GB543474A - Improvements in television transmitters - Google Patents

Abstract

543,474. Television transmitters ; wireless transmitting systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Sept. 30, 1940, No. 14752. Convention date, Sept. 30, 1939. [Classes 40 (iii) and 40 (v)] In a television transmitter in which the synchronizing signals 11, Fig. 3, are combined with picture signals 13 to extend beyond black level 12 in the white to black direction, and the carrier wave is modulated negatively with the combined signal, the output of the final radio frequency amplifier stage is brought to or beyond its maximum continuous rating by the synchronizing signals, and the output may be automatically reduced if the carrier becomes unmodulated. As shown in Fig. 1, the combined picture and synchronizing signal coming from a terminal equipment 10 which may be as described in Specification 395,499 or 448,065, is fed through amplifier valves 16, 17 and by a direct current re-inserting diode circuit 27 which may be as described in Specifications 422,906 and 422,914 to a modulator valve 18 also fed with a carrier from a source 21. The modulated carrier passes from valve 18 to a push-pull power amplifier 19 by a direct coupling which may be as described in Specification 462,530 but for the sake of simplicity is shown as a battery 26. In addition the combined picture and synchronizing signal is supplied to a valve relay device 31, which holds contacts 43 closed so long as at least the periodic synchronizing pulses affect the valve. The contacts 43 when thus closed short circuit the right-hand section of a voltage divider 44 causing the bias applied to valve 18 to produce full loading of the amplifier 19. If the signal from the device 10 ceases, the valve relay 31 opens the contacts 43 and the negative bias applied by the voltage divider 44 to the valve 18 is therefore reduced, whereby the carrier amplitude output of the amplifier 19 is reduced to a safe value such as the amplitude representing black.