Patent GB-1179591-A (GB)
Title: Improvements in or relating to Systems for Distributing Television Programmes by Cable
Inventor: BURKE ALAN SAMUEL
IPC: H04N7/16
Filed: 1966-10-27
Abstract: 1,179,591. Wired television systems. BRITISH RELAY Ltd. 29 Jan., 1968 [27 Oct., 1966], No. 48273/66. Heading H4R. In a colour television wired distribution system a pilot signal is transmitted with the television programmes, the pilot signal frequency being a sub-multiple of the chrominance sub-carrier frequency and a sub-multiple of the vision carrier frequency. The pilot is used at the receivers for demodulating the vision and chrominance signals. The pilot signal phase may be reversed at line frequency to provide simple demodulation of a P.A.L. system transmission. The vision signal may be scrambled for transmission, and unscrambled at the receiver with the use of the pilot signal, under the control of a coin-operated mechanism or charge recorder. Fig. 1 shows a transmitter 10 and one receiver 12 connected by a distribution network 11. The transmitter feeds two programmes to the network, the two video signals being phase synchronized at line frequency by the delay line 40 and control network 41 as described in Specification 1,179,592. An oscillator 20 oscillates at the chrominance sub-carrier frequency, i.e. 4À43 MHz. and is phase locked to the chrominance bursts on the back porches of the incoming video 1 signal. The output from oscillator 20 is used as vision carrier for the video 1 signal in modulator 21 and, after multiplying by three in 22, as the vision carrier for the video 2 signal in modulator 23, this vision carrier being 13À29 MHz. The output of oscillator 20 is also fed to the pilot carrier generator 28 to produce the pilot carrier, e.g. at 2À215 MHz. For the P.A.L. system the pilot signal is reversed in phase on alternate lines by modulator 35 under the control of synch. separator 34. In addition the pilot signal is fed to the sound carrier generator 32 where it is mixed with an offset frequency from oscillator 33 to modulate the two audio signals, one with the pilot signal plus the offset frequency and the other with the pilot signal minus the offset frequency, in modulators 30 and 31. At the receiver 12 video 1 signals are fed direct from the amplifier 50 to the receiver 51 while the video 2 signals are modulated by an appropriate carrier, derived from the pilot signal, in frequency changer 52. In addition the pilot signal frequency, extracted by the selective amplifier, is used to provide a vision carrier frequency signal applied at 58 to the receiver to enhance the vision carrier before demodulation, in addition the line modulation of the pilot signal is detected in 56 to provide synchronization of the chrominance signal in the receiver. Alternative receiver system for use where normal broadcast receivers are used in which 405 line signals are transmitted over the line system with positive modulation and have to be converted to negative modulation for reproduction on a standard receiver, employs, Fig. 2 (not shown), the pilot signal to provide a reference signal which is combined in antiphase with the vision carrier to reverse the polarity of modulation. Scrambled system uses vision signals in which zero carrier level corresponds to grey level in the picture. The receiver, Fig. 3 (not shown), employs the pilot signal to provide a reference signal at vision carrier frequency to shift the zero level of the picture. Vision carrier frequency determination where the effects of cross talk between two transmissions on separate line pairs is minimized by employing vision carriers offset by an integral multiple of the line scan frequency. The carrier generators are synchronized by a signal derived from the chrominance burst signals, Fig. 4 (not shown). Two pilot signals may be transmitted, one unswitched to demodulate the unswitched chrominance signal and the other phase reversed at line frequency to demodulate the switched chrominance signal.