilaterally conducting device being so arranged as to pass only the picture.

or the like signals through said potentiometer device, the arrangement being such that when said potentiometer device is adjusted to. vary the amplitude of the picture or the like signals, the, amplitude of the synchronising'or the likesi nals remains substantially unafiected.

In one modification of the above embodiment, in order to reduce the amplitude of the picture signals to zero, the residual signal which is due to the finite resistance :of the unilaterally conducting device is neutralised by combining with it an equal signal in opposite phase.

The neutralising voltage may be derived from 3 a potential dividing means shunting the source of picture synchronising or the like signals, and

in a further modification a valve which acts as a so-called cathode follower, is included in the circuit in such a manner as to allow the resistance of said potential dividing means to be increased.

According to another embodiment of themesent invention there is provided a circuit arrangement for use with television or the like signals comprising two channels, one channel passing the picture or the like signals and includingmeans for varying the amplitude of said pictureor the like signals, and a second channelwhich includes a unilaterally conducting device which is so arranged as to pass only the synchronising or the like signals unaffected by said amplitude varying means, the outputs from the two channels being combined in a suitable manner.

According to a further embodiment of the present invention there is provided a circuit arrangement for fading from a first source of television I the like signals'of the second source is increased.

- on continued movement of said common control.

In order that the said invention may be clearly understood and readily carried into effect it will now bemore fully described with reference to the accompanying drawingin which:

, Fig. 1 illustrates a circuit constructed in ac cordance with one form of the invention, Fig. 2.is a circuit constructed in accordance with another formof the invention, and

Figs. 3 and 4 illustrate modification of the in vention shown in Fig. 1. I

Withthe circuit shown in Fig. 1, picture-signals 5 in the positive direction interspersed with synchronising signals in the negative direction are appliedto.

input terminals

3 and 4,, the signals with the D. 0. component present being fed to the input terminals from a low impedance source, 0 such as from a circuit which includes a so-called cathode-follower valve., The terminal 3 is connected through a, potentiometer 5to the anode of a diode 6, whilst thetermina-l t is connected to the cathode of the diode through a biassing bat- 55 tery land to one output terminal 8, the other output terminal 9 being connected to' the adjustable tapping point of the'potentiometer 5. The cathode of the diodefi is biassed by the battery 1 to a potential equal to the potential of the black 60 level of the signals applied to the

terminals

3 and 4 so that the diode is only rendered conducting vwhen picture signals are applied thereto, whilst the synchronising signals render the diode nonconducting. For example if the peaks of the syn- 5 chronising pulses are at zero potential, the'biassing potentials will require to be positive and the potentiometer 5, the amplitude of the picture signals can be reduced substantially to zero or increased from zero upwards, whilst the amplitude of the synchronising signals is substantially unimpaired. The diodeB biassed in the manner above describedfunctions as discriminator means whereby the amplitude 'of one set of signals can be varied without substantially affecting the amplitude of anotherset of signals. Instead of em- 1 ploying a diode, any other suitable construction of unidirectional conducting device may be employed.

With the arrangement described in connection with Fig. 1, it will be appreciated that a picture may be faded out whilst holding the picture in synchronism during such fading process and, at an appropriate time, the scanning device or W transmitter from which the signals are being faded out is switched off and another scanning device or transmitter switched in simultaneously, the picture from the second scanning device or transmitter. being faded in by means of an arrangement similar to that shown in Fig. 1. The arrangement shown alsopermits of adjustment of the amplitude of thepicture,signalsrelatively to the synchronising signals whilst maintaining the latter signals of substantially constant amplitude.

The arrangement shown in Fig. 1 may, in some circumstances, not prove altogether satisfactory, since itmaybe difilcult to reduce the picture signals .to less than about 5% of the maximum amplitude without loss of. the higher frequencies.

