Patent GB-555565-A (GB)
Title: Improvements in and relating to colour television
IPC: H04N9/07
Filed: 1942-09-11
Abstract: 555,565. Colour television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 11, 1942, No. 12859. Convention date, Jan. 31, 1941. [Classes 39 (i), 40 (iii) and 40 (v)] Relates to colour-television systems in which images are projected on a charge storage electrode of a cathode-ray tube in several primary colours in succession, and consists mainly in discharging the residual charge after each scan before the next illumination, for example by the retrace of the scanning ray, which for this purpose is made stronger during the retrace period. The cathode-ray tube may be of the high-velocity-beam or low-velocity-beam type. In the form shown in Fig. 1, the cathode-ray tube 1 is a low-velocity-beam one having an electron gun 2, line deflection plates 3, a focus ing coil 5, a frame-deflec tion coil 6, and a photo sensitive mosaic electrode 4 An image of an object 15 is projected on the mosaic electrode 4 by means of a lens 16 and a mirror 17. The object is illuminated with white light from a source 13 ; and a rotating light filter 18 is provided to cause the successive images on the mosaic electrode to be in a series of primary colours. The light filter may be a drum, Figs. 1 and 2, or a disc, Fig. 3, comprising one or more sets of primary colour sections according to the speed at which it is desired to rotate the filter. This rotation is performed by a synchronized motor. As shown in Fig. 1, the filter sections pass through the light beam close to the mosaic electrode 4. When there is a considerable distance between the mosaic electrode and the envelope of the cathode-ray tube, the filter is preferably located at another image plane in the optical system. Each colour section of the filter is separated from the next by an opaque portion may be constituted by an overlap of the adjacent colour sections. Each opaque portion produces an unilluminated strip 30, Fig. 5, on the mosaic electrode 4, which strip gradually moves up during the scanning of a frame. Each line 50 of the electrode is scanned while it is in this dark strip. During the line-retrace period the ray is intensified and blurred and deflected backwards to travel along a line 51 to wipe out any residual charge before the next illumination in a different primary colour. The successive frames are scanned in interlaced fashion. Means are provided for compensating for differences of colour response and fluctuations of intensity of light sources. A zero datum level is provided by arranging that a vertical strip 36, Fig. 5, of the mosaic electrode remains unilluminated. for example by means of a masking strip 35, Fig. 1, against the wall of the cathode-ray tube. The response of the apparatus to the different primary colours is measured by a source of white light 40 arranged to illuminate a vertical strip 45 of the mosaic electrode through the rotating colour filter, this strip being screened from the light coming from the source 33. The strip 45 may be adjacent the strip 36, as shown; or these strips may be on opposite sides of the mosaic electrode. The source 40 of white light may contain three sources 41, 42, 43 of different primary colours, the intensities of which may be separately adjustable. For measuring fluctuations of intensity of the light source 13, part of the light therefrom may be deflected to illuminate the strip 45 in place of the light source 40. The signals produced by the arrangement shown in Fig. 1 across the output impedance 55 of the cathode-ray tube is indicated by the waves denoted I in Fig. 6. The part R of these waves represents a portion of the signals produced while lines are being scanned in red. The portion a is the zero datum produced by the dark strip 36, Fig. 5. on the mosaic electrode 4. The portion b is the measure of the response of the apparatus to red light, as produced by the strip 45 of the mosaic electrode. the portion c is the red video signal proper, and the portion d, the signal produced during the line retrace period by the wiping out of the residual charge of the mosaic electrode by the intensified ray. The similar parts B and G of the waves are for the blue and green images respectively. Portions of the vertical return period are shown between the parts B and G. The signals represented by these waves are fed by a condenser 57, Fig. 1, to an amplifier 56 ; and the output of this amplifier is passed by another condenser to the control grid 62 of a valve 59. This valve is triggered to pass only the pulses b, b<SP>1</SP>, b<SP>11</SP> representing the response of the apparatus to the several primary colours. The triggering is controlled by a grid 63 receiving pulses from the synchronizing circuits. The pulses passed by the valve 59 are rectified by a diode 66 and integrated by a circuit comprising a condenser 67, a resistance 69, and a valve 68 triggered by pulses from the synchronizing circuits. The integrated voltage of these pulses is combined with an adjustable voltage from a battery 80 and is applied as a gain control bias to the amplifier 56 to equalize the output response of this amplifier for the several colours. This output is fed to a suitable network or transmitting amplifier 58, the unwanted parts of the waves shown at I, Fig. 6, being blanked out, and synchronizing pulses being inserted as required. Reference has been directed by the Comptroller to Specification 627,592, [Group XL (b)].