Patent GB-597461-A (GB)
Title: Television transmitting systems
IPC: H04N7/08
Filed: 1945-08-20
Abstract: 597,461. Television ; multiplex pulse signalling. STANDARD TELEPHONES & CABLES, Ltd. Aug. 20, 1945, No. 21268. Convention date, Aug. 25, 1944. [Classes 40 (iii) and 40 (v)] In a multiple television transmission system, the vision signals from a number of television cameras are transmitted on separate carrier frequencies and the accompanying sound signals, for each camera, together with synchronizing signals, common to all vision transmitters, as time-modulated pulses on a single carrier frequency. Horizontal and vertical synchronizing pulses 15, 19 and multi-channel pulses 21 are produced by appropriate shaping, multiplying and dividing of the basic pulse train 13, Fig. 2 (a). The multi-channel pulses provide three channels 1, 2, 3, Fig. 2 (d), for the transmission of sound signals accompanying the three separate vision transmissions and one channel n for transmission of vertical and horizontal synchronizing pulses. This is accomplished by applying the pulses 13 after suitable shaping through three delay devices producing time delays t1, t2, t3, Fig. 2 (e, f, g), to produce three keying pulses, each of which controls an amplifier associated with each sound source, the amplifiers being normally blocked except on the application of a pulse. The sound pulse trains 29, 30, 32 produced by the keying pulses, the envelopes of which are proportional to the signal amplitude of the sound sources, are applied to a time modulator together with the pulse train 21 and the action of the modulator is such that the pulses 21 are displaced in time position according to the instantaneous values of the pulses 29, 30, 32. The time modulator may comprise a network including iron cored coils, the inductance of which is varied according to the amplitude of the applied signal energy or the pulses 21 may be made of sawtooth shape and clipped in a clipping gate at levels controlled by the signal energy and then differentiated to produce time-modulated pulses. The output pulses of the time modulator h, together with the horizontal and vertical synchronizing pulses 15, 19, are applied to a mixer circuit having a limit clipping level 35. The output of the mixer pulse train i is then applied to a transmitter for transmission on a carrier frequency f0 over an associated antenna, Fig. 1 (not shown). The horizontal and vertical synchronizing pulses 15, 19 are also applied together with blanking pulses HB, VB, produced from the synchronizing pulses by suitable shaping circuits, to control the picture scanning of the three television cameras, each of which produces an output waveform which is transmitted on frequencies fl, f2, f3 from separate antennas, Fig. 1 (not shown). A receiver suitable for use with the system described is arranged to receive simultaneously two carriers, the multi-channel carrier of frequency f0 and, by selective tuning, one of the other carrier frequencies f1, f2, f3 and in addition includes means for selecting the appropriate sound channel. The system provides increased transmission efficiency due to the possibility of locating all antennas at the highest point in the area to be covered and economy of apparatus by utilizing a single set of synchronizing equipment ; also, since the synchronizing pulses are not superimposed on the blanking pulses of the video signal, the entire amplitude swing of the transmitters may be employed for the vision signals. Specifications 605,109 and 610,464 are referred to.