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Publication numberUS2194514 A
Publication typeGrant
Publication dateMar 26, 1940
Filing dateApr 12, 1934
Priority dateApr 13, 1933
Publication numberUS 2194514 A, US 2194514A, US-A-2194514, US2194514 A, US2194514A
InventorsCasling White Eric Lawrence, Osborne Hilda E F, Spencer Percival William, William Willans Peter
Original AssigneeEmi Ltd
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Television and like system
US 2194514 A
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

March 26, 1940 RWMLLANS HAL 2,194,514

TELEVISION AND LIKE SYSTEM Filed April 12, 1934 2 Sheets-Sheet 1 BLACK ,8

Mom/m 16R 2; 4 BLHCK ,6 9 7 6 I REI/YSERTING AMPL we 1 OSCILLRTOR l4 ELEc'r/P/cnL Z ZERO INVENTORS 36 as I B 53 3 PETER WILLIAM WILLANSQ 3 WILLIAM SPENCER PERCIVAL, 1; ERIC LAWRENCE CASLING WHITE,& y T CHARLES PERCY OSBORNE, DE- A BY CEASED, I I v HILDA E. F. OSBORNE ADMINISTRATRIX ,6. 6'. v

March 26, 1940. P. w. WILLANS r AL 2,194,514

TELEVISION AND LIKE SYSTEM Filed April 12, 1954 2 Sheets-Sheet 2 AUXILIARY AMPLIFIER DEMODULATOR Mm MoouLA-roR\ MDDULl-ITDR M c AUXILIARY 2 \Ag ARRIER Oscuumnz I stun-ran 4 SAw T0o'rH b2 GENERATOR 66 '11 NFL! PIER I INVENTORS 5 P ER WILLIAM WILLANS,

WILLIAM SPENCER PERCIVAL, ERIC LAWRENCE CASLING WH|TE,&.

- CHARLES PERCY OSBORNE, DE-

L Fig CEASED, BY

HILDA E. F. OSBORNE,ADMINISTRATRIX BY ATTORNEY Patented Mar. 26, 1940 UNITED STATES TELEVISION AND LIKE SYSTEM Peter William Willans,

Hampstead, William Spencer Percival, Hanwell, Eric Lawrence Casling White, Hillingdon,

and Charles Percy sborne, deceased, late of Hillingdon, England,

- by Hilda E. F. Osborne, don, England, assignors administratrix, Hillingto .Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application April 12, 1934, Serial No. 720,206 In Great Britain April 13, 1933 15 Claims.

The present invention relates to television, picture transmitting and the like systems of the kind in which signals representative of the brightness of elemental areas of an object are transmitted together with synchronising signals along a single channel which is usually constituted by a radio frequency carrier wave. The synchronising signals are often in the form of pulses, one pulse, known as a line impulse, being transmitted in the interval between the scanning of each strip of the object and one pulse, known as a frame impulse being transmitted in the interval between two successive scannings of the object. The line impulses are usually made of different form from the frame impulses so that the two sets of impulses can be separated readily at the receiver.

At the receiver the impulses may be used to control the frequency of a scanning device. In the case of a receiver embodying a cathode ray tube, each set of impulses is used to control the frequency of a generator of oscillations of sawtooth wave form which serve in known manner to deflect the ray across the screen so that the ray scans the screen in close parallel strips.

Where the composite signals transmitted, comprising picture signals and synchronising impulses, are generated by means which do not produce components having frequencies down to 80 and including effectively zero frequency or where the signal is passed through any device,'such as a condenser or a transformer, which suppresses the D. C. and low frequency components of the signal, the composite signal, when plotted against time, will be seen to consist of fluctuations above and below a zero line so situated that the average area enclosed by the fluctuations above the zero line is equal to the average area enclosed below the line. This zero line may be referred to as the electrical zero line and, even if the D. C. and low frequency signal components are transmitted, when the composite signals are received on re-- ceivers of known type, the electrical zero line will be the one about which the fluctuations in effect 45 take place. This electrical zero'does' not represent any fixed value of picture brightness and if the average brightness of the object changes the brightness represented by the electrical zero also changes. A serious difficulty arises on account 50 of the fact that the amplitude of the synchronising pulses with respect to the electrical zero line also changes. Assuming that the synchronising pulses are in such a sense as to correspond to an increase in blackness they are usually arranged to extend from a level corresponding to black in the opposite sense to the picture signals, that is, they extend in what may be called a blacker than black direction. In this case the effect of an increase in average picture brightness is to increase the amplitude of the synchronising 5 pulses with respect to the electrical zero and the effect of a decrease in average brightness is to decrease the amplitude of the pulses.

In order that accurate synchronising may be obtained, the synchronising signals must be separated from the picture signals by some suitable device such as a suitably biased rectifier. The bias of this rectifier, if no D. C. component is present in the signals, must be fixed relative to the electrical zero line so that the rectifier will pass synchronising signals and not picture signals. Because the amplitude of the synchronising signals relative to the electrical zero line varies and also because the amplitude of the picture signals corresponding to the blackest parts of the picture at any moment also varies from time to time, it is necessary to provide large synchronising signals which will under all circumstances project sufficiently from the electrical zero line to allow some at least of their amplitude to be reliably separated without either the separated synchronising pulses being unduly small during conditions of minimum picture brilliancy, or there being anychance of picture signals being passed to the synchronising channel during conditions of maximum picture brilliancy. Thus the synchronising impulses transmitted must be given a relatively large amplitude, for example two or more times the maximum picture minimum ampli- The use of synchronising signals of relatively large amplitude is of course uneconomical in that a relatively large fraction of the total available change of carrier amplitude has to be used for the synchronising signals.

It is an object of the present invention to provide a television or the like system in which the ratio of maximum be increased.

the separating means at the receiver, are such that, with respect to some point in the receiver, the amplitude of the synchronising signals fed to the separating means is substantially unaffected by changes in the average amplitude of the picture signals. In other words it is arranged that the composite signal fed to the separating means contains the appropriate components of low frequency down to effectively zero frequency which will for convenience be referred to as the D. C. component. Thus any particular picture signal voltage applied to the. separating means with reference to some fixed point represents an absolute value of picture brightness.

The present invention further provides a television or the like system comprising a transmitter adapted to transmit, by modulated carrier, composite signals comprising picture signals interspersed with synchronising signals and a receiver for the signals, having means for separating the synchronising signals from the picture signals, wherein, in the composite signals used to modulate the carrier at the transmitter the ratio of the amplitude of the synchronising signals to that of the maximum picture signal is less than 1-5 and wherein, at the receiver, the circuits between the aerial or equivalent input terminals and the separating means are such that the synchronising signals fed to the separating means are of substantially constant amplitude with respect to some point in the receiver.

The modulation at the transmitter is preferably effected in such a way that the peaks of the synchronising pulses are represented by substantially zero carrier amplitude. One advantage of this is that the chance of producing false synchronising signals, in the form of pulses, by an interfering signal is reduced because if an interfering signal'of suitable wave form to produce a false pulse has an amplitude greater than a certain value it fails to reduce the carrier amplitude substantially to zero (as is required for the production of a synchronising pulse) and, instead, reverses the phase of the carrier oscillations. If the interfering signal, on the other hand, has too small an amplitude, it still fails to produce an interfering synchronising pulse.

The present invention further provides a television receiver adapted for use in receiving a carrier wave modulated with a composite signal comprising trains of picture signals interspersed with synchronising signals, the receiver having means for separating the synchronising signals from the picture signals, wherein the circuits between the aerial, or equivalent input terminals of the receiver, and the separating means are such that the separation is unaffected by variations in the picture signals.

Where the separation is effected after the signal modulation has been extracted by a detector, the receiver according to this invention may have a conductive coupling between the detector and the separating point so that the D. C. component of the modulation is present at the separating point. Alternatively the separation may be effected at radio frequency.

The invention will. be described more particularly with reference to a system for transmitting motion pictures from film but is of course also applicable to systems for transmitting directly from objects.

The invention will be described by way of example with reference to the accompanying drawings in which Fig. 1 is a diagrammatic representation of a transmitter according to the present invention.

Figs. 2 to 6 are circuit'diagrams showing various forms of re-inserting devices which may be used in the arrangement of Fig. 1,

Fig. 7 is a. diagrammatic representation of an alternative form of transmitter according to this invention, and

Figs. 8 and 9 show circuit arrangements of receivers according to the present invention.

Referring to the schematic diagram of Fig. 1,

a film I is moved at a uniform speed in a direction normal to the surface of the paper and is scanned in a series of transverse strips by suit able means such as a mirror drum 2. Images of the strips are thus swept over an apertured photo-electric cell 3 and generate picture signals therein. The form of these signals is shown graphically at l, the line 5 representing complete black.

At the end of the scanning of each strip of the picture, a line synchronising pulse is generated, for example, with the aid of a separate light source 5 and photo-electric cell I co-operating with the mirror of the drum which has just scanned the strip orpreferably as shown with the mirror 8 which will next scan a strip. Similarly, by any known or suitable means, a frame impulse is generated at the end of each complete scanning of one frame. During the generation of these synchronising signals the light arriving at the photo-cell 3 generating the picture signals is suppressed so that the output intensity in the picture channel corresponds to full black.

This may be done with the aid of a shutter mechanically coupled to the drum 2 or by render-- ing the edge of the film I opaque.

If desired the synchronising pulses (of wave form indicated by the curve 9) may be amplified in an amplifier l0. Similarly the picture signals may be amplified if desired in an amplifier II. The synchronising impulses are then mixed with the signals in the picture channel so as to give in effect signals in a blacker than black sense, that is to say, the sense of the synchronising signals with respect to a signal level corresponding to black is opposite to the sense corresponding to increased brightness. Care is taken that the synchronising signals are of constant amplitude.

The composite signal, thus constituted, is then passed through a resistance coupled amplifier I2 which on account of the condenser I3 is incapable'of passing direct current signal components and components of very low frequency below the frequency of the line synchronising impulses.

These components will be referred to herein for simplicity as the D. 0. component. This condenser l3 thus acts as a suppressor of the D. C. component. The signal in the output of amplifier I2 is thus of the form indicated at 14, the oscillations taking place about an electrical zero line l5 such that the average areas enclosed by the curve [4 above and below the line I5 are equal. Thus when the average brightness of the film I changes, the distance from the peaks of the maxima I6 constituted by the synchronising impulses to the line I5 will change.

On account of the particular features of the receivers according to the present invention with which the transmitter has to operate, the ratio of synchronising signal amplitude to the maximum picture signal amplitude can be made much 0-5. Any further decrease of the synchronising signal amplitude is not found to be of great assistance although, in cases where the modulation hasbeen in such sense that the synchronising signals reduce the carrier amplitude, a ratio as low as 0-3 has been used successfully. The maxi- 'mum picture signal amplitude is of course the .omy in transmitter power, and the suitability of the transmitter to co-operate with varying types of receiver, to radiate all the components of the original light modulation down to zero frequency,

that is to say including the D. C. component.

For this purpose it is necessary to ensure that in some manner a signal corresponding to the D. C. component of the light arriving at the photo-cell in the picture channel is correctly applied to the modulator tubes of the radio frequency transmitter.

For this reason the composite signal is fed to a re-inserting device I1, certain forms of which will be described later, whereby the D. C. component is re-inserted into the signal. The signal may thus be of the form shown at l8. The effect of the re-insertion of the D. C. component is that,

in spite of changes in the average brightness of,

the film I, the peaks IE will represent a constant voltage and will be constantly related to the voltage corresponding to picture black indicated by the line iii. The signal with its D. C. component is then passed to a modulator2|l wherein carrier oscillations generated by an oscillator 2| are modulated by the signal. Because the D. C. component is present, it can be arranged that any given voltage, such as that corresponding to black for example, is represented by a fixed value of carrier amplitude whatever may be the average value of the picture brightness. Thus the peaks It may be made to correspond substantially to zero carrier amplitude.

The re-inserting device I! may be of a variety of different forms. Preferably it is of the kind set forth in the copending application Ser. No. 720,205. Certain forms of device according to that specification are shownin Figs. 2 to 6, wherein the input terminals A and output terminals B correspond to terminals A and B in Fig. 1.

The signals are applied to the terminals A in such a sense that the synchronising impulses tend to make the grid of the triode 22 positive. The condenser 23 and the resistance 24 are arranged to have a time constant which is longer than the intervals between successive synchronising impulses and shorter than the time constant of any circuit which the signals have traversed since they contained their D. C. component, for example shorter than the time constant of condenser l3 and leak resistance 25 of Fig. 1.

Assuming first of all that no picture signals are present, synchronising pulses of the kind shown at 9 in Fig. 1 are applied to the grid of valve 22 and the first of these causes grid current to flow and charge the condenser 23. When the impulse ceases the grid will be more negative than before and this negative charge will leak away only slowly through leak 24. The next impulse again caused grid current to flow thereby increasing the negative charge on the grid after the impulse has ceased. This process continues until a steady state is reached in which grid current only Just flows at the peak of each synchronising impulse. When picture signals are present they make the grid more or less negative and do not-cause any grid current to flow. They do-not therefore affect appreciably the charge on the condenser 23. Thus, however, the average value of the picture signals may change, the grid of valve 22 will be held at such a value that the peaks of the synchronising impulses just cause grid current to flow and the signal at the output terminals B will therefore contain the D. C. cmponent. The re-inserting means will thus be seen to comprise a unidirectionally conducting device, constituted by the grid circuit of valve 22, in combination with the circuit 23, 24 of time constant which is longer than the intervals between successive signal maxima in one direction, in this case the positive direction. In the form of reinserting device shown in Fig. 3, the

condenser 23 and resistance 24 as before constitute a circuit of long time constant. In this case it is arranged that the synchronising im- I pulses are in a negative sense and. the picture signals in a positive sense. The necessary change canof course be obtained by the insertion of an additional stage of thermionic valve ampliflcation. tionally conducting device and the anode of the diode is connected to a tapping point on a potential di.ider 21 bridging a voltage source 28. The negativ: maxima, constituted by the synchonising impulses, cause current to fiow in the diode and after the impulse has ceased the cathode of the diode is more positive than the anode.

The processcontinues with each succeeding impulse until current only just flows in the diode at the peaks of the maxima. In the signals at the terminals B, therefore, the D. C. component will appear. The datum line of these signals can be adjusted with the aid of the potential divider 21.

The circuit of long time constant need not necessarily include a condenser and resistance as in Figs. 2 and 3. For example in the device of Fig. 4 it is constituted by an inductance 29 and a resistance 30 (together with the resistance of the diode 3| when passing current). The unidirectionally conducting device is, as in .Fig. 3, the diode 3|. Whereas the terminals A of Figs. 2 and 3 are preferably fed from a low impedance amplifier output, the terminals A of Fig. 4 may A diode 26 constitutes the unidirecbe fed from a high impedance amplifier output.

In operation, the signals are applied to the re-inserting device of Fig. 4 in such a sense that the synchronising impulses tend to make the upper end of inductance 29 more negative. The negative maxima cannot pass through the rectifier 3| and they therefore charge" the inductance 29, that is to say they initiate in the inductance 29 a flow of current which tends to persist after the impulse has ceased. This persistence of current flow is equivalent to the discharging of a condenser and the only path through which it can flow is through the diode 3| and the resistance 30 in series. This current supplies the necessary D. C. component.

a drop of applied voltage thus occurs. The efiect.

of this is that the whole of the missing D. C. component is not re-inserted. In some cases this may not be material. Where it is desired to obtain more complete re-insertion, means may be provided at some suitable point for favouring the D. C. signal component (this expression being used as elsewhere in this specification to include low frequency signal components) relatively to the remainder of the signal frequency band. One arrangement of this kind is shown in Fig. 5.

The circuit is the same as that of Fig. 2 excepting that across the anode resistance 33 of the valve 22 thereis shunted a circuit comprising a resistance 34 in series with a condenser 35. The

' circuit 34, 35 serves to favour the low frequencies compared with the high frequencies appearing at terminals B because thelatter are by-passed to a larger extent through circuit 34, 35 than the former.

Another re-inserting device favouring the low frequencies is shown in Fig. 6. Here the condenser 23 and the resistance 24 function as in Figs. 2, 3 and 5. A diode 36 is connected in parallel with the resistance 24. The signals are applied to terminals A-with the synchronising signals in a positive sense. When the steady state is reached the condenser 23 is charged with the right hand plate negative with respect to the left hand plate to such an amount that current just flows through the diode 36 on the peaks of the maxima. A suitable tapping point on the resistance 24 is connected through a resistance 31 to the grid of an amplifying valve 38.. A circuit comprising a resistance 39 in series with a condenser 40 is connected across thegrid circuit of the valve 38 and serves to favour the low frequencies at the expense of the high frequencies applied between the grid and cathode of the valve 38.

Instead of using a re-inserting device at H in Fig. 1 it is possible to provide a circuit capable of passing direct current between the photo-cell 3 and the modulator 20. This is however usually inconvenient and an alternative arrangement is shown diagrammatically in Fig. '7. Here the circuit to the left of terminals C and D in Fig. 1 is omitted and may be the same as in Fig. 1. The signals containing the D. C. component are fed to an auxiliary modulator 4i and are used to modulate an auxiliary carrier generated at 42. The modulated auxiliary carrier is amplified as desired by amplifier 43 and is then applied to a demodulator 44. The amplified picture and synchronizing signals with their D. C. component are then used in modulator 45 to modulate a carrier generated at 46.

Clearly if desired the demodulator and main modulator 44 and 45 can be omitted if desired and the auxiliary oscillator 42 is then arranged to supply the carrier frequency which is to be radiated.

Since the synchronising signals are purely A. C. in character and contain no components below the frame frequency, there is no need, in the case of these signals, to provide D. C. amplification and where a separate amplifier is used for the synchronising signals, this may be of the A. C. type. This may be explained with reference to Fig. l. The synchronising signals are shown at 9 and it is desired to superimpose them upon the picture signals 4 in such a way that the synchronising impulses occur in the gaps between trains of picture signals where the picture signals correspond to black. As shown in this figure, this is done by feeding the impulses through a transformer into the picture signal channel, which cannot pass any D. C. component with means for,

' value-for example about 3 volts.

of the synchronising signals. In spite of the fact that no D. C. is passed from the terminals D, D into the picture channel nevertheless impulses in the primary of the transformer will give rise to corresponding impulses in the secondary. The impulses shown at 9 are the line impulses and at the end of every frame there occurs a framing impulse which is usually of the same amplitude asa line impulse but of longer duration.- These framing signals can equally well 7 be applied through the transformer, and the desired superposition-can be effected therefore satisfactorily without a. D. C. coupling because the lowest frequency of interest in the synchronising signals is the component of frame frequency.

When modulation is carried dut in any of the ways above described so that the D. C. component is present at the modulating point, the amplitude of the carrier corresponding to black and that corresponding to the peak of a synchronising pulse will remain constant independently of any changes in average picture brightness.

It should be noted that, in modulating a' carrier in this way, use can be made of the lower curved part of the anode current-grid voltage characteristics of thermionic valves, that is the part corresponding to low anode currents, so long i as it is the synchronising signals which operate upon these parts of the characteristic and-so long as these synchronising signals are initially given a sufilciently great amplitude so that after their amplitude has been reduced by the curvature of the characteristic it has the desired value relatively to the picture signals.

In one form of receiver according to this invention shown diagrammatically in Fig. '8 the modulated radio frequency signals are received on a suitable aerial system and amplified as required in an amplifier indicated at 41. If desired, a supersonic heterodyne receiver may be used.

In any case the radio frequency carrier modulated by the composite signal (that is the intermediate frequency in the case of a superhete'r' odyne). is applied to the control grid-cathode circuit of a pentode 48 arranged to operate as an anode bend detector. The various voltages required for the receiver are preferably derived from some form of stabiliser as many of these voltages should be maintained as steady as possible. One suitable known form of stabiliser shownat 49 comprises a number of electrodes arranged equally spaced apart within an enclosed envelope containing neon. The pressure within the envelope is arranged to be such that the striking voltage between the electrodes has the desired value. In the case illustrated the arrangement issuch that five electrodes are provided with, in operation, a voltage drop of say seventy volts between each adjacent pair. The stabiliser is arranged in series with a. resistance 50 across a suitable voltage source, for example of about 350 volts and provided the current taken is below a predetermined limit the voltage of each electrode will remain substantially constant.

Between the negative terminal of the stabiliser and the negative terminal of-the source is arranged a small resistance 5| which is shunted by a copper-copper oxide rectifier 52 to limit the voltage drop across this resistance to a desired The resistance is connected in the control grid-cathode circuit of the pentode in such a way as to maintain the control grid about 3 volts negative relatively to the cathode. The grid nearer the con- .trol grid is connected to a variable tapping point 63 of a potential divider 53 connected between say the 140 and 210 volt electrodes of the stabiliser. The outer grid is connected to the cathode and the anode of the pentode is connected to the 280 volt electrode 54 of the stabiliser through'a resistance 55. A variable tapping point on this last mentioned resistance is connected to the cathode 51 of a cathode ray tube shown diagrammatically at 56 (or to an electrode of a sodium lamp for example if mechanical scanning means are provided at the receiver) whilst the control grid 59 of the cathode ray tube (or the other electrode of the sodium lamp) is connected to a variable tapping point 65 on the potential divider 53.

The connections to the control grid and cathode .of the cathode ray tube are suitable for the present circumstances but in some cases may require to be reversed in order that the grid may be swung in the positive direction by picture signals corresponding to increased brightness as is obviously required.

The anode of the pentode I6 is also connected directly to the control gridof a triode 59, the cathode of which is connected to a variable tapping point 65 on the potential divider 53. The

anode of the triode 59 is connected through a resistance 60 to the positive stabiliser 49 and through a condenser 6| to a frame and line synchronising signal separator 62.

The last mentioned potential divider tapping point 65 is so adjusted that current flows in the anode circuit of the triode 59 when no carrier is received and the amplitude of the signals applied to the pentode detector 48 is arranged to be such that current flow in the triode 59 justceases when the carrier amplitude is increased to a value corresponding to 'black. No current therefore flows in the triode 59 in response to picture signals and the triode acts as a separator valve for separating the synchronising signals from the picture signals. The synchronising signals fed through the condenser 6| to the frame and line separator 62 are thus free from picture signals, and because of the conductive coupling between the detector 46 and the separating triode 59, the amplitude will remain constant in spite of changes in the average picture signal amplitude.

The frame and line signal separator 62 serves to separate line synchronising impulses from frame impulses. The separator 62 maybe of any 4 known or suitable kind, for example it may operate by virtue of the frequency diflerence between the line and frame impulses.

The line synchronising impulses thus appear at terminals 66 which may be connected as shown to a device 61 which serves to generate oscillations of saw tooth wave form under the control of the line impulses. These saw tooth oscillations may then be fed to deflecting coils 66 serving to deflect the ray in'one scanning co-ordinate.

. g Th frame impulses appear at terminals 69 and may be applied to a second saw tooth wave generator which is not shown and this second-saw tootliwave may be applied to deflecting coils (not shown) serving to deflect the ray inthe co-ordinate at right angles to that of coils 66.

It will be seen that the modulation of the carrier at the transmitter has been carried out in such a way that black is represented by a flxed value of carrier amplitude and at the receiver, by providing a D. coupling between the detector and the separating point, the voltage across the terminal 54 of the of the synchronising signals grid circuit of the triode corresponding to black" is also independent of the average picture amplitude. In this way a substantially constant amplitude of synchronising pulses is obtained and the amplitude of these pulses can therefore be made smaller in relation to the picture signals than hitherto.