Fig. 2 of the drawing illustrates an arrangement in which the signals can be faded as above described, and in which the picture signals can be reduced to zero without substantial attenuation of the higher frequencies. In this figure separate channels are provided in parallel, one for the picture signals and the other for the synchronising signals, the picture signal channel having means for varying the amplitude of the picture signals. The picture signals interspersed with synchronising signals in the blacker than black direction are applied to input terminals I I! and H and to the grid of a triode valve 12 which acts as a linear amplifier through a condenser I 3, a leak resistance l4 being provided between grid and cathode as shown. The anode of the valve l'2 is connected through a potentiometer l5 to the positive terminal'source of anode .current, not shown, whilst the tapping point in the potentiometer is connected through a condenser l6 to the grid of a further triode valve H which acts as a linear amplifier'and has aleak resistance l8, as shown. The cathodes of the valves I2 and I! are associated with biassing resistances l9 and'Zll, each shunted by by-pass condensers'Zl and 22. The path through valve i2, potentiometer l5, condenser l6'to the grid of the valve I1 constitutes'the channel for the picture-signals and a further valve 23 is provided which constitutes an element of the channel for the synchronising signals. The anode of valve i2 is connected through a condenser 24 to the grid of the valve 23 a leak resistance 25 being provided between grid and cathode as shown, valve 23, condenser 24 and resistance 25 are. arrangedto function in the manner described" in the specification of British Patent No. 419,441, so that in this manner the valve 23 which operates in non-linear manner only conducts when synchronising signals are applied to its control grid; The anodes of the valves 23 and I! are connected together and through a load resistance 26 to the positive terthe negative terminal of the associated source of anode current to which the cathodes of the valves are connected as shown. It will be seen from the arrangement described, that the amplitude of the picture signals can be varied on adjustment of the potentiometer I5, and'since the synchronising signals are not only fed through the potentiometer rs, but are also fed directly from the anode of the valve I! to the Valve 23, the amplitude of the synchronising signals cannot be reduced below a predetermined amount by adjustment of the potentiometer l5.

consequently, amplified and added to the signals appearing in the anode circuit of valve ll, the output from the terminals 2! and 29 will consist of picture signals and synchronising signals the latter being variable but always of greater amplitude than the synchronising signals fed to the input terminals l0 and H. In order to reduce the amplitude of the synchronising signals to the required constant level, the signals are stabilised with reference to black level by the use of for example the means described in the specification of British Patent No. 449,242, and the signal passed through a limiting stage in order to restore the synchronising signal to the required amplitude.

In this example also it will be seen that the circuit functions to discriminate by the provision of separate channels between the picture signals and the synchronising signalsso that the former signals may be varied in amplitude without sub stantial decrease in amplitude of the synchronising signals below their original value.

Where the arrangement shown in Fig. 2 is employed for fading out a picture from one source and fading in a picture from another source, two circuits similar to that shown may be associated with the two transmitters and the two potentiometers may be arranged to be operated from a common control knob so that on initial movement of the knob the picture from one transmitter is faded out and at the mid-position of the knob the first transmitter is switched off and the other transmitter switched on, the finalmovement of the knob fading in picture signals from the second transmitter.

Fig. 3 of the drawing illustrates a further circuit in which the amplitude of the picture signals can be reduced to zero. It will be appreciated that the reason for the inability of the circuit shown in Fig. 1 to reduce the picture signals to zero is due to the finite resistance of the diode 6 when conducting and also by reason of the fact that it may be necessary to insert resistance in the cathode of the diode in order to mask variations of this finite resistance. Fig. 3 shows a circuit embodying a diode operating in accordance with Fig. 1 and in which the residual signal is neutralised by combination in opposite phase with an equal and opposite signal. As shown, the tapping point 30 is connected to the grid of a triode valve 3|, which if desired may be biassed in any suitable manner, the anode of which is connected through a load resistance 32 to the positive terminal of a source of anode current, not shown, whilst the cathode is connected to an adjustable tapping point 33 on a potentiometer 34 connected across the

input terminals

3 and 4, such that the Since the synchronising signals are fed to the valve 23 and are,

signal amplitude on the cathode of the triode is substantially the same .as the signal amplitude at the anode of the diode, that is to say the lowest signal amplitude applied to the grid. As result the signal output obtained at the output terminals 35 and 3B is zero when the tapping point 30 is in its lowermost position. r

With the arrangement shown in Fig. 3, the resistance of the potentiometer 36 cannot be made so high as-to constitute a negligible load on the input, since the portions of the resistance above and below the tapping point 33 are in a given ratio depending upon the residual voltage to be neutralised, and since the lower portion cannot be large as it constitutes the biasing resistance for the valve 3|.

