GB415118A - Improvements in or relating to television and the like systems - Google Patents

Abstract

415,118. Television. ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, Middlesex, and BROWNE, C. O., 29, Monk's Drive, West Acton, London. Feb. 15, 1933, No. 4689. [Class 40 (iii).] In television systems in which the synchronizing signals become delayed or advanced in transmission as compared with the picture signals, means are provided for advancing or delaying the production of the synchronizing signals at the transmitter ; in this way the need for a long interval between successive line-scannings is avoided. Thus a line-synchronizing signal may be superimposed at the transmitter as a pulse a of sub-carrier frequency on the picture signals b, and framesynchronizing signals may be similarly superimposed. The synchronizing signals are generated by momentary illumination of a photo-electric cell at instants so timed that the line-synchronizing signals a are generated earlier than the intervals 1 which separate the line-scannings. The output valve 26 receives picture signals from valve 2, and sub-carrier oscillations modulated by line-synchronizing and frame-synchronizing impulses from valves 12, 21 respectively, these valves being fed with modulating signals through the circuits 3, 5, and with sub-carrier oscillations through the circuit 7. The acceptor circuit 18, 19 shunted across the output terminals of valve 12 byepasses the line frequency and its lower harmonics, and leaves only the sub-carrier impulses. The frame frequency, on the other hand, is not removed from the output of valve 21, but is retained for use at the receiver for blacking out the picture between frame. scannings. A rejector circuit 14 keeps the subcarrier away from the picture-current valve 2. At the receiver, Fig. 5, the demodulated signals are fed through a valve 30 to parallel circuits one of which includes an inductance 33 which resonates with its inherent capacity to the sub-carrier frequency, while the other includes a rejector circuit 37 tuned to the same frequency ; the tuned inductance 33 eliminates the sub-carrier from the grid circuit of the cathode-ray receiver, which is shunted across the impedance 34 and receives picture signals and frame-synchronizing signals only, the latter serving to black out the picture during intervals between frames. The rejector 37 applies sub-carrier impulses to rectifier 38 which controls synchronization. The sub-carrier impulses are delayed by circuit 14 at the transmitter (by the time taken to build up a given amplitude), by the tuned inductance 33 at the receiver and by the synchronizing unit (not shown) at the receiver ; the total delay is that which is compensated by generating the linefrequency impulses a, Fig. 2, in advance of the intervals 1 between the line-scannings. The circuit 34 may be replaced by a resistance shunted upon the inherent capacity of the oscillograph (Fig. 6, not shown).