US2243079A

US2243079A - Television transmitting apparatus

Info

Publication number: US2243079A
Application number: US173550A
Authority: US
Inventors: Blumlein Alan Dower; Browne Cecil Oswald
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1936-11-10
Filing date: 1937-11-09
Publication date: 1941-05-27
Anticipated expiration: 1958-05-27
Also published as: NL63851C; GB490205A; DE760333C; FR829278A

Description

y 1941- A. D. BLUIMLEIN ETAL 79 TELEVISION TRANSMITTING APPARATUS Filed NOV. 9, 1937 2 Sheets-Sheet-2 l lAlVE/VTORS 414m 0. Bil/MAFIA CECIL 0. a nwp A TTORNEY Patented May 27, 1941 TELEVISION TRANSIVHTTING APPARATUS Alan Dower Blumlein, Ealing, London, and Cecil Oswald Browne, West Acton, London, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application November 9, 1937, Serial No. 173,550 In Great Britain November 10, 1936 14 Claims. (01. 178-72) This invention relates to signal transmitting systems and more especially to television transmitting apparatus, and is directed to the reduction or elimination of spurious or undesired signals of irregular wave form and of relatively low frequency.

The invention is particularly, although not exclusively, applicable to transmitting apparatus embodying a cathode ray tube picture scanner in which a mosaic or other screen structure is provided composed of a plurality of photo-sensitive mutually-insulated elements capacitatively associated with a signal plate, the elements acquiring an electrostatic charge according to the intensity of the corresponding elementary areas of the picture to be transmitted. The elements are restored to an equilibrium potential under the action of a scanning beam of cathode rays, restoration of the elements to the equilibrium potential generating picture signals which are fed to a suitable amplifier for transmission.

In such transmission apparatus the spurious or undesired signals give rise to what has become known as tilt, and the tilting effect is manifested in the picture signals by the signals having superimposed upon them a saw-tooth wave so that the picture signals generated when scanning a line of constant brightness are such as to correspond to a line having a progressively changing brightness along its length. In a picture, reconstituted by means of a cathode ray tube, the image on the fluorescent screen appears to have a progressively increasing brightness towards one corner of the reconstituted image.

In order to reduce the effect of tilt, it has been suggested to apply an equal and opposite saw-tooth wave to counteract the saw-tooth wave upon which the picture signals are imposed. Such proposal, whilst resulting in a reduction of tilt, is not altogether satisfactory and, accordingly, it is the chief object of the present invention to provide an improved manner of reducing the effect of undesired signals.

According to the invention, a method of removing spurious or undesired signals of relatively low frequency from signals extending over a relatively wide range including said low frequency undesired signals is provided which comprises interspersing said signals with other signals of a datum value and recurrent at a frequency higher than the frequency of the undesired signals, attenuating frequencies below a predetermined frequency so as to remove said undesired signals from the signal chain, and subsequently components of the desired signals by reference to the recurrent signals of datum value. In the case of a television transmitting apparatus of the above described kind to which the invention is particularly applicable, the picture signals generated by scanning the mosaic or other screen are interspersed with signals of datum Value, preferably corresponding to black, of a frequency higher than the frequency of the tilt component,

v.low frequency signals up to a frequency or higher than or equal to that of the tilt component being attenuated so as to remove such signals including the tilt component from the picture signals, the suppressed low frequency or D. C. components of the picture signals being subsequently reinserted by reference to the recurrent signals of datum value. The picture signals from which .ing to the invention may be produced by projecting the image of the object of which a picture is to be transmitted on to the mosaic or'other screen through a line grid which casts a shadow on said screen forming black lines on the screen preferable at right angles to the direction of the line scan whereby when said screen is scanned the desired picture signals so produced are interspersed with signals of datum level, i. .e. signals corresponding to, the black lines formed .by said grid.

The grid may be formed on the support carrying the mosaic or arranged at any other suitable point with reference to the image producing optical system.

The effect of the grid in the receiver may be minimised or rendered relatively innocuous by moving or reciprocating the grid or by suitable treatment of the signals during transmission.

In order that the invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic illustration of a picture scanning apparatus employed in the preferred form of the invention, and

Figures 2, 3 and 4 are explanatory diagrams.

Figures 5, 6a, 6b, 6c, 6d and 6e are further explanatory diagrams,

Figure 7 illustrates another form of scanning re-inserting suppressed low frequency or D. C.. apparatus employed in the invention,

Figure 8 is a circuit diagram of a convenient arrangement for transmitting signals produced according to the invention, while Figure 9 shows in block diagram form a wave shaping and combining circuit.

In the following description of the invention with reference to Figures l-7, the invention will be described in its application to the reduction or elimination of tilt and, by way of example, to a television picture scanning device of the kind in which an image of the object for transmission is projected onto one side of a mosaic screen, the screen being scanned from the same side.

As shown in Figure 1 the picture scanning device comprises an evacuated envelope 4 in which is mounted a mosaic screen 5 composed of a multiplicity of photo-electric mutually-insulated elements capacitatively associated with a signal plate 5. The envelope 4 also contain a cathode ray gun I for generating a beam of electrons, the beam being focussed in known manner and deflected by electrostatic or electromagnetic means, not shown. An image of the object for transmission is projected onto the mosaic screen through a lens system indicated at 8, and on scanning the image by the beam picture signals indicated by the wave form 9 (Fig. 2) are established across a resistance 60.. The picture signals should be electrical variations having a zero or horizontal axis as at l l in Figure 2, but, owing to the effect of tilt, the electrical variations occur relatively to an inclined axis I2 so that in effect the picture signals represented by the wave form 9 are superimposed upon a saw tooth wave. These saw-tooth oscillations, which are recurrent at about the line frequency will, unless special steps are taken, result in an uneven shadingof the reconstituted picture as stated above.

In accordance with one embodiment ofthe present invention there is interposed between the lens 8 and the light source, or at any other suitable position, a grid 10 which has the effect of dividing the image projected onto the screen 5 into a plurality of vertical strips separated by black lines disposed at right angles to the direction of scanning. This grid I is placed close enough to the object or virtual object imaged by the lens 8 to cause definite black and white lines to appear in the image produced on the mosaic screen 5. For example, when transmitting from film, the grid 8 may be inserted close to the film and likewise for direct vision the grid 8 may be so placed in front of the object that it is also in focus. In an alternative arrangement a double lens system may be used, one lens of the system forming an image of the original object to be transmitted. This image is formed on the grid close to which is placed a condensing lens which re-condenses the light into another lens such as lens 8, which throws an image of the first image, which constitutes a virtual object, and of the grid 10 on the mosaic screen 5. This alternative, of course, involves an additional inversion of the image and loss of light and it may be preferable in carrying out the invention to utilise a grid actually applied to the carrier, as described more fully below with reference to Fig. 7 of the accompanying drawings. The width of the black lines formed on the screen should be such that the lines will be resolved during the scanning action. On scanning a horizontal line of the image the picture signals generated will be interspersed with black signals of datum value as shown in Figure 3.

The signals derived from the scanning action are fed to an amplifier which is arranged to have such a low frequency cut-off as by the use of a resistance-capacity combination in the amplifier, that frequencies of the order of the line frequency, that is to say, of the order of the tilt component, are not transmitted. For example, if the line frequency is 10,000 cycles er second, then it should be arranged that the amplifier has a cut-off at a small multiple of the line frequency as, for example 40,000 cycles.

' Attenuation of the lower frequencies imparts to the signals appearing at the output of the amplifier a wave form similar to that diagrammatically shown in Figure 4. It will be seen from this figure that the low frequency tilt component has been removed and since certain low frequency components of the picture signals have also been removed, the black signals extend below the electrical axis in the negative sense by an amount dependent upon the amplitude of the attenuated low frequencies.

At the transmitter or receiver it is necessary correctly to relate the picture signals with reference to their true zero axis, that is to say, to re-align the negative peaks of the black signals relatively to a horizontal axis. This is effected by reinserting the suppressed picture frequencies at a suitable point in the transmission channel, by reference to the recurrent black signals in a manner similar to that referred to in the specification of British Patent No. 422,906. For the proper functioning of the apparatus, the time constant of the resistance-condenser combination of the reinserting device is long compared with the interval between the successive black signals but short compared with the highest of the suppressed frequencies. For example, if the number of black lines produced by the said grid is and the line frequency is 10,000 cycles per second, the frequency of the recurrent black lines will be one megacycle, and the interval between successive black signals will be one micro-second. If the time constant of the reinserting device is of the order of 1-10 micro-seconds, it will be seen that the time constant is long compared with the intervals between successive black signals, i. e. one micro-second, but short compared with the period of 25 micro-seconds of the lowest frequency of 40,000 cycles effectively transmitted.

