GB1158918A - Colour Television System - Google Patents

Abstract

1,158,918. Colour television; wired distribution systems. COMMUNICATIONS PATENTS Ltd. 3 Oct., 1967 [7 Oct., 1966], No. 44826/66. Headings H4F and H4R. A colour television system of the type described in Specification 1,086,352, in which transmission of luminance information takes place as an amplitude modulation of a carrier wave and the transmission of colour information takes place as an amplitude (or frequency) modulation of a suppressed carrier wave, the carrier waves having the same frequency but a phase quadrature relationship, is characterized in that the transmission of the accompanying sound information takes place as a frequency modulation of a third carrier wave having a frequency which differs from the frequency of the luminance and colour information carriers (vision carriers) by a predetermined amount which is related to the absolute frequency of the vision carriers so that at a receiver it may be used for the generation of a re-insertion carrier wave for effecting demodulation of the colour suppressed carrier wave. At the transmitter sound signals are frequency modulated on a carrier having a frequency which corresponds to the desired frequency spacing between the vision carrier waves and the sound carrier wave as present at the output terminals of the transmitting apparatus. The frequency of this carrier is maintained at the desired value by comparing it with a multiple of the frequency of the vision carrier frequency. The actual transmitted frequency for the sound signals is obtained by applying the latter frequency modulated carrier and the vision carrier wave to a mixer, the resulting sum frequency being the transmitted frequency. As described in Specification 1,086,352, the luminance signal is corrected before it amplitude modulates the carrier to minimize any visible effects of distortion introduced into the signal during transmission so that a correct luminance signal is displayed at a colour or monochrome receiver, the chrominance signal comprises two colour difference signals transmitted line sequentially or at a faster rate, the synchronizing signals are amplitude modulated on the same carrier as the luminance signal, and keying or identifying signals are transmitted for operating colour recognition circuits in the receivers. In a receiving apparatus, Fig. 3 (not shown), for wired broadcasting the amplified vision carriers are passed to an envelope detector to derive a composite signal which is utilized as the luminance signal, and to a synchronous detector from which the colour signals are derived by means of an oscillator (including a 90 degree phase shifter) the frequency of which is controlled by the received sound carrier wave frequency after it has been suitably divided. In a receiving apparatus, Fig. 4 (not shown) for radio broadcasting the received high frequency signals are applied to a mixer (95) together with a high frequency wave from an oscillator (96), and the resulting intermediate frequency signal is demodulated (99) after amplification (98). The demodulated output signals comprise the luminance signal, which after amplification (100) is applied to the matrix (105) of the display tube, and a frequency modulated carrier wave in respect of the sound signals which after demodulation (102) is applied to a loudspeaker. An additional signal is obtained from the intermediate frequency amplifier (98), having a bandwidth corresponding to the quadrature colour signal, which is passed to a synchronous demodulator (109), the re-insertion carrier input being derived from the sound signal carrier. To this end the sound intermediate frequency carrier is applied via a narrow band pass filter to a second mixer (110) which also receives oscillations from the oscillator (96). The mixer (110) produces an output wave having a frequency which corresponds with that of the transmitted sound signal and which is formed in a particular relationship to the carrier waves of the luminance and colour signals. The output wave of the second mixer (110) is divided (111) and multiplied (112) to provide an output wave having a frequency which corresponds to that of the carrier waves of the transmitted luminance and colour signals. This output wave is then passed to a third mixer (113) which also receives oscillations from the oscillator (96) and provides on its output line a wave having a frequency which corresponds to that of the intermediate frequency luminance and colour signals. This output, after phase shifting (114), is applied to the demodulator (109) as the re-insertion carrier wave.