US2222934A - Television transmitting and receiving system - Google Patents

Description

Nov. 26, 1940. A, M N Q 2,222,934

TELEVISION TRANSMITTING AND RECEIVING SYSTEM Filed Oct. 12, 19-38 I I II I I mAA/sM/rrER, f 4 555% 6 26 v 27 6; i osmmron I8 20 /ZZ I HORIZONTAL VERTICAL AUXILIARY DEFLELT/ON DEFLECT/ON DEFLECT/ON MoouzAro/z as/vsmro/z GENERATOR ssumam VER 77514 L DEFZ E677 GEN AUXILIARY Patented Nov. 26, 1940 UNITED STATES TELEVISIONTRANSIVHTTING AND RECEIVING SYSTEM Alan Dower'Blumlein, Ealing; London, :England, -assignor to Electric & Musical,-Industries Limited,- Hayes, Middlesex, England, a company 0 Great Britain ApplicationiOctOber 12, 1938, Serial N0'.234;560

In Great Britain October 14; 1937 1 Claim.

'This invention relates to television transmitting and receiving systems and has for its chief object to provide an improved transmitting and receiving system for use therewith in which the picture detail of a transmitted image is increased thereby permitting the detail in the reproduced image at the receiverto be correspondingly increased.

The present transmitting system employs a cathode ray transmitting tube in which an. object of an image is projected onto a 1mosaic screen composed of a multiplicity of photo-electric mutually insulated elements which'acquire an electrostatic charge according to the intensity of elementary areas of the incident image and a cathode ray is caused to scan the mosaic screen, scanning restoring the elements to a datum value and generating in an associated signal plate picture signals for transmission. It will be understood that with a system of this "kind. the detail of the picture transmitted depends upon the number of lines into which the picture is resolved and the consequent diameter of the scanning spot of cathode rays. *It is clear that detail can be improved providing that the-number of lines is increased" and the scanning spot size reduced accordingly, 'but this introduces diificulties not only on account of the fact that the frequency of the transmitted signals is thereby considerably increased but also on account of the fact that many receiving systems might be in operation'which would necessitate reconstruction for the reception of signals from a transmitter in which the frequency of the-scanning lines is increased as'sug-gested above.

"For these reasons therefore it=-is desirable to provide an improved transmitting system in which detail of'the transmitted picture can be increased withoutincreasing the line scanning frequency thereby enabling existing receiving systems to receive the improved transmission but without the increased detail whilst permitting receivers specially designed for the improved transmission to reproduce the received pictures with greater detail than receivers as constructed heretofore.

According toone feature of the present invention, there is provided a television transmitting system in which an image for transmission is resolved into picture signals by scanning in a plurality of lines by a suitable scanning spot, and wherein in addition to normal movements executed by the scanning spot a further component of motion is imparted thereto so that in scanning a line of the image the spot is caused to oscillate about the axis of theline in such a manner that the picture detail'of the transmitted transmitter-to oscillateabout the axis of a line.

With the existing transmitting system it will *be vappreciated that the'diameter of the spot determines the width of the line scanned and consequently if this line embraces a horizontal black portion and a horizontalwhite portion the resultant picture signals generated on scanning the line-instead of-being resolved into black and white signals-willbe resolved into a composite grey signal.

'With a system according to the invention, however, the diameter of the'spot may be made to correspond to one-half the difference between the axes of adjacent lines and since the scanning spot oscillatedabout the'axis of the line, it will be appreciated that with a line containing horizontal black and white edges the scanning spot whilst resolving the picture into the same number of lines-as an existing system will, nevertheless, resolve individually the black and white horizontal edges so that the picture signals generated will'be truly representative of the image so increasing the detail in the transmitted signals. The oscillating motion imparted to the scanning spot is preferably of a sinusoidal form, although other forms of oscillation motion may be imparted'thereto; such as motions of a sawtooth or similar form. r

The invention 'may be applied to interlaced transmitting and receiving systems. It will, of course, be necessary in the transmitted signals to transmit suitable synchronising signals or other signals of suit-able wave formin addition to those normally transmitted for the purpose of synchronising the oscillating motion imparted to the reproducing spot at the receiver with that at the transmitter.

This invention is applicable to television transmitting and receiving systems which employ a Figure 1 is a diagram which is used for explanation purposes,

Figure 2 shows a portion of a'raster out by the scanning or reproducing spot,

Figure 3 shows schematicall'y a television transmitter embodying the present invention, and

Figure 4 shows schematically a television receiver for use therewith.

The invention will now be described with, reference to a transmitting system in which apicture is resolved into picture signals by scan,- ning with 400 lines non-interlaced and employing a cathode ray transmitting tube l2 having a mosaic screen M of 'the'kind'referred-to above. With such a system employing 25- frames per second, the line frequency is 10,000-cycles per second and the so-called dot frequency is two megacycles per second. The cathode ray beam I6 is caused to scan the mosaic screen l4 by applying to the beam the usual line I 8 and frame 20 frequency oscillations. In accordance with the invention, the diameter of the scanning spot is made smaller than that normally employed as, for example, bymaking the-spot diameter equal to half the distance between the axes of the lines normally scanned, and instead of the scanning spot scanning a line substantially rectilinearly as is usual, the scanning spot is caused to oscillate about the axis of the line during its progression from one edge ofthe mosaic screen to the other. 'I'he'frequency of the additional oscillation motion imparted to the scanning spot by the auxiliary deflection generator 22 may be, about three to four megacycles per second, and

may be of sinusoidal form, the amplitude of oscillation being approximately one-half the distance between the axes of the lines normally scanned.

Figure 1 of the accompanying drawing diagrammatically shows a portion of the raster traced out by the scanning or reproducing spot. I, 2 and 3 represent portions of consecutive lines of the picture when scanned ina normal fashion, the axes of these lines being reproduced by the lines 4, 5 and 6 respectively. When the diameter of the spot is reduced to'a diameter equal to half the distance between the axes of the lines normally scanned and an additional sinusoidal oscillation imparted to the spot of an amplitude of one half the distance between themes of the line normally scanned, the spot'then traces out a raster, a portion of 8 and 9.

Since, however, with the additional sinusoidal oscillation having a frequency to afford'the configuration shown in Figure 1, comparatively large portionsof the picture are leftunscanned, it is desirable to choose such a frequency for the additional oscillation that the spot covers practically the whole o-fgthe area of the line. If the frequency of the additional sinusoidal oscillation. shown in Figure 1 is increased three times the resulting area covered by the spot will be as shown shaded in Figure 2. i

Although the diameter of the spot is only one half of that normally used, since additional oscillation motionis imparted thereto, iteffectively which is represented by 1,

scans substantially the whole of the area scanned by a spot with a diameter of the normal size and consequently it is capable of resolving a line of the image with greater detail than that of a normal system. If desired the scanning spot may be madesmaller than half the line spacing thus tending to enhance the picture detail. In television systems which employ cathode ray scanning means at the transmitter and receiver, the scanning spot has no definite boundaries but consists of a distribution having a centre portion of relatively high intensity, the intensity falling away rapidly to a comparatively low value outside the centre portion. It is the usual practice to make the width of the major portion of the spot smaller than line spacing, or for the purposes of this invention less than half the line spac ing. With mosaic screens as at present in use it is not practicable to reduce the size of the scan.- ning spot to a very small value owing to the resultant increase in the line scanning frequency although it will be appreciated that with the present invention the smaller the diameter of the spot, together with the use of an oscillation motion of suitable frequency, so the detail in the maintaining a practical number of lines for each frame scanned. The picture signals generated in the transmitting tube I2 are transmitted in the normal manner by transmitter 24 although, ini view of the increased definition, it is necessary for the band width of the transmitted signals to be wider than that corresponding to a normal 400 line scanning system, a suitable band width being five or more megacycles of modulation frequency.

The oscillating motion of the scanning beam at the transmitter may be effected by superimposing, for example, by coupling means 26,

upon the frame frequency oscillations sinusoidal oscillations of the frequency of the order above mentioned. The voltage variations from the auxiliary deflection generator are also supplied to a modulator 28 where a sub-carrier frequency from the oscillator 2'! is modulated, the output from the.

chronism thereto so that the movement of the cathode ray beam at the receiver is substantially the same as that of the scanning beam at the transmitter. The transmitted signals are caused to modulate the cathode ray beam at the receiver through the use of a grid 34, the receiver being designed to handle a wider frequency range than heretofore in view of the increased band width of the picture signals. Providing synchronism is preserved between the oscillation motion ofthe cathode ray beam 32 at the receiver with that of the scanning beam I6 at the transmitter, the reproduced picture will be integrated with the increased detail resulting from the improved scanning at the transmitter.

As stated above, with a normal system, assuming that a line scanned embraces a horizontal white edge and a horizontal black edge, the scanning spot instead of resolving such a line into its component shades will resolve the line into a shade, corresponding to a combination, of the picture transmitted can be improved, whilst still shades of the image which in the present example,

will be grey.