In the case of a 400 line picture the grid [0 would probably have somewhere in the neighbourhood of 400 bars or lines, the limit to the number of bars being set by the resolving power of the scanning system. The grid 50 is shown in Fig. 1 as comprising a small number of bars only for convenience in drawing.

It will be appreciated that the resistance-capacity combination which is usually associated with a diode for reinserting purposes, may, in some cases, be utilised to afford the required cut of the lower frequencies.

It will be understood that, providing the recurrent black signals are caused to appear in the output of the picture scanner, the tilt component can be removed either at the transmitter in which case the suppressed frequencies can be reinserted at a suitable point at the transmitter or receiver, or the frequencies may be both suppressed and reinserted at the receiver.

"tilt effect is eliminated or substantially reduced. I

If desired, steady light, that is to say, a uniform auxiliary illumination apart from the projected image, may be projected onto the mosaic screen through the same grid I to enable signal of the grid frequency to be obtained even when the picture is dark. This enables the D. C. reinserting device to work more satisfactorily for the dark parts of the picture.

It is possible that the black lines produced on the mosaic screen may have some noticeable effeet in the reconstituted picture and for the purpose of rendering such lines less noticeable it may be arranged that the position of the grid relatively to the mosaic is continuously changed during the scanning action. Thus, by the pro vision of suitable mechanism and by mounting the grid in a suitable manner, the grid may be intermittently or continuously reciprocated so that the position of the :black lines cast upon the screen are changed or interlaced during appropriate periods. Suitable means for reciprocating the grid may be an eccentric IUI and I03 shown in Fig. 1 after the fashion shown by the scanning arrangement in Fig. 3 of page 18 of the textbook entitled Das Elektrische Fernsehen und das Telehor by Mihaly, published in Berlin by M. Krayn in 1926.

It will usually be advisable to provide a filter to eliminate the electrical frequency resulting from the scanning of the black lines, such frequency in the case considered above being 1 megacycle.

An alternative method of removing the efiects of the black lines on the received picture is to use in the transmitter an arrangement in which the width of the black lines is equal to the width of the spaces between them, and utilise prior to transmission or at the receiver two sets of picture currents in which the D. C. components have been re-inserted, one set being derived from the other and delayed by an amount corresponding to the width of a black line. The two currents added together produce an effect as shown in Figure 5 of the drawings in which the D. C. component has been added and in which the full line a represents the wave form of the undelayed current and the dotted line b represents the wave form of the delayed current. It will be seen that in the wave form produced by adding the two currents, the valleys corresponding to black in the original current are largely filled up, though as the component currents do not entirely match, the valleys are by no means eliminated.

A more nearly complete-elimination of the valleys can be obtained by using the method illustrated in Figures 6a to 6e of the accompanying drawings. Figure 6a shows a typical wave form for a picture line without any interruptions due to interspersed black signals. Figure 6b shows the wave form of a current of a quarter the amplitude of that shown in Figure 6a with interruptions due to interspersed black signals produced from black lines of the same width as the spaces between them. Figure 6c shows the wave form of a current similar to that of Figure 6b but half the amplitude of the desired wave shown in Figure 6a and delayed by an amount equivalent to the width of a black signal, and Figure 6d shows the wave form of a current similar to that of Figure 6b except that it has been delayed by an amount equivalent to the width of two black signals. If the currents with the wave forms of Figure 1.

shown in Figures 6b, 6c,and 6d are added together, a current with a wave form as shown in Figure 6e will be obtained. The wave form in Figure 66 will be seen to correspond very closely with that of Figure 6a., and will thus give a picture from which the effect of the black bars has been practically completely eliminated. The delaying and combining of the impulses may be effected after the fashion and the method disclosed in Patent No. 2,132,655 which issued to J. P. Smith for System for producing electrical impulses on October 11, 1938, which was filed in February 28, 1935. Alternatively, an arrangement of attenuators and phase delay networks may be used, as described in the Wiener et al. Patent No. 2,024,900 entitled Electrical network system.. Such an alternative circuit is shown in Fig. 9 where the input is fed in parallel to a series of attenuators I5I, I53 and I55 for providing the proper amplitudes of the currents to be combined, while the phase delay network I57 provides a delay equal to the width of a black signal and the phase delay network I55 provides a delay equal to two black signals.

