US2212199A - Television and the like transmitting system - Google Patents

Description

Filed NOV. 19, :1935

(M r fr ,TELEvIlsIpN ANDTHE' LIKE TRANsmTT-ING SYSTEM Aug. 2o, 1940.

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Patented Aug. 20, 1940 UNITED STATES PATENT OFFICE TELEVISION AND THE LIKE TRANSMITTING SYSTEM Cecil Oswald Browne, West Acton, London, John Hardwick, West Drayton,

Frank Blythen,

Application November 19, 1935, Serial No. 50,560 In Great Britain November 19, 1934 10 Claims.

The present invention relates to television and the like transmitting systems.

In most known television systems an object to be transmitted is scanned in contiguous strips and electrical variations are derived from the Changes in brightness of the elemental areas of the object. When the object has been completely scanned, the operation is repeated. This complete scanning may take place 24 times per second for example. Since the absolute level of brightness has significance to the eye it is necessary to reproduce in the final picture all changes in brightness of the object, however slowly these changes may occur, if the final result is to be an undis It will be evident that in order'to convey always the correct impression of absolute brightness it is necessary to transmit and receive frequencies down to and including zero frequency. Usually, however, the amplifiers serving to amplify the signals prior to transmission have a low frequency ycut-oli, `for example they may be capableof amplifying from `about 10 'cycles per second to 400,000 or more Cycles per second, and below l0 cycles per second the ampliiication falls rapidly.

Moreover some picture signal generators, 4for example certain cathode ray tube transmitters, `do not develop the direct current and very low frequency components of the picture signals.

In order to'insert accurately the direct and low frequency components, hereinafter referred -to for shortness as the D. C. component, it is found to be essential that the amplitude of 'the picture, or combined picture and `synchronising signals should attain a fixed maximum or minimum value, relative to the value corresponding to black in the picture, at suitable intervals `of lower frequency eldl the frame scanning iield. The ray is preferably extinguished during the return strokes to avoid the .generation of picture signals ,at these times. Y f

rEhe higher frequency field may be called the line scanning field and the It has been found, however, that spurious unwanted signals arise in the picture signals during the return strokes of both fields and during the scanning strokes at the beginning or end of a line or frame justfollowing the turning on or oi of the cathode ray. Thus, for example, unwanted signals may occur at the beginning or end of a line scanning stroke or at any time during a line return stroke. The amplitude of the unwanted signals depends upon the intensity of the scanning ray and also, in a somewhat obscure manner, upon the nature of the object being transmitted. Unwanted signals of a similar nature are also developed under some conditions at the beginnings or ends of the frame scanning strokes or at any time during the frame return` strokes. The cutting oif of the cathode ray during return strokes reduces the magnitude of the disturbing signals 'but itself introduces other disturbances. The unwanted signals may consist of excursions in both the black and white directions and their presence may make it dflicult or impossible to reinsert direct current and low frequency components into the picture signals in the manner referred to above.

lt has already been proposed to remove spurious signals, such as those above described, Vby superimposing upon the Vpicture signal intervals in which the spurious signalsoccur pulses which drive the signal amplitude beyond the `peak level of 'the picture signals and by then limiting the signal amplitude.

It is an object of the present invention, therefore, to provide an improved method and means for partially or wholly removing the spurious sig'- rious signals from aliecting the insertion of the` D. C. component.

According to the present invention, there is provided a method of television transmission inwhich spurious signals occur in the intervals between groups of picture signals and in which the spurious signals Vare removed, wherein rthe D. C. component of said picture signals is arranged to be present at the point at which the spurious `signals, are removed, the spurious signals being replaced by a signal having an amplitude rcorresponding to or at least xedly related to picture black.

- The D. C. component ofthe picture signals is presentat the point at which the spurious signals `are removed when, at that point, all parts ofthe picture signals which correspond to black in the object are represented by the same potential.

According to a feature of the present invention there is provided a method of television transmission in which scanning is eifected with the aid of a cathode ray tube, the cathode ray being periodically deflected, by means of a suitable electrostatic or electromagnetic scanning eld,

from one edge of the area scanned to the opposite edge thereof, the scanning field being then reversed in sense and the ray being deflected, in a return stroke to the first-named edge at a comparatively high speed, the picture signals so generated being passed to a suitable amplifier, wherein, during the return strokes of the cathode ray, the output of the amplier is substantially isolated froin the input thereof, the D. C. component being arranged to be present at the point where isolation is effected. Thus the amplification of the amplifier may be reduced substantially to Zero during the'return strokes, for example by the application thereto of suitable pulses which bias one or more control grids thereof to anode current cut-oli. The cathode ray is also preferably cut olf 4and in this case the cutting-off and turning-on of the ray is advantageously eiected at such timesv that, allowing for any delay which the picture signals may undergo before reaching the point at which isolation is effected, any spurious signals which may arise due to the cuttingo orr turning-on of the ray arrive at the isolating point after the amplification has been reduced, and before the amplication is restored to its normal level.

