GB515474A - Improvