Patent GB-603080-A (GB)
Title: Improvements relating to television apparatus
IPC: H04N11/00
Filed: 1944-06-10
Abstract: 603,080. Television. COSSOR, Ltd., A. C., and STEVENS, W. H. June 10, 1944, No. 11146. [Class 40 (iii)] [Also in Group XL (a)] A colour television system employs at the transmitter and/or receiver a coloured filter grating having narrow coloured strips and scanned in a direction substantially at right angles to the length of the strips forming the grating. The coloured strips may be of unequal width to compensate for differences between the spectral sensitivity of the pick-up camera .and the human eye. The varying intensities of the colour components, e.g. green, red and blue may be transmitted as amplitude variations lying within specified amplitude ranges of a carrier wave-blue, for example, occupying 10 per cent to 40 per cent of the full width of carrier modulation. The synchronizing impulses are allocated to the range 0 to 10 per cent of the amplitude width. The amplitude width may be obtained optically by using a grating which is printed on to a glass plate lightly etched on the red lines and more heavily etched on the green lines. Lamps are arranged to impose an even illumination on that arising from the image so that the light transmitted through any part of the grating will depend on the depth of the etching at that part. The sharp changes in amplitude indicated in Fig. 2 may be utilized to control a saw-tooth oscillator at the picture-point frequency and the sawtooth oscillations are mixed with the demodulated received signals to produce variations corresponding to the colour components as indicated in Fig. 4. Correct colour registra- . tion between the scanning spot and the colour filter is obtained by the use of a colour monitor comprising three small photo-electric cathodes and an anode, the cathodes having filters which pass light of wavelengths except those of green-red and blue respectively. The cathodes are swept by a beam of electrons deflected in synchronism with the saw-tooth oscillations of Fig. 3 and the colour monitor is illuminated at every instant with the colour the screen is emitting. If, for example, it is emitting green when it should be red, the " minus red " cathode will be energized and the resulting impulse is utilized to adjust the scanning control. In an alternative arrangement the coloured grating has irregularly arranged black strips, and the grating at the receiver has correspondingly arranged strips of an additional colour, e.g. yellow, associated with a yellow filter and a photo-electric cell. When the system is operating in colour synchronism no signal is received when the beam is at the yellow strip, since the transmitter scanning spot is on a black strip, but if the system is out of synchronism the yellow strip will be illuminated and the output of the cell is utilized to control the time-base circuits to restore synchronism. The colour grating for the receiver tube may be formed by printing a glass plate with heat resisting stains and sealing the plate to form the screen end of the tube. According to the Provisional Specification, a modified monitor comprises three cathodes and two separate and screened anodes-the cathode co-operating with the red components having blue and green filters separately associated with the anodes which are connected in circuits to accelerate or retard the scanning spot. The rate of scanning may be varied so that the spot travels relatively slowly over wanted colour strips and at the receiver the colour part of the signal is separated by gating circuits and the variable width strips are applied to the scanning control and colour monitor to provide the velocity modulated component. The electron gun assembly may be modified to provide a focussed short line in lieu of the normal circular spot.