In the case of a mechanical scanner at the receiver, the synchronising signals may of course be used to synchronise the mechanical scanning. device.

In another form of receiver according to this invention illustrated diagrammatically in Fig. 9, the modulated carrier is applied after suitable ampliflcation to terminals and therefore across the control grid and cathode of a triode 1| arranged as an anode bend rectifier. The anode is connected through a resistance 12 to the 280 volt electrode of astabiliser 13 and the cathode is connected toeearth and to the negative terminal of the stabiliser.

In this case oscillations of saw-tooth wave form are generated for example in the manner set forth in the specification of British Patent No. 400,976 by means of discharge tubes of the known type in which the anode-cathode impedance remains high so long as its grid is maintained at a suitable negative potential relative to its cathode but falls to' a low value when thenegative potential on the grid is suitably reduced-- and remains at a low value irrespective of any potentials subsequently applied to the grid until conductively through a resistance 16 to the grid of the line frequency discharge device 16 and through a low-pass filter represented diagrammatically at 11 to the grid of the frame frequency.

discharge device 15. The cathodes of the devices 14 and 15 are connected to variable tapping points 8|, 82 on a potential divider 18 arranged across the 140 and 210 volt electrodes of the stabiliser 13. The resistance 50 and the resistance and rectifier 5| and 52 perform the same functions as in Fig. 8.

The modulated carrier oscillations are also fed to terminals 60 and thereby to a second detector 19 which serves to pass the picture signals to the cathode ray tube or other device used for reproduction indicated diagrammatically at 56. Thus a tapping point on the anode resistance 83 of valve 19 is connected to the cathode 51 of the cathode ray tube 56 and the control electrode 58 of this tube is connected to a variable tapping. point on the potential divider 18.

The connections in the anode circuits of the devices 14 and 15 are not illustrated because they are already well known.

ture amplitudes do not appear as corresponding pulses inthe detector output. Such partial separation is, if the transmitter is radiating the D. C. component of the picture modulation, effected under stabilised conditions as regards the n. c. component of the modulation, since particular carrier values correwond to particular picture signal amplitudes. It will be seen that separation of this type embodies the essential feature of the invention. It should also be noted that to take full advantage of separate detectors for synchronising and picture signals, it should be arranged that each detector operates over the'most favourable part of its detection characteristic for the amplitude range over which it is required to work. For example the picture detector should be most efficient between radio frequency amplitudes corresponding to the range between full black and full white and the synchronising detector should operate most efiiciently for radio frequency amplitudes in the range corresponding to blacker than black. By suitably choosing the operating range of a synchronising detector, a further partial degree of separation can be obtained.

The system above described is arranged, by adjusting the tapping points 8| and 82 on the potential divider, so that the discharge devices 14 and 15 oscillate at a faster rate than the correct rate when no carrier is being received and the arrangement is further made such that the discharge devices run slower than the correct rate when the received carrier amplitude corresponds to complete black. The synchronising signals then serve to hold the discharge devices to the correct frequencies.

The grids of the discharge devices are driven more negative by the picture signals and are therefore unaffected by them. Consequently the discharge devices themselves act as separators and, although preferable, it is not necessary that separation should have been effected previously in the amplifier preceding the synchronising signal detector or in this detector itself as above described.

The line synchronising impulses are prevented from affecting the frame frequency discharge device by the low-pass filter TI which may consist of series resistances and shunt condensers. The frame frequency pulses on the other hand do not affect the line frequency discharge device I4 provided that they are of suitable form. They may for instance be in the form of lengthened line impulses, that is to say a number of line impulses (for example three) are lengthened so that they last for nearly the whole interval between successive line impulses, the abrupt commencements of the line impulses being left un- Y changed. These so-called broadened pulses operate the frame frequency discharge device satisfactorily but do not produce any other effect upon the line frequency discharge device 14 than is produced by the ordinary line impulses. If desired, however, a high pass filter may be provided in the input to the line frequency discharge device.

In the above described receiver arrangements the separation of synchronising signals from picture signals is effected at least partially after detection. The separation may, however, be effected wholly before detection and the effect is similar to that produced by a D. C. coupling between the detector and the separating point. In other words the amplitude of the synchronising signals is independent of average picture amplitude. The sepal tion at radio frequency may be eifected by feeding the modulated radio frequency oscillations to a triode arranged as a limiter. For example the conditions may be made such that the valve is swung past its lower .(or if desired upper) bendby the smallest picture signals and remains on the relatively straight portion only for synchronising signals. Alternatively it may be arranged that the smallest picture signals in crease grid voltage to such a point that grid current commences to flow.

Some of the advantages of the present invention can be obtained even if the signal used to modulate the carrier at the transmitter has been deprived of its D. C. component, so long as this component is arranged to be present at the point at the receiver at which separation is effected. For example the D. 0. component may be re inserted at the receiver with reference to recurrent signal maxima or minima, such as the peaks of the synchronising signals in the manner described in connection with Figs. 2 to 6. Where this is done the low ratio of synchronising signal amplitude to maximum picture signal amplitude may still be used. There is, however, the disadvantage that a greater range of carrier amplitudes is required since the carrier amplitude corresponding to any particular value of picture brightness varies with change 0 average picture brightness.