According to a further feature of the present inventionl there is provided a method of television transmission 'in which spurious signals occur in the interval between groups of picture signals and in which there are superimposed upon the picture signals during said intervals suppression pulses of such sense and magnitude as to displace the spurious signals beyond the level of picture signals, the spurious signals being then removed by limiting the amplitude of the picture signals, wherein it is arranged that the D. C. component of said picture signals is present at the point at which said superimposition takes place. To enable the D. C. component to be reinserted if it has'been lost, the picture signals may be caused, during a part of each line scanned, to assume'a value corresponding to black, these recurrent values being utilised for reinsertion purposes. The whole of the signal in the interval between successive lines is preferably, after the elimination of the 'unwanted signals, caused to assume a value corresponding to black, for example by superimposing impulses of suitable nature upon the signals, and synchionising signals may then be superimposed upon these black interval signals.

The present invention also provides a method of television transmissionin which the D. C.

' component is inserted with reference to a recurrent, fixed amplitude value of the signal and in which there occur in intervals between groups of picture signals spurious signals which may interfere with the insertion of the D. C. component, wherein the device by which the insertion is effected is rendered unresponsive to signals of said amplitude value during .said intervals.

The invention will now be described, by way of example, with the aid of the accompanying diagrammatic drawings, in which Fig. 1 is a schematic drawing-of a television transmitter,

Fig52 showsthe waveform of picture signals developed by the cathode ray tube 3 in the apparatus of Fig. 1 and containing the unwanted signals referred to above,

Fig. 3 shows the wave form of synchronising signals before they are mixed with the picture signals,

Fig. 4 shows the wave form of the signals used for cutting off the amplifier and eliminating the unwanted signals,

Fig. 5 shows the wave form of the signals which are used in cutting on the cathode ray,

Fig. 6 shows the wave form of the picture signals combined with the suppression signals of Fig. 4, after suitable amplification of the latter,

Fig. 7 shows the wave form of the combined picture and suppression signals after elimination of the unwanted signals,

Fig. 8 shows the interval signals reduced to black level and Fig. 9 shows the wave form of the signals of Fig. 8 combined with the synchonising signals of Fig. 2,

Fig. l() shows apparatus for generating suppression and neutralising pulses,

Figs. l1 and 13 show detailsof the apparatus of Fig. 10', K

Fig, 12 is an explanatory diagram,

Fig. 14 shows modified apparatus according to the invention,

Fig. 15 illustrates a method of generating neutralising pulses,

Fig. 16 is a circuit-diagram of apparatus for carrying out the present invention, and

Figs. 17 to 24 are explanatory drawings illustrating the wave forms of signals at various points in the apparatus of Fig. 16.

In all the figures containing graphs, with the exception of Fig. 12a, ordinates represent current or voltage and abscissae represent time.

Referring to Fig. l, picture signals corresponding to an object 2 to be transmitted are generated by means of a cathode ray tube 3, having a cathode 4, a modulator electrode 5, two anodes 6 and "I, two pairs of deflecting plates 8, 8 and 9, 9 and a screen Il? comprising a mosaic of photo-electric elements disposed upon a signal plate. The anodes and l' are held at positive potentials relative to the cathode li by means of a source of current indicated at il, the positive terminal of which is earthed. The mosaic screen is connected to earth through a signal resistance I8.

The deflecting plates 8 and 9 are connected to earth: plate B is connected to a line-frequency scanning oscillation generator I I, while plate S is connected to a frame-frequency scanning oscillation generator I2. Generators II and l2 are fed with synchronising pulses from sources I3 and I4, the impulses from which occur at the line and frame frequencies respectively. The form of the impulses generated by source I3 is shown in. Fig. 3; source It generates similar rectangular impulses. The line and frame frequencies are so chosen that the cathode ray scans the mosaic screen III in a series of substantially straight parallel lines, each scanning stroke being followed by a relatively rapid return stroke.

An'optical system indicated generically at I5 is provided to project an image of the object 2 upon the mosaic screen I0.

The object 2 is provided with a black border indicated at I6, so that the cathode ray, after scanning a line of the object 2, scans a black zone corresponding to the border I6; the border may be provided in any known or suitable manner. Means which are further described below, are provided for cutting on? the cathode ray during the return strokes, and for turning it on again at the beginnings of the scanning strokes.

A portion of the signal generated by the apparatus described and appearing across resistance I8, is illustrated in Fig. 2, in which the signal between A and C represents the signals generated in scanning one line, while the part CD corresponds to a return stroke; AB represents picture signals, BC represents thepart of the signal due to the black border l, and 'CD repre.- sents the spurious signals which are generated,

during the return stroke, while the ray is cut off.

The portion BC due to the black border constitutes a recurrent signaly of fixed amplitude value which is suitable for use in the re-insertion of the D. C. component of the picture signals. A method by which-this reinsertion may be accomplished will be described later with reference to Fig. l, and it will be suilcient to indicate here that the method involves the application of the signal of Fig. 2, the D. C. component of which it is assumed that it is 'desired to rei-insert, to a circuit comprising a unidirectionally conducting device, and a condenser provided with a leak resistance, it being arranged that current just flows to the condenser through the unidirectionally conducting device when the part BC of the signal is present, the picture signals AB producing no ilow of current.

It will be evident that the spurious signals indicated at CD in Fig. 2 will interfere with the reinsertion or" the D. C. component by the method outlined. since the excursions of the signal of Fig. 2 in the upward direction from the straight line ABCD will cause current to flow in the unidirectionally conducting device.

It will be understood that the recurrent xed amplitude value which serves for reinsertion of the D. C. component need not correspond to black so long as it is ixedly related to black, and so long as suitable steps are taken tomake a pre-determined xed current or voltage correspond with black in the signal.

It is desired iirstly to eliminate the unwanted signals CD, and secondly to insert signals suitable for synchronising purposes.

Normally, synchronising pulses derived from the synchronising pulse sources (i3 and i4, Fig. l.) at the transmitter are mixed with the picture signals in such manner that one line synchronising pulse occupies between one half and the Whole of the interval between two successive line trains of picture signals, and in order that the synchronising pulses may be easily separated from the trains of picture signals at the receiver the synchronising pulses are inserted into the transmission channel in the blacker-thanblack direction, that is, in the opposite direction from a datum line corresponding to black to that corresponding to an increase in brightness of the object. f

It is desirable, therefore, that the signals BCD of Fig. 2 should be converted into signals of the shape TUXYZ shown in Fig. 9 and this may be done in the following manner:

Suppression pulses of the kind shown at VILW f Vgreater than that occupied by part CD of Fig. 1, that is, the spurious signals.

A set of pulses of the form shown in Fig. 5 is also generated, by means of source 2E, in xed phase relationshipto the line synchronising signais; the pulses of Fig. 5 which last for a time at least equal to that during which the spurious signals may be developed, will be referred to as switching pulses, and are applied between the modulator 5 and the cathode Il of the tube 3 in such a sense, and at such an amplitude level, that they serve to reduce the intensity of the cathode ray substantially to zero, or, in other words, to cut ofi the ray.

The signals set up across resistance i8, which have the form shown in Fig. 2, are fed through picture signal amplier 2|, which ,will be assumed to be an amplifier which is incapable of amplifying the D. C. component of the picture signal to the co-ntrol grid circuit of a tetrode valve 22, which has a grid condenser 23 and grid leak 245 in its grid circuit, and a biasing resistance 25 shunted by a by-pass condenser 2G in its cathode circuit. Suppression pulses from source ltare fed to the grid circuit of a tetrode valve 2l having a grid condenser 28, grid leak 29, biasing resistance 3l? and by-pass condenser 3l. The anodes of valves 22 and 2l are connected through a common anode resistance 32 toy the positive terminal of a source 33 of anode current, the negative terminal of which is earthed;

the screening grids of these valves are also connected to a suitable point in source 33 by connections which, for the sake of simplicity, have been omitted from the drawings.

The picture signals, Fig. 2, are applied to valve 22 in such sense that an increase picture brightness corresponds to a decrease in the negative potential of the control grid of that valve, while the pulses of Fig. 4 are similarly arranged to drive the control grid of valve 2l less negative. The signals of Figs. 2 and 4 are in this way mixed together, the form of the mixed signal, which is set up across resistance 32, being as indicated in Fig. 6, except that the D C. component of the picture signals is missing, having been lost in amplifier 2l and in the coupling to valve 22. The amplitude of the suppression pulses is made such that the combined amplitude (MNOP, Fig. 6) of suppression pulses and spurious signals is always brighter than black, and preferably greater than the maximum amplitude occurring in the picture signals.

As has already been pointed out, the signals set up across resistance 32 lack the Di. C. cornponent. This component is now reinserted by means of tetrode Valve Si, which has a grid condenser and a grid leak 36 in its control grid circuit.

The signals applied to valve 34 consist of variations about a datum line such that the areas enclosed by the signal envelope above and below the datum line are equal, and it is arranged that signal excursions in the downward direction of 6 cause the control grid of valve 32% to become more negative. The applied signal causes grid current to ilow in valve 3d, and condenser 35 charges up, the control grid potential' thus becoming more negative. A state is eventually reached in which` only the parts QM? (Fig. 6) of `the signal will cause grid current to ilow, the time constant of condenser 35 and leak 3S being made such that only a slight change in control grid potential takes place between successive parts QM?. The datum line is thus changed to one through the peak portions QMP of the signal, and the D. C. componentis thus restored.

