GB793351A - Improvements in and relating to colour television receivers - Google Patents

Abstract

793,351. Colour television. SYLVANIATHORN COLOUR TELEVISION LABORATORIES, Ltd. Oct. 6, 1955 [Oct. 11, 1954], No. 29294/54. Class 40 (3). Relates to a colour television receiver for use in systems wherein there is transmitted a luminance signal together with a chrominance signal in the form of a carrier whose phase represents colour, and which utilizes a cathoderay tube with a screen formed of a repeating sequence of parallel red, green and blue stripes scanned with the line direction transverse the stripes. According to the invention, the luminance signal is applied to control the intensity of the cathode-ray tube beam whilst the chrominance signal is applied to control the velocity of line scan, the mean value of which is controlled by a colour-synchronizing signal derived from the scanning of stripes of a given colour. Thus, as shown in Fig. 1, cathode-ray tube 19 formed with a screen 20, as shown in Fig. 2, is scanned in the line direction 21 by time base 14 coupled to scanning coil 18 and receives a luminance signal at its intensity control electrode 23 from video amplifier 22 whilst a chrominance signal from channel 36 in the form of a 3.6 mc/s. phase and amplitude modulated sub-carrier is coupled to the scanning coil so as to vary the velocity of scan. The effect of varying the scan velocity is to cause the beam to " dwell " for a longer period on stripes of one colour, the colour depending on the phase of the chrominance signal and hence the colour represented by that signal. In order to increase the maximum saturation which may be produced it is necessary to increase the period during which the scan is at minimum velocity and hence the period during which the beam " dwells " on the selected colour. This may be achieved by passing the chrominance signal through an amplitude limiter 31 which clips the half-cycles corresponding to a decrease in velocity. In order that the now asymmetrical wave should not alter the mean velocity of the scan it is passed through a capacitor 33 to remove the D.C. component. A selective filter 34 may be arranged so that the high-frequency components of the luminance signal are also applied to vary the velocity of scan leaving only the low-frequency components to modulate the control electrode 23. In order to maintain the velocity of line scan at the proper mean value to ensure that the correct colour stripes are impinged in step with the colours represented by the phases of the chrominance signal, a photoelectric cell 24 is arranged to respond to the light from the stripes of one colour so as to develop a signal indicating the rate of scanning the stripes. This signal after amplification and limitation at 25 and 26 is then compared in phase discriminator 27 with a reference signal at sub-carrier frequency to develop a signal which is applied to control the time base. The reference signal is obtained from an oscillator 17 synchronized by bursts of signals at the subcarrier recurring at the beginning of each line and extracted from the received signal by a gate 16 controlled by blanking pulse generator 13. Conveniently photo-electric cell 24 may respond to light from the blue stripes. By utilizing the violet or ultra-violet component of the light from the stripes, a filter 35 non-transmission to such light may be included in front of the tube screen to prevent room lighting from reaching the cell. In an alternative arrangement, a signal is derived in response to secondary electron emission from the stripes, the stripes of one colour having different secondary electronemitting properties from the others. To ensure that a signal is obtained under all conditions of luminance signal, a feedback may be provided from amplifier 25 to a suitable point in the circuit feeding the intensity control electrode 23 so that brightness is increased if no output is obtained from photo-electric cell 24. Specification 793,353, [Group XL (a)], is referred to in connection with a method of manufacturing a cathode-ray tube with a striped fluorescent screen.