Whilst picture signals in the specific example described are interspersed with black signals through the interposition of a grid between the light source and the lens system, other means, may, however, be employed for the same purpose, as for example, by using a transparent photo cathode with opaque lines provided on its non-sensitive side through which the image is projected.

By way of illustration, a modified form of picture scanning apparatus according to the invention is illustrated in Figure 7. In this arrangement the tube 4' is arranged with the mosaic screen Ib'perpendicular to the axis of the neck in which the cathode ray gun I is housed, the image projecting optical system 8 being arranged on this axis but on the opposite side of the screen I6 from the gun I.

It will be seen that in the arrangement of Figure 7, the grid Iii in front of the lens system 8 is omitted and a screen I6 shown in perspective is provided, to one side of which a grid having lines 20 is applied. In this arrangement the screen I6 is transparent and formed, for example, of a sheet of mica on one side of which is provided with a multiplicity of mutually insulated photo-sensitive elements constituting the mosaic 5 whilst the other side, facing the lens system 8, the sheet is provided with the lines 20 which correspond in function to the grid I0 These lines may be formed by depositing a suitable metal such as silver on the surface of the mica, as for example, by evaporating silver thereon through a suitably shaped grid. The grid is removed after deposition of the silver. Alternatively, in some cases, the required number of lines may be preformed as a grid and secured in any suitable manner to the mica sheet. In either case the lines will be formed of conducting material and so arranged as to constitute the necessary conductive signal plate from which the picture signals are obtained on scanning the screen. The image is projected on to the surface of the screen I6 carrying the lines 20 and the screen scanned in the usual way without the necessity for any keystone correction arising, the provision of the lines 20 causing the photo-sensitive elements in front of the lines to be maintained at an equilibrium potential so that on scanning the screen, those elements 0pposite the. lines cause excursions in the black direction in the required manner.

The arrangement according to the invention also enables an accurate representation of average brightness to be obtained, as the direct current component is reinserted. If (as for example, clue to the use of a large number of lines in the grid It) the scanning device cannot resolve the lines efficiently the excursions towards black may be of too small an amplitude, so that on reinserticn of the low frequencies, these are reinserted at a false and too low amplitude, then a correction may be made after reinsertion, whereby the low frequencies are relatively boosted to their correct relative amplitude.

Referring again to Figure 4 of the drawings it will be seen that the wave form of the picture signals generated on scanning the screen of the transmitting tube after frequencies of the order of the tilt component have been removed, is composed of picture signals interspersed with black signals recurrent at a frequency depending on the number of strips into which the image is divided and the frequency of the scanning beam, the amplitude of the black signals extending above and below the zero axis owing to the fact that certain low frequencieshave been removed. Now this wave form may be employed in a sense as a carrier frequency for the transmission of the picture signals through the transmitting apparatus, the frequency of the carrier wave form being the frequency of the black intervals. In some cases it may be desirable to divide the image into a number of strips the width of each strip being equal to the spaces between the strips to produce a wave form as shown for example by curve a. of Figure 5. With such an arrangement the use of the black intervals as a carrier is particularly advantageous. The carrier wave in this case is of substantially rectangular form, the peaks of the Wave form being in eifect modulated by the true picture signals. The generated signals may be amplified by an arrangement such as that shown in Figure 8 of the drawings comprising low-pass or bandpass couple'd amplifying stages including valves 31 and 32, shown as being of the screened-grid pent-ode type, the mid-frequency of the bands being centred at the carrier frequency. The

amplified signals are then rectified, the rectifier signals so obtained containing the required picture frequencies together with their D. C. component but being substantially free from frequencies of the order of the tilt component. For the rectification of the signals it is preferred to employ a full wave rectifier such as shown at 33 in Figure 8, whereby both positive and negative portions of the carrier are rectified so that there appears in the output of the rectifier no signals of the frequency generated by'scanning the black lines at the transmitting tube. If the black lines in the wave form generated are represented by excursions in the negative direction, those negative excursions are converted into positive signals by the full wave rectifier. They therefore appear of the same brightness as the white signals immediately before them, and hence will be less noticeable in a picture reproduced from signals transmitted in accordance with the invention.

The arrangements of the circuits of valves 3| and 32 and of the rectifier 33 of Fig. 8 form no part of the present invention and may be of any suitable form known to those skilled in the art. The interrupted or chopped picture signals produced according to the invention may be applied to the input terminals shown at 34 and the output or the arrangement taken from output terminals such as 35. Valves 3| and 32 in the arrangement shown are arranged in a low pass filter amplifier, having shunt input capacities constituted by the capacity between the control grid and cathode of valve 3|, the anode circuit of valve 3| including an inductance 36 and resistance 31 connected to a source of potential indicated by the sign to the right hand side of the drawing and thence to earth. Inductance 35 is tapped down and connected by coupling condenser 38- to the control grid of valve 32, the anode of valve 32 being connected to the said source of positive potential through the primary of an output transformer 39, which is shunted by resistance 43. The grid leak resistances M and 42 are provided for valves 3! and 32 respectively, together with grid biasslng resistances 43 and 44" arranged in the respective cathode leads and shunted by the appropriate by-pass condenser 45 or 46.

Each end of the secondary of transformer 39' is connected to an anode of rectifier 33 and a midpoint connection is provided from the secondary of the transformer to the output of the rectifier which is taken from across the combination including resistance 48 and condenser 49 arranged in parallel. The positive potential for'the. anodes of rectifier 33 may be applied through a connection to the upper of the output terminal 35. The cathodes of all the valves 3|, 32, and 33 are connected to a common earth lead 50 as shown.

It will of course be understood that the rectifier output will be fed after further amplification, if desired, to a modulating stage for modulating a high frequency carrier prior to transmission.

Although the invention in the specific description has been applied towards a reduction of the tilt component arising in a cathode ray transmitter tube, it will be understood that the invention is not limited thereto, since it can be empioyed to eliminate or reduce other undesired signals of relatively low frequency arising in. any manner.

We claim:

1. The method of removing spurious signals of relatively low frequency from desired signals generated simultaneously with said spurious signals extending over a relatively wide range including said spurious low frequency signals, which comprises the step of combining said spurious and desired signals with signals having at least one predetermined datum value and recurrent at a frequency higher than the fundamental frequency of the spurious signals, attenuating frequencies below a predetermined frequency so as to remove said spurious signals from the combined signals, and subsequently reinserting suppressed low frequency components of the desired signals by reference to the recurrent signals of predetermined datum value.

2. In a picture transmission system in which an image of the object of which a picture is to be transmitted is resolved into picture signals by projecting the image onto a mosaic having elements which acquire an electrostatic charge according to the intensity of elemental image areas, which mosaic is periodically scanned to discharge said elements to generate picture signals, the method of removing spurious signals having low frequency components, which comprises the steps of combining the picture signals generated on scanning the mosaic with signals of a predetermined minimum value and of. a. frequency higher than the fundamental frequency of the spurious components, attenuating all frequencies below a predetermined value, said predetermined value of frequency being higher than the frequency of the spurious signals to be removed, and subsequently reinserting the attenuated low frequency components of the picture signals in accordance with the value of the recurrent signals of predetermined minimum value.

3. In a picture transmission system in which an image of the object of which a picture is to be transmitted is resolved into picture signals by projecting the image onto a mosaic having elements which acquire an electrostatic charge according to the intensity of elemental image areas, which mosaic is periodically scanned to discharge said elements to generate picture signals, the method of removing spurious signals having low frequency components which comprises the steps of combining the picture signals generated on scanning the mosaic with signals of a predetermined minimum value and of a frequency higher than the fundamental frequency of the spurious components, feeding the combined signals to a transmitter, attenuating at the transmitter all signals below a predetermined value of frequency, said predetermined value of frequency being higher than the frequency of the spurious signals to be removed, and thereafter reinserting the attenuated low frequency components of the picture signals.

4. The method claimed in claim 2 in which the combined signals of predetermined datum level have a time duration substantially equal to the time duration of the picture signals between them and comprising the further step of rectifying the signals following attenuation to provide the reinserted low frequency component.

5. In a picture transmission system in which an image of the object of which a picture is to be transmitted is resolved into picture signals by projecting the image onto a mosaic having elements which acquire an electrostatic charge according to the intensity of elemental image areas, which mosaic is periodically scanned to discharge said elements to generate picture signals, the method of removing spurious signals having low frequency components, which comprises the steps of combining the picture signals generated on scanning the mosaic with signals of a predetermined minimum value and of a frequency higher than the fundamental frequency of the spurious components, the duration of said combined signals being substantially equal to the interval between successive signals, attenuating all frequenciesbelow a predetermined value, said predetermined value being greater than the frequency of the spurious signals to be removed, rectifying the at.- tenuated signals, and reinserting signal energy proportional to the rectified energy and the attenuated signals.

6. In a picture transmission system in which an image of the object of which a picture is to be transmitted is resolved into picture signals by projecting the image onto a mosaic having elements which acquire an electrostatic charge according to the intensity of elemental image areas, which mosaic is periodically scanned to discharge said elements to generate picture signals, the method of removing spurious signals having low frequency components, which comprises the steps of combining the picture signals generated on scanning the mosaic with signals of a predetermined minimum value and of a frequency higher than the fundamental frequency of the spurious components, the duration of said combined signals'being substantially equal to the interval between successive signals, attenuating all frequencies below a predetermined value, said pre- :determined value being greater than the frequency of the spurious signals to be removed, full transmitted is resolved into picture signals by projecting the image onto a mosaic having ele- .-ments which acquire an electrostatic charge according to the intensity of elemental image areas, which mosaic is periodically scanned to discharge said elements to generate picture signals, the method of removing spurious signals having low frequency components, which comprises the steps of combining the picture signals generated on scanning the mosaic with signals of a predetermined minimum value and of a frequency higher than the fundamental frequency of the spurious components, the duration of said combined signals being substantially equal to the interval between successive signals, feeding the interspersed signals to a transmitter, attenuating the combined signals at the transmitter selectively below a predetermined frequency, said predetermined frequency being higher than the frequency of the spurious signals to be removed, rectifying the attenuated combined signals, and modulating the transmitter with both the attenuated combined signals and energy proportional to the rectified signals.

8. The method claimed in claim '7 and comprising in addition the steps of delaying a series of picture signals by a predetermined amount, and mixing a set of substantially undelayed picture signals with the delayed picture signals.

9. The method claimed in claim 7, comprising in addition the step of mixing undelayed picture signals with signals of the same amplitude delayed by a predetermined amount, said predetermined amount of delay being substantially equal to the time duration of a single said combined signal.

10. The method claimed in claim 7 comprising in addition the step of mixing together undelayed picture signals having interspersed signals of a predetermined minimum value, picture signals of twice the amplitude of the undelayed signals and delayed by an amount equal to th time duration of a single interspersed signal of predetermined minimum value, and picture signals of the same amplitude as the undelayed signals, but delayed to an extent corresponding in time duration to twice the duration of a single interspersed signal of predetermined minimum value.

11. A picture transmission system comprising in combination a picture signal generating device including a mosaic having elements which acquire an electrostatic charge according to the intensity of elemental image areas, means for periodically scanning said mosaic to discharge said elements to generate picture signals, means for projecting an image to be transmitted onto the mosaic, a grid in association with said image projecting means, said grid being adapted to cast a shadow on said screen in such manner that when said screen is scanned interspersed signals of a predetermined level and frequency are produced with said picture signals. means for removing signals below a predetermined frequency lower than the frequency of said inter-- spersed signals, and means for subsequently rcinserting suppressed low frequency components of the picture signals by reference to the recurrent signals of datum value.

12. A television or picture: transmission system according to claim 11 wherein said grid is adapted to be moved reciprocally during the scanning of the mosaic whereby an image of said grid will be less apparent in the received picture.

13. A television or picture; transmission system according to claim 11 in which said mosaic screen is transparent the optical image v0i the object 135 of which a picture is to be transmitted projected onto the opposite side of the screen from :that on which the mosaic is formed, said grid lacing applied to said opposite side and being formed of opaque electrically conducting material which forms the conductive signal plate of the scanner.

14. A television or picture transmission system according to claim 11 in which said mosaic screen is translucent the optical image of the obj of which a picture is to be transmitted being projected onto the opposite side of the screen from thatch which the mosaic is formed, said grid being applied to said opposite side and being formed of opaque electrically conducting material which forms the conductive signal plate of the scanner.

ALAN DOWER BLUMLEIN. CECIL OSWALD BROWNE.