Patent GB-1005707-A (GB)
Title: Improvements relating to colour television systems and to synchronous demodulator arrangements
Inventor: JAMES IVANHOE JOHN PENFOUND
IPC: H04N5/60
Filed: 1960-12-08
Abstract: 1,005,707. Multiplex radio signalling. ELECTRIC & MUSICAL INDUSTRIES Ltd. Dec. 7, 1961 [Dec. 8, 1960; Dec. 20, 1960], Nos. 42269/60 and 43683/60. Heading H4L. In a television transmitting system wherein a colour signal is transmitted by modulating a colour oscillation during line forward stroke periods, at least one sound or other low-frequency signal is transmitted by modulating said oscillation during the line fly-back periods. In the N.T.S.C. waveform two quadrature components of a colour sub-carrier wave are modulated by colour difference signals R - Y and B - Y, respectively, and one component which is modulated during line forward strokes by, for example, the B - Y signal, is modulated during the fly-back time by the sound signal. At the receiver the sound signal may be demodulated by the B - Y chrominace demodulator by gating the output of the demodulator via a low-pass filter to the sound channel by means of line flyback pulses. To increase the band-width of the sound signal sampling may take place at twice the line frequency, the mid-line sample being delayed so that it occurs during the fly-back period, before or after the end-line sample, a complementary delay being provided at the receiver. Alternatively the mid-line sample may be delayed to coincide with the end-line sample, the two samples respectively modulating the phase-quadrature components of the sub-carrier. Stereophonic left and right or sum and difference signals may be transmitted in this way, or the sub-carrier may be phasemodulated by the left (or the difference) signal and amplitude modulated by the right (or sum) signal. In another arrangement the sub-carrier may be arranged to transmit a signal representing the position or direction of a sound signal during the line fly-back period, the sound signal being transmitted on a separate carrier. The monophonic signal may be received on a modified monochrome receiver. In one arrangement, during the line fly-back period a burst of reference phase unmodulated sub-carrier is followed by a burst of sub-carrier modulated by the sound. At the receiver a delay network brings the two bursts of sub-carrier into coincidence so that they may be utilized in a synchronous detector. Sound signals may be transmitted by modulating the colour subcarrier during line fly-back periods where R - Y and B - Y signals are transmitted either by amplitude or frequency modulation during alternate lines, or where R - Y and B - Y signals are transmitted respectively by simultaneous amplitude and frequency modulation of the sub-carrier. Sound may be transmitted by modulating the colour synchronizing burst in such manner that no phase variation is produced. Instead of sound signals, scrambling or like signals employed in subscription television may be transmitted by modulating the colour sub-carrier, or a slow-run television display employing a long-persistence display device may be provided. The extra channels may also be used to provide bi-lingual or trilingual transmission.