The invention has been described in connection with modulated carrier transmission systems but is of course not limited, in many of its aspects, to such systems. For example in systems where the connection between the transmitter and receiver is by wire, it is arranged that the circuits of the system preceding the separating means at the receiver are such that the D. C. component of the modulation is present at the separating point. Either the circuits may be made conductive or some means may be provided for reinserting the D. C. component before separation takes place.

The transmitterdescribed is one employing a mechanical scanning device. The invention can also be applied where other forms of transmitter are used. For example the transmitter may comprise a mosaic photo-electric structure and may even be of a type in which no D. C. component is generated. Where the D. C. component is generated the invention can be applied as already described. Where the D. C. component is not generated by the scanner itself, the D. C. component may be generated separately, for example with the aid of a separate photo-electric cell exposed to light for the whole object to be transmitted, and combined with the picture signals.

We claim:

1. Television apparatus comprising a transmitter having means for generating a carrier oscillation signal, means for generating picture signals having direct current and low frequency components, said picture signals being interspersed with synchronizing impulses, the ratio of the amplitude of said synchronizing impulses to the maximum amplitude of said picture signals being substantially less than two, means for modulating said carrier with said signals and impulses including the direct current and low frequency components, means for transmitting said modulated carrier, a receiver adapted to receive the transmitted modulated carrier, amplitude selection means for separating said impulses from said signals. coupling means between oscillation, signal generating means for generating picture signals having direct current and low frequency alternating current components interspersed with synchronizing impulses, said synchronizing impulses having their polarity opposite to that of said picture signals, means to amplify said pictur'e signals and synchronizing impulses while suppressing the direct current and the low frequency alternating current components of said picture signals, means for modulating said carrier with said amplified signals and impulses in such a manner that said synchronizing impulses serve to reduce the carrier amplitude substantially to zero, and coupling means connected between said signal generating means and said modulating means to supply signals representative of the suppressed direct current and low frequency alternating current components of said signal to said modulating means.

3. Television apparatus comprising a transmitter having means for generating a carrier oscillation, signal generating means for generating picture signals interspersed with synchronizing impulses whose polarity is opposite to that of said picture signals, means for modulating said carrier with said signals in such a sense that said synchronizing impulses serve to reduce the carrier amplitude, coupling means incapable of transmitting direct current and low frequency alternating current signal components between said signal generating means and said modulating means, and electrical means preceding said modulating means to supply energy representative of said direct current and low frequency alternating current signal components.

4. Television apparatus comprising a transmitter having means for generating a carrier oscillation, means for generating picture signals and synchronizing impulses interspersed therewith where the polarity of the synchronizing impulses is opposite to that of the picture signals, means for modulating said carrier with said signals and impulses in such a sense that said synchronizing impulses serve to reduce the carrier amplitude, means incapable of transmitting direct current and low frequency alternating current signal components between said signal generating means and said modulating means, and means responsive to the peaks of impulse maxima in one direction to supply said direct current and low frequency alternating current signal components.

5. Television apparatus comprising a transmitter having means for generating carrier oscillations, signal generating means for generating picture signals including directcurrent and low frequency alternating current components interspersed with synchronizing signals whose amplitudes are greater in the direction of black than the amplitude of the picture signals representative of the blackest black of the picture to be transmitted, means for amplifying said picture and synchronizing signals while suppressing said direct current and low frequency alternating current components, means for modulating said carrier by both of said signals in such a manner that the maximum signals in one direction serve to reduce the carrier amplitude substantially to zero, and means between said amplifying means 7 and said modulating means to modulate said carrier oscillations under the control of signals representative of the suppressed direct current and low frequency alternating current component 6. Television receiving apparatus comprising means for receiving a carrier wave modulated with a composite signal comprising picture signals interspersed with synchronizing signals, said synchronizing signals rier wave, said demodulatedmeans includinga thermionic tube having at least a cathode, control electrode and anode, said picture signals from said synchronizing signals, means for coupling said demodulator means to said separator means, a cathode ray tube having a cathode, a control electrode, an' anode, and a luminescent screen for reproducing pictures, and a direct connection from the anode of said thermionic tube to the control electrode of said cathode ray tube.

7. Television apparatus comprising a transmitter having means for generating a carrier oscillation, signal generating means for generating picture signals having direct current and low frequency components which atspaced intervals assume a fixed value, means for generating synchronizising impulses, means for superimposing said synchronising impulses upon said intervals to form 'a composite signal, amplifying said composite signal without the direct current and low frequency components, means for modulating said carrier with said composite signal,

means for separating and a circuit between said generating means and said modulating means for maintaining peaks of said impulses proportional to a fixed value of carrier amplitude and for reinserting the direct current and low frequency components.