The grid base of valve 34, that is, the change in potential required to swing the control grid of valve 3d from zero potential (relatively to the cathode) to the potential corresponding to anode current cut-off, is made such that the signal of Fig. 6 is converted in valve lill into a signal of the form shown in Fig. '7; in other words, it is arranged that parts of the signal of Fig. 6 beyond a line corresponding to the line RS of Fig. '2, lie in the region of ycontrol grid potentials more negative than that corresponding to anode current out-off, and are thus removed. The valve Sli thus serves not only to re-insert the D. C. component, but also as an amplitude limiting device.

vThe anode of valve 3d is connected to the positive terminal of source 33 through anode resistance 3l, and the signals set up across this resistance, Whichhave the form indicated in Fig. '7, are fed to the control grid circuit of a further valve 38, which has a grid condenser 39, a grid leak Gil, a biasing resistance lll and by-pass condenserr 42. Suppression pulses from source i9 are applied to the control grid of a valve 43 having a grid condenser dll, grid leak 1&5, biasing resistance i6 and by-pass condenser 4l, in such a sense as to drive the control grid of valve d3 in the positive sense. The valves i3 and. 34 both have the same anode resistance 3l, and the outputs of Valves 13 and 3.1i are mixed together across this resistance, it vbeing arranged that the pulses fed to valve i3 are substantially equal in amplitude and opposite in sense to the parts MRS? of the signal oi Fig. 7. The signal :ted to valve 38 thus has the form shown in Fig.

' 8, the parts MRSP of the signal of Fig. 7 being replaced by intervals of black, or substantially black level. The pulses of Fig. 4 serve in the valve 153 as neutralising pulses.

The signal of Fig. 8, which isfree from spurious signals, is now mixed with synchronising pulses;

the latter are derived from generators i3r and ld, and are applied to a valve d3 having a biasing resistance lid, oy-pass condenser 5, grid leak 5l and gr-id condenser 52. Valves 38 and i8 have a common anode resistance 53; the synchronising pulsesare fed to valve 48 in lsuch sense that they are mixed with the picture signals inthe blackerthan-black sense, the mixed signals set up across resistance 53 being of the form illustrated in Fig. 9. The mixed signals are then fed to a radio transmitter indicated at 54.

In the arrangement described,` the re--insertion of the D. C. component is eiected at the point where the superimposition of the suppression pulses takes place. If desired, the D. C. com ponent may be re-inserted after the point at which the suppression pulses are superimposed on the picture signals, and for this purpose, the valve 22 and 2l of Fig. 1 may be omitted, the picture signals and suppression pulses previously mixed being fed directly, in suitable sense, to the control grid circuit of valve Bit. It is not necessary that amplitude limitation should be effected by the re-inserting valve, and the func tions of valve 34 of Fig. 1 may be performed by two D. C. coupled valves, the first effecting reinsertion, and the second, amplitude limitation. It will be seen that in order to obtain the signal of Fig. 8, in which groups of picture signals are separated by black, or substantially Vblack intervals, independently of changes in average picture brightness, it is important that the D. C. component should be present in the signals fed to .this component is not re-inserted simultaneously with the super-imposition of the suppressing pulses of Fig. 4, it must be re-inserted before limitation is eiiected. When the D. C. component is present at the limiting valve, the part MRSP of the signal of Fig. 7 is the same in every interval, and a neutralising pulse of constant amplitude can thus be employed. The pulses of Fig. 4 can be used not only as suppression and neutralising pulses, but also to reduce the amplification of one or more of the valves of the picture signal amplifier 2l, preferably substantially to Zero, in this way isolat-. ing the output of the amplifier from the input thereof. For this purpose, the pulses of Fig. 4 may be fed over lead 55 from source i9 to ampliiier 2l, and applied to bias the grids of the amplifier valves to or beyond anode current cut-off. Preferably at least the valve nearest the source of picture signals is out 01T in this way and other later valves may also be out off if desired.

It will be noted that owing to the shapes of the pulses of Figs. 4 and 5, the amplifier 2l is out off at rV (Fig. 4) before the cathode ray is `cut off at I (Fig. 5) and the ray is again turned on at L (Fig. 5) before the amplifier is turned on at W (Fig. e). In determining the `shapes of the pulses of Figs. 4 and 5, due regard must be had to anyidelays which the picture signals may undergo up tothe point where the amplifier 2l is cut off, so that the spurious signals always arrive at that point after the amplifier is out ofi and cease before it is turned on again. spurious signals arising from the cutting ol and'turning on of the cathode ray are in this way at least substantially suppressed.

