GB867087A - Improvements relating to wired relay systems for colour television - Google Patents

Abstract

867,087. Distributing music &c. ELECTRIC & MUSICAL INDUSTRIES Ltd. Aug. 14, 1957 [Aug. 21, 1956], No. 25458/56. Class 40 (4). In a colour television distribution system it is arranged to transmit colour television signals including brightness information, a colour carrier wave modulated with colour information and a further carrier wave modulated with scanning information, the frequencies of the further and the colour carrier waves being simply related, e.g. in the ratio of two integers both less than ten, and means are provided at each receiver for synchronously detecting the colour information with reference to the further carrier wave. As described, the invention is applied to the distribution of signals received as broadcast television transmissions in the N.T.S.C. colour television system in which colour information is transmitted as modulation of a subsidiary carrier wave which is in turn transmitted as modulation of a carrier wave also modulated with brightness information. Transmitter.-The radio-frequency and demodulator stage of a colour television receiver is indicated at 1 (Fig. 1), one output from which is applied to high-pass filter 2 to separate out the colour information as modulation of the sub-carrier wave, and a second output of which is applied to low-pass filter 3 to separate out the brightness information as a video signal, which is then applied to modulator 4 to modulate, as a vestigial side-band, a carrier wave derived from a local oscillator 5. A second output from the low-pass filter 3 is applied to a synchronization pulse separator 6 which controls a scanning waveform generator 7 to produce a combined line and frame scanning waveform output as described in " Wireless World," February, 1955, at pages 83 to 87. The output from the high-pass filter 2 is also applied to block 9 which gates the phase reference signals, or so-called " colour bursts," under the control of the output of the synchronization pulse separator 6, so as to drive a locked oscillator the output of which is applied to a frequency dividing circuit 10 arranged to produce an output the frequency of which bears a simple relation to (e.g. is one half of) that of the colour carrier wave. The scanning waveform output from 7 modulates the output of divider 10 in the modulator 8. The outputs from the high-pass filter 2, modulator 4, and modulator 8 are applied to a combining network 11 and thence to the relay network. Receiver.-Filters 12 (Fig. 2), separate the three carrier waves. The brightness carrier wave is fed to a detector 13, the output of which is applied to the control electrode of a three-gun colour television cathode-ray tube 14. The scanning information carrier wave is detected by 15, the scanning information being amplified at 16 and applied to the deflection coils, and E.H.T. being derived as described in the above-mentioned article. The scanning information carrier wave is also applied to an amplitude limiter 18 which removes the scanning information, the resultant oscillation being applied to a multiplying circuit to derive the desired homodying frequency which is applied in the appropriate phase relationship to the two synchronous detectors 19 and 20 which are fed with the colour carrier wave from filters 12. The I and Q colour components are thus detected and are fed to a matrix 21 to produce the colour signals R-Y, G-Y and B-Y which are then applied to the three cathodes of the tube 14 in normal manner. In a modification the chrominance information output from the high-pass filter 2 of Fig. 1 is detected and modulated on to a carrier wave of different frequency, and the scanning information modulates a carrier wave of a frequency equal to the difference between those of the carrier waves on which luminance and chrominance information are modulated so that the limited scanning waveform signal output from 18 of Fig. 2 is of the desired frequency for homodyning with the chrominance signal after detection. Alternatively, the luminance information may be modulated on a first carrier wave and the chrominance information on a second subsidiary carrier wave which is modulated on to the first carrier wave.