8. Television apparatus comprising a transmitter having means for generating a carrier oscillation, signal generating means for generating picture signals which at spaced intervals assume a fixed value equal to the threshold value of the blacker than black region, an amplifier incapable of transmitting direct current for amplifying said picture signals, means at the output side of said amplifier said carrier by the amplified picture means at the output side of said amplifier responsive to peaks of said blacker than black values adapted to re-inserting a direct current component into said signals.

9. In a television transmission system comprising means to generate picture signalling im pulses representative of an image to be transmitted, means to generate synchronizing impulses, means to generate a first source of carrier wave energy, means to modulate the carrier wave energy by both the picture signals, including diproducing picture signals including the direct current and low frequency components representative of an image to be transmitted, producing synchronizing signals, producing a first source of carrier wave energy, modulating the produced carrier wave energy oy both the picture and synchronizing signals, amplifying the modulated carrier wave energy without direct current and low frequency components, demodulating the amplified carrier wave energy, producing a second source of carrier wave energy, modulating the carrier wave energy of the second source by the demodulated energy including the direct current and low frequency components, and transmitting the modulated carrier energy of the second source.

11. Television comprising a transmitter having means for generating a carrier oscillation, signal generating means for generating trains of picture signals having direct current and low frequency components with intervals between said trains, means to suppress said direct current and low frequency components, means for modulating said carrier with said picture signals whereby an increase in picture brightness is represented by an increase in carrier amplitude, means for modulating said carrier by signals representative of the suppressed direct current and low frequency components, means for generating synchronizing impulses, means for modulating said carrier in said intervals with said synchronizing impulses whereby said synchronizing signals reduce the amplitude of said carrier, means to linearly amplify the demodulated picture carrier wave, and means to non-linearly amplify the synchronizing demodulated carrier wave.

12. In the method of transmitting television signals, the steps of generating signalling energy representative of an object to be transmitted, generating synchronizing energy, combining the signalling energy with the synchronizing energy in opposite phase relation, removing energy components from the combined signalling and synchronizing energies representative of the average value of the combined energies, generating carrier wave energy, modulating the carrier energy by the combined signalling and synchronizing energies from which the energy components representative of the average value of the combined energies have been removed while regulating the average amplitude of the carrier wave energy by the removed energy, and transmitting the modulated regulated carrier wave energy.

13. A television transmitting system comprising means for generating signalling energy representative of an object to be transmitted, means for generating synchronizing energy, means for combining the signalling energy with the synchronizing energy in opposite phase relation, means for supressing the energy representative of the average value of the generated signalling energy, means for deriving energy representative or the average value of the combined synchronizing and signalling energy, means for generating carrier wave energy, means for modulating the carrier energy by the combined signalling and synchronizing energies and for regulating the average amplitude of the carrier wave energy by the derived energy, and means for transmitting the modulated regulated carrier wave energy.

14. A television, transmitting system comprising means for generating signalling energy representative of an object to be transmitted, means for generating synchronizing energy, means for combining the signalling energy with the synchronizing energy in anti-phase relation, means for removing energy components from the combined signalling and synchronizing energies representative of the average value of the generated signaling energy, means for generating carrier wave energy, means for modulating the regulated carrier energy by the combined signalling and synchronizing energies from which the energy components representative of the average value of the generated signalling energy have been removed and for regulating the average amplitude of the carrier wave energy by energy representative of the removed energy components,.

and means for transmitting the modulated regulated carrier wave energy.

15. Television apparatus comprising a transmitter having means for generating carrier oscillations, signal generating means for generating picture signals including direct current and low frequency alternating current components, means for generating synchronizing signals, means for combining the picture signals and the synchronizing signals so that maximum amplitude of the picture signal is of opposite polarity to the maximum amplitude of the synchronizing signal, means for amplifying the combined picture signals and synchronizing signals while supressing the developed quency alternating current components, means for modulating the generated carrier oscillations by both of said picture and synchronizing signals in such manner that the maximum intensity signals in one direction serve to increase the carrier amplitude substantially to amaximum and means between said amplifying means and said modulating means for reinserting signals representative of the suppressed direct cur rent and low frequency alternating current components to control the modulation of the said carrier oscillations.

PETER WILLIAM WILLANS. WILLIAM SPENCER PERCIVAL. ERIC LAWRENCE CASLING WHITE. I-IILDA E. F. OSBORNE, Administratria: of the. Estate of Charles Percy Osborne, Deceased.

direct current and low fre-

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US5941505 *May 8, 1996Aug 24, 1999Arca Regler GmbhValve
Classifications
U.S. Classification348/469, 348/691, 348/E05.99
International ClassificationH04N5/445
Cooperative ClassificationH04N5/445
European ClassificationH04N5/445