With the improved transmission system since the scanning spot oscillates about the axis of the .5 line, upward and downward excursions of the scanning spot will resolve the black and white edges individually and these black and white edges will consequently be reproduced in a receiving tube with the component shades instead of, in the normal case, into a single shade of grey.

For the purpose of ensuring synchronism between the scanning spot at the receiver with that of the scanning spot at the transmitter, it is necessary to transmit suitable synchronising or other signals from the transmitter. This may be effected in a variety of manners. For example, a separate sub-carrier frequency 21 on the main transmitter carrier frequency may be used or, alternatively, the synchronising signals or signals of the required wave-form may be radiated directly as a medium wave signal. The preferred method, however, is to transmit the necessary oscillation frequency which imparts the desired motion to the beam as a phase or frequency modulation of the transmitted vision signals. Such phase modulation may be effected in known manner as, for example, by deriving an amplitude modulated signal and injecting suflicient carrier frequency oscillations of suitable phase in order to turn the effective carrier oscillations through 90 degs. so that the resultant signals are phase-modulated. For example, in a transmitting system employing a final push-pull modulated amplifier the vision signals for modulating the carrier are applied as a bias to the control grids of the valves in the pushpull amplifier. With the present invention the oscillations which are to be modulated with the picture signals are 40 previously phase-modulated with the additional frequency which is effected in earlier stages of the amplifier in known manner. In a system in which the line and frame synchronising signals are represented by zero carrier frequency, i. e., 45 the synchronising signals are in the blacker-thanblack sense the modulation frequencies which are applied to the last stage of the amplifier will suppress the carrier wave, and likewise the phasemodulation, entirely during the synchronising 50 signals. With such a system the picture signals have a minimum value higher than zero carrier frequency and consequently the picture signals will have the necessary phase-modulation from which the signals for oscillating the scanning 55 beam at the receiver can be derived. The necessary phase-modulation may amount to some At a receiver for receiving the improved trans mission with signals transmitted as above described the transmitted signals are arranged to 50 be received with both side bands and are demodulated in the normal manner for deriving signals for modulating the cathode ray beam. A portion of the unmodulated signals may be passed through a suitable circuit employing an amplitude limiter 65 for aifording a constant amplitude output at a level slightly lower than that corresponding to black in the transmitted signals. The signals so obtained may be passed through a sharply tuned resonant circuit tuned off the carrier frequency in such a manner that the phase-modulation is partially converted into a single side band amplitude modulated oscillation. The signals so obtained are then demodulated and added to the frame scanning oscillations so that the required additional oscillations are applied to the scanning spot. In order that the phase of the additional oscillations at the receiver may be made the same as the additional oscillations at the transmitter a suitable phase-changing device may be provided at the receiver.

The invention may also be applied to an interlaced scanning system the additional oscillations of the cathode ray beam at the transmitter being applied as above described. With such an arrangement the effective lines in a single frame, i. e. 800 lines, are scanned in pairs and consequently if the reproduced picture is observed closely, flicker maybe observed. To obviate this the amplitude of the additional oscillations at the transmitter and at the receiver may be increased to that corresponding to between two thirds and one-and-a-half times the distance between the axes of the lines in a picture normally scanned in an interlaced system. With such an arrangement the scanning produced in one line will overlap the scanning produced by the next line so that flicker will not be produced and without loss of detail in the reproduced picture providing that the beam at the receiver traverses the fluorescent screen with a similar amplitude of movement.

With receivers designed for the reception of normal transmission, such receivers may be operated with the improved transmission without structural alteration since the line and frame scanning frequencies are the same and since the transmitted signals representing the additional oscillation of the scanning beam and the modulation frequencies produced as a result of this additional oscillation will be outside the band width of the existing receivers.

I claim: I

A television transmitting and receiving system in which an optical image is resolved into aseries of picture signals comprising a transmitting tube having a mosaic electrode, means for generating a cathode ray beam in the transmitting tube, means for deflecting the cathode ray beam in a vertical direction across the mosaic at a relatively slow rate, means for deflecting the cathode ray beam in a horizontal direction across the mosaic at a more rapid rate whereby the mosaic may be scanned, means to very rapidly deflect the cathode ray beam in a verticaldirection, the degree of the last named deflection being approximately,

the distance between successive horizontal traversals of the beam and the cross-sectional dimension of the cathode ray beam at the mosaic electrode being materially less than the distance be-

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