Fig. 4 illustrates an arrangement having the. propertiesof the circuit shownin Fig. 3, but in which the resistance of the potentiometer 34 can be considerably increased compared with the resistance of the potentiometer 34 of Fig. 3. In this case, the voltage at the tapping point 33 is. applied to the grid of a triode valve 31, which, if desired, may be biased in any suitable manner, the cathode of which is connected to the cathode of a triode valve 38 to the grid of which the tapping point 30 is connected, the cathodes of'the two

valves

31 and 38 being connected together and through a cathode resistance 39 to the'negative terminal of the associated source of anode current, not shown. The resistance 39 maybe returned with advantage to a potential more negative than that of the negative terminal of the source, 'allowing the resistance 39 to be made larger than would otherwise be possible. The anodes of the two valves are connected through load resistances 4i! and 4| to the positive terminal of the source of anode current. The magnitude of the resistance 39 is such that the valve 38 is correctly biassed as an amplifier and so that the potential'of the cathode follows substantially potential variations of the grid of that valve. The tapping point 33 on the potentiometer 34 is such that, with the tapping point 30 of the potentiometer 5'in its lowermost position, the voltage applied to the grid of the valve 31 is equal to that appearing at its cathode and hence there is substantially no change in the anode current in valve 31, that is to say, there is substantially no picture signal output. When synchronising pulses are applied to the

input'terminals

3 and 4, the voltage'on the'grid of'the valve 38 falls by the full magnitude of the synchronising pulses, whereas the voltage of the grid of the valve 3'! falls by only a fraction of this amplitude.

The effect of the common cathode resistance 39 is to provide synchronising pulses in push-pull relation at the anodes of the two valves. During decrease of the amplitude of the picture signals by adjustment of the tapping point 30 of the potentiometer 5, picture signal will also be obtained in push-pull at the anodes of the two valves 3? and 38. The

output terminals

35 and 36, therefore, provide picture signals, the amplitude of which can vary down to zero, and synchronising pulses of substantially constant amplitude in the same sense as the synchronising pulses applied to the

input terminals

3 and 4.

In the above description reference has been made to synchronising signals in the blackerthan-black direction, but it must be understood that the invention can also be applied in cases where synchronising signals are in the whiterthan-white direction. If, for example, the synchronising signals are in the positive direction and the picture signal in the negative direction then the diodes of Figs. 1, 3 and 4 are reversed. Fig. 2 will, however, only work with the synchronising signals in the negative direction.

We claim:

1. In a television system wherein picture and synchronizing signals are present and constitute a single series of signals, the picture signals being present between synchronizing signals and having an amplitude greater than a predetermined datum value and the synchronizing signals occurring at substantially regular intervals and having an amplitude less than the predetermined datum value, and wherein the picture signals may be altered in intensity to the exclusion of and by an amount different from the change in intensity of the synchronizing signals, comprising a unilateral conducting device and an impedance connected in series, means for impressing the picture and synchronizing signals across said device and impedance, means for biasing said device against conductivity for amplitudes of less than substantially the said predetermined datum value, and an output circuit including said device and a portion of said impedance, the portion of the impedance included in the output circuit being less than-the entire impedance.

2. In a television system wherein a single series of signals including picture and synchronizing signals are present, the'picture signals being present between synchronizing signals and. having an amplitude greater than the predetermined datum value and the synchronizing signals occurring at substantially regular intervals and an output circuit connected to said cathode and an adjustable point along said resistance.

3. In a television system wherein a series of signals are present, a portion of the signals having an amplitude in excess of a predetermined value, .the remaining portion of the signals having an amplitude less than a predetermined value, and wherein the portion having an amplitude in excess of the predetermined value may be altered in intensity with respect to the amplitude of the remaining portion of the signals, comprising a discharge device having a cathode and an anode, a resistance, means for connecting one end of said resistance to said anode, means for applying said series of signals to said cathode and the other end of said resistance, means for maintaining said cathode positive with respect to said anode by an amount substantially equivalent to said predetermined value, and an output circuit connected to said cathode and an adjustable point along said resistance intermediate the ends thereof.

, 4. In a television system wherein a single series of signals including picture and synchronizing signals are present, the picture signals having an amplitude greater than the predetermined datum value and the synchronizing signals having an amplitude less than the predetermined datum value, and wherein the picture signals alone may. be altered in intensity to the exclusion of the synchronizing signals, comprising a' first discharge tube including a cathode and an anode, a first resistance, means for connecting one end of said resistance to said anode, a second ,re-, sistance connected in parallel with said first discharge tube and said first resistance, a second discharge tube including a cathode, a control electrode and an anode, means for connecting the cathode of said second discharge tube to a point along said second resistor, means for conmeeting said control electrode'to a point along said first resistor, means including a load impedance for maintaining the anode of said sec-- ond discharge tube positive with respect topsaid cathode, means for maintaining the cathode of said-first discharge tube normally positive with respect to said anode by an amount substantially equivalent to said predetermined datum value,

means for applying said single series of signals across said second resistance and said seriesconnected first dis-charge tube and first resistor, and an output circuit coupled to the cathode and anode of said second discharge tube.

ERIC LAWRENCE CASLING WHITE. OSBERT LINN RATSEY.