GB723122A - Improvements relating to television apparatus especially television relay apparatus - Google Patents

Abstract

723,122. Distributing music, &c. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Dec, 8, 1952 [Dec. 20, 1951], No. 29816/51. Class 40 (4). In a television relay system of the type in which vision signals and scanning signals are transmitted over a cable to a television receiver or receivers, the scanning signals are transmitted as modulation of a carrier wave the frequency of which is at least approximately an odd multiple of half the line frequency, so as to minimise interference caused by the carrier wave. As shown, the output of receiving channel 3 is applied in parallel to synchronizing signal and vision clipping circuits 5, 6. The output of clipper 6 feeds a scanning waveform generator 7, the output of which may be of the form described with reference to Fig. 3 of Specification 704,141. This waveform modulates a subsidiary carrier provided by oscillator 9, the frequency of which is 2.2325875 Mc/s. (viz. line frequency multiplied by 441 and divided by 2, with half the frame frequency added). The signals produced by modulator 8 are combined with the vision signal output of clipper 5 and the whole modulates a principal carrier wave of 5.4 Mc/s. The sidebands produced by the subsidiary carrier then fall mainly between the visionsideband zones. Filter 13 suppresses the upper sideband and the remaining signals pass over cable channel 14 to the receivers such as 2. There the signals are amplified, demodulated, and applied to the grid of amplifier tube 17. Filter 19, tuned to the subsidiary carrier, feeds the further demodulator 25, the scanning signal output of which is amplified and applied to the scanning circuit which may be of the type described with reference to Fig. 2 or Fig. 4 of Specification 704,141. Vision signals are applied from a tap on potential divider 20 to the control electrode of the cathode ray tube. The subsidiary carrier wave and its sidebands cause modulation of the cathode ray beam, but as the subsidiary carrier wave is approximately an odd multiple of half the line frequency, the interference pattern produced is largely self-destructive. The invention may be applied to apparatus of the type described in Specification 723,123 in which alternative television programmes are simultaneously distributed.