Instead of producing a black signal BC (Fig. 2) at the end of each line, a whiter-than-white signal may be produced. By a whiter-than-white signal is meant a signal value in the white direction from black and further removed from black than full White in the image of the object. Such a signal may be producedfor example by forming an image of an elongated source of light upon one side of the image upon the mosaic screen, and for this purpose, the black border i6 of Fig. 1 is arranged to be replaced by this bright image. The brightness oi the marginal image is made such that it will always be greater than that of the brightest part of the image of the object 2 formed upon the screen iii. The D. C. component of the picture signals is .re-inserted with reference to the recurrent whiterthan-white,l signal value instead of black value QM in Fig. 6.

Suppression pulses from source i9 are arranged, as before, to drive the unwanted signals so far in the white direction that they can be removed by an amplitude limitation, in a manner such as has already been described. The resultant interval signal, which then has a whiter-than-white value, is brought to the condition TUXYZ, shown in Fig. 9, by superimposing upon the picture signals synchronising pulses of the form shown in Fig. 2, these pulses being derived from sources i3 and lil and being superimposed upon the picture signals in such a sense, and at such an amplitude level that signals of the form of Fig. 9 are obtained. In this way, the whiter-than-whte pulses are removed from the signals as well as the spurious signals. If it is found that the spurious signals extend beyond the whiter-than-white signals in such a way that they would interfere with the re-insertion of the D. C. component, pulses of the kind shown in Fig. i may be superimposed upon the signals at the reinsertion point, these signals being derived from source i9 and being arranged tc be in such sense as to drive the spurious signals to a sufficient extent in the black direction. The procedure thereafter may be as before.

The pulses of Figs. 4 and 5 may be generated by means of a disc having two series of apertures of suitable shape arranged on concentric circles, the disc rotating between a pair of light sources and two co-operating photo-sensitive cells. Apparatus of this kind will be described later with reference to Figs. l and l1. The synchronising signals of Fig. 2 may be generated by a second series of apertures in the same disc, arranged to co-operate with a separate cell.

In a further arrangement, the D. C. component is not re-inserted with reference to a black or whiter-than-white signal arranged to occur at the end of each line, in the manner described above, but is re-inserted with reference to peaks of the picture signal in the black direction. 'I'he black border or bright image i6 referred to above may then be omitted, and it should be noted that in this further arrangement, the durations of the pulses of Figs. 2, 4- and 5 must be chosen with the fact in mind that the interval BC of Fig. 2 is no longer present.

It has been found diiiicult to ensure that the pulses employed as neutralising pulses have exactly the same duration and shape as the suppression pulses. If they have a diierent duration, narrow, peaky pulses in one sense or the other will remain after the superimposition of the neutralising pulses. If these residual pulses are in the white direction, they may cause white lines to appear at the edge of the final picture. If the residual pulses are in the blacker-thanblack direction they are likely to disturb the synchronisation, since the synchronising signals are usually in this direction in the transmitted signal.

It may therefore be arranged that the durations of the suppression and neutralising pulses are different and such that the residual pulses are in the blacker-than-black direction. The residual pulses are then removed by means of a suitable limiting device before the admixture of the synchronising pulses. Usually the suppression pulses are applied in the white direction and they are then made of shorter duration than the neutralising pulses.

One method of generating suppression and neutralising pulses of different duration will now be described with reference to Figs. 10 to 13 of the accompanying drawings. Referring to Fig. 10, a disc 56 is driven, by means of a motor 5l, to rotate between a light source 53 and an apertured diaphragm 59, behind which is a photo-electric cell G9. The pulses set up in the cell 69 are fed to an apparatus Si, which comprises an amplier and means, which will be described with reference to Fig. 13, for dividing these pulses into two separate sets. Fig. 11 is a front elevation of the disc 56, which has a plurality of substantially rectangular apertures G2 formed therein.

Referring to Fig. 12, the graph a represents the intensity of the light falling on the diaphragm 59 plotted against time as abscissae, while b represents the form of the aperture in the diaphragm 59. The curve c represents the form of the pulses set up in cell 69, amplitude of pulse being plotted against time as abscissae. Each aperture B2 in disc 5S gives rise to a tapered pulse ABCDEF of the form shown in Fig. 12a, and these pulses, after amplification in amplifier 6I, are fed to the control grid circuit of a valve 63 through a grid condenser 61% with which is associated leak resistance 65. d

The valve S3 is biased by means of biasing resistance G9 and condenser lil to a point which is more negative than that corresponding to anode current cut-off, and the pulses of Fig. 12o are arranged to drive the control grid of valve 63 less negative. vThe value of the grid bias is so chosen that only the upper portions BCDE of the pulses cause anode current to now; the anode of valve 63 is connected through resistance 68 to the positive terminal of a source 'I9 of anode current, and pulses of the forml BCDE appear at terminal 69, and are employed as suppression pulses.

'Ihe pulses ci Fig. 12o are also applied to a valve li, having a grid condenser l2, leak I3 and anode resistance lli, which is so biased by means of resistance 'l and condenser 'i6 that it effects no amplitude limitation of the pulses, but reverses them in sense and applies them to valve 'Vl by means of condenser 1S and leak resistance 19. Valve l1 is so biased by meansl of resistance 80 and condenser 8l that anode current only flows Vtherein for the parts ABEF of the pulses of Fig.

12e. The anode or valve il is connected to the positive terminal of source 'i9 through resistance 82, and pulses of the form ABEF appear at the terminal 83, whence they are led away to serve as neutralising pulses.

The neutralising pulses, which are constituted by the lower portions of the tapering pulses of Fig. 12e, are thus arranged to be of longer duration than the suppression pulses, which are constituted by the upper portions. The suppression pulses are conveniently mixed with the picture signals in the white sense, so that when, after the reinsertion of the D. C. component, and subsequent limiting, the neutralising pulses are superimposed, the residual impulses are in the blackerthan-black sense, and can be removed by an amplitude limitation by a method such as has already been fully described.

Instead of generating the suppression and neutralising pulses as above described from a single set of tapering pulses, the two sets of pulses may be generated separately. For example, two sets of pulses may be generated optically in a manner such as that described with reference to Figs. and 11, with the aid of two sets of apertures of different widths. Alternatively, the pulses may be derived from two separate electrical pulse generators.

It is however, advantageous to use what is effectively a single set of pulses, namely the two parts of the tapering pulses, for both suppression and neutralising, becausea change in the tapering pulse, for example in its length, affects both suppression and neutralisation equally.

A modified circuit arrangement, applicable whether the suppression and neutralising pulses are of equal or unequal durations will now be described with reference to Fig. 14. Referring to Fig. lli, the signals ci the form of Fig. 6, containing their D. C. component, are applied to the inner control` grid of a hexode valve 84 through grid condenser and leak 86, the neutralising pulses being applied to the outer control grid through grid condenser 8l, with which is associated leak 98 and a source 89 of biasing potential. The inner grid is biased by means of source 90. The picture signals are applied in such sense that they render the inner grid less negative, whilst the neutralising pulses applied to the outer grid render it more negative. A resistance 9| providing negative reaction is arranged in the cathode lead of the valve 84 to straighten the characteristic and sharpen the bottom bend. The anode of valve 84 is connected to the positive terminal of a source 92 of anode current through a resistance 93, and the screening grids are connected to a point at a positive potential in source 92. It is arranged that, with the outer control grid at the potential it assumes in the intervals between neutralising'pulses, picture black corresponds to anode current cut-oi; signals of the form of Fig. 8 can thus be derived from terminal 94.

Where the neutralising pulses are of greater duration than the suppression pulses, the residualpulses are in the negative sense beyond the point corresponding to anode current cut-01T in valve 84, and produce no change of anode current.

Since the amplitude (with respect to black) of the residual pulses before they are removed is dependent solely upon the amplitude of the neutralising pulses, which is kept constant, the D. C.

component may be reinserted with reference to the peaks of these residual pulses before the latter are removed. It may be desirable to do this, for example, when the D. C. has been Wholly or partially lost between the point at which it is rst inserted and the point at which the residual pulses are removed.

Where these residual pulses are applied in the negative sense to the grid of a valve, the D. C. may be re-inserted with the aid of a diode rectifier in parallel with the grid circuit of the valve as described in BritishPatent specication No. 422,906. l

In an alternative method of generating suppression and neutralising pulses of different durations, suppression pulses are generated in any convenient manner, and neutralising pulses are generated from the suppression pulses with the aid of a hexode valve. Referring to Fig. 15, the suppression pulses are applied directly and in the negative sense from terminals 99 to the outer control grid of a heXode valve 95 and through a delay network 96 which comprises inductances 91 and shunt condensers 98, and also in a negative sense, to the innercontrol grid. The outer control grid has a condenser |08 and a leak Il, while the inner grid has a condenser |02 and a leak |03. The anode is connected through resistance |04 to a source |05 of anode current, to apoint in which the screening grids are also connected. It is arranged that the directly applied pulses cause the anode current of the hexode to fall to zero and that the pulses arriving through the delay network serve to hold the anode current at zero. The resulting pulses at the terminal |06 are therefore of longer duration than the suppression pulses. The suppression pulses, which are to be superimposed upon the picture signals, may be subjected to half the delay to whichV the signals applied tothe inner grid of the hexode are subjected, so that the centre lines of the suppression and neutralising pulses are substantially coincident.

'I The methods above described for eliminating spurious signals from the line intervals may also be applied to remove spurious signals from the frame intervals. vThe frame intervals are usually longer than the line .intervals and the suppression and neutralising pulses are then made of suitably longer duration.

A further example of the present invention will now be described with reference to Figs. 16 to 24.

In Fig. 17 is shown a part of a picture signal. In this iigure, as in Figures 19, 21, 22 and 24, the sense corresponding to an increase in brightness is indicatedV by an arrow W. In Fig. 17 the periods P are periods of scanning complete lines of the object and the periods P1 are periods of scanning half lines of the object, the signals being those developed in interlaced scanning where the object is completely scanned in two traversals thereof, the lines traced out in one traversal interlacing with those traced out in the next traversal. The periods LI are the line intervals and the periods FI are the frame intervals. During these intervals spurious signals S may occur.

Ihe signal of Fig. 17 does not contain the D. C. component and the Zero line o therefore is such that the areas enclosed by the wave form above and below it are equal to one another.

The signal of Fig. 17 is applied to the grid of a valve |07 in Fig. 16 in such sense that signals in the direction W make the grid more negative. The valve |01 has a resistance |08 in its cathode circuit, and is arranged in such a way that the oscillations derived from the cathode substantially correspond to those applied to the grid. The anode of valve |07 is connected to the positive terminal of a source |89 of anode current. Oscillations from the cathode of valve |01 are applied to the control grid of a valve ||0 through a condenser A tetrode valve ||2 is connected as shown between the grid and cathode of the valve H0, a resistance ||3 being connected between the anode and cathode of the valve. Ihe cathode potential of valve ||2 is adjusted so that valve I8 is biased to a desired point on its characteristic, and for this purpose is connected to earth through a bias resistor ||4 shunted by a condenser ||5.

The anode and cathode of the tetrode |2 acts as a diode, and current flows through the diode on the positive peaks of the signal (that is the peaks in the black direction) thereby charging condenser l|| and making the grid of valve ||9 more negative. and resistance E3 is made long compared with the intervals P, (Fig. 17). When the steady state is reached, current will flow through valve ||2 only on the extreme tips of the signals and D. C. will be established on the grid of valve H0. It will, however, be clear that parts of the spurious signals S may project beyond black into the blacker-than-black region and interfere with the insertion of D. C. by the device H2, ||3. To prevent this, negative pulses such as are shown in Fig. 18 are applied'to the control grid of the valve ||2 from a source H6. These pulses serve to stop current flow in the anode-cathode path of the valve H2 during the intervals LI and FI of Fig. 17 and so render the re-inserting device unresponsive to blacker-than-black peaks which may occur in these intervals. The re-insertion of D. C. is thus unaffected by the spurious signals and takes place with reference to the blackest parts of the picture signals in the intervals P and P1. The resulting signal at the grid @of valve ||9 is shown in Fig. 19, the Zero axis O corresponding to picture black.

The anode oi valve Il@ is connected to the anode of ay tetrode valve andthrough resistance Yl I9 and a condenser ||8 in parallel to The time constant of condenserV the grid of a valve |20. The anodes of valves ||0 and lil are connected to the positive terminal of source its through resistances |2| and |22, a decoupling condenser |23 being also provided. A resistance lili is connected between the grid of valve 82d and earth, and the anode of this valve is connected to the positive terminal of source idg.

To the control grid of valve is applied a signal which is of the form shown in Fig; 20 and is derived from source H6. The signal at the common anodes of valves HQ and H7 will thus be as shown in Fig. 2l, the spurious signals S being displaced wholly beyond black level represented lby the axis O in the blacker-than-black direction.

These signals are conveyed to the grid of valve l2?) through coupling means, including elements il, E22, Sie, 52d, lit? and |23, which serve to pass all frequency components of the signal, including zero frequency or D. C. The valve iZO is so'adjusted that anode current ceases to flow at a signal -level corresponding toblack, so that at the catho-de of this valve (which is connected to earth through resistance E25) the signal voltage has the forni shown in Fig. 22, the axis O corresponding to black. This signal is applied to the grid of a valve the cathodeof which is connected to the cathode of another valve |2`| and thence to earth through resistance t28. To the grid of valve l2? are applied synchronising signals of the form shown in Fig. 23 and the signal output from the cathodes of valves |26 and |21 will then have the form shown in curve 2d. The signals of 23 are derivedy from source |29.

It will be seen that the spurious signals have been removed and the D. C. component has been inserted.

We claim:

l. In a television transmission system in which video signals having a direct current component and spurious signals are developed and in which the direct current component is suppressed in the passage of the video signals through the transmission apparatus, the method of re-inserting the direct current component exclusive of the spurious signals which comprises the steps of developing suppression waves, combining the suppression waves with the spurious signals to raise yso the amplitude level thereof, limiting the value of said combined waves to a value outside the range of the spurious signals, and re-inserting the direct current component at a portion of the system through which passes only the limited combined wave substantially simultaneously with the limiting thereof.

2. The method in accordance with claim 1 wherein there is provided the additional steps of developing neutralizing Waves and combining the neutralizing waves with the limited Waves in such as to remove substantially the limited waves.

3. In a television system wherein there is included means incapable of transmitting direct current low frequency alternating current, means for developing trains of video signals having a'direct current component from anobject to be scanned, and in which there are included spurious signals, means for developing a signal xedly related to a xed shade in the object to be scanned, means for developing suppression pulses, means for combining said suppression pulses with said spurious signals whereby the' level of said spurious signals is changed, and means for limiting the value of the combined suppression pulses and spurious signals passed to the system to a value outside the range of said spurious signals'said latter means also acting to re-insert rthe direct current component of the video signal with respect to the signal xedly related to a fixed shade in the object to be canned. 4. Apparatus in accordance with claim 3 wherein said means for limiting the amplitude ci the combined suppression pulse and spurious signal and also acting to re-insert the direct current component of the video signals comprises a thermionic tube having the input circuit thereof of such parameters as to block the current now in the tube at an amplitude short of the spurious signal level and to maintain a substantially fixed ing to the direct current component.

5. Apparatus in accordancey with claim 3 wherein there is provided in addition means for cancelling the limited pulses said means comprising means for developing neutralizing pulses, and means for combining said neutralizing pulses with said limited pulses in such phase as to substantially cancel said limited pulses.

6. In a television system wherein there is included means incapable of trans itting direct current and low frequency alternating current, means for developing trains of, video signais having a direct current component from an object to be scanned and in which there are included spurious signals, means for developing a signal xedly related to picture black in the object to be scanned, means for developing suppression pulses, a thermionic vacuum tube having anode, cathode and atleast one control electrode, means for combining said suppression pulses and said spurious signals in such phase as to increase the amplitude level of said spurious signals, means for impressing said combined spurious signals and suppression pulses onto the control electrodecathode circuit of. said thermionic tube, means for impressing said video signals and said signal xedly related to picture black onto the control electrode-cathode circuit of said tube, a time constant circuit connected in the control electrode-cathode circuit 'of said tube having parameters of such value that said tube is blocked for wave amplitudes of the range of the spurious signal amplitude, and said control electrode is maintainedat a substantially constant bias for waves corresponding to the signal Xedly related to picture black, whereby -the spurious signals are removed, and the direct current component of the videosignals is re-insertedby the same thermionic tube.

7. Apparatus in accordance with claim 6 wherein there is provided in addition means for cancellingthe limited pulses said means comprising means for developing neutralizing pulses, and means for combining said neutralizing pulses with said limited pulses in such phaseras to substantially cancel said limited pulses.

8-. In a television system wherein there is included means incapable of transmitting direct current and low frequency alternating current, means for developing trains of. video signals having a direct current component from an object to be scanned and in'Which there are included spurious signals, means for developing a signal rlXedly related to picture black in the object to be scanned, means for developing suppression pulses, a thermionic vacuum. tube having anode, cathode and at least one control electrode, means for combining said suppression pulses and said bias on said input circuit of a value correspondspurious signals in such phase as to increase the amplitude of said spurious signals, means for impressing said combinedspurious signals and suppression pulses ontothe control electrodecathode circuit of. said thermionic tube, means for impressing said video signals and said signal Xedly related to picture black onto the control electrode-cathode circuit of said tube, a time constant circuit connected in the control` electrodecathode circuit of said tube having parameters of such value that said tube is blocked for wave amplitudes 0f the range of the spurious signal amplitude, said control electrode is maintained at a substantially constant bias for waves corresponding to the signal xedly related toy picture black, whereby the spurious signals are removed, and the direct current component of the video signals` is re-inserted by the same thermionic tube, means for developing neutralizing pulses, means for combining said neutralizing pulses with the limited combined suppression pulses and spurious signals in such phase as to substantially cancel the limited combined suppression pulses and spurious signals, means for developing synchronizing signals in the intervals between trains of picture signals, and means for inserting said synchronizing signals during at least a portion of the interval during which the cancelled ysuppression pulses have been present.

9. Apparatus in accordance with claim 6 wherein there is provided in addition means for cancelling the limited pulses said means comprising means for developing neutralizing pulses and means for combining said neutralizing pulses with said limited pulses substantially 180 .degrees out of phase therewith, the neutralizing pulses and the suppressio-n pulses having different durations.

10. Television transmitting apparatus comprising picture signal generating means of such nature that spurious signals may be generated in intervals between groups of picture signals, means for generating suppression pulses of predetermined suitable amplitude, means or mixing said pulses with said picture signals and spurious signals in the presence of the D. C. component of said picture signals, a heXode valve, means for feeding superimposed picture signals, spurious signals and suppression pulses to one control grid of said Valve in such sense that an lincrease in picture brightness is represented by a positive signal, means for generating neutralising pulses of predetermined suitable amplitude, means for feeding said neutralising pulses to the other control grid of, said valve in such sense as to drive the potential of that grid in the negative sense, and means for ensuring that, in intervals between successive neutralising pulses, a picture signal representative of black causes the flow of anode current substantially to cease.

CECIL OSWALD BROWNE.

JOI-IN HARDWICK.

FRANK BLYTI-IEN.

ERIC LAWRENCE CASLING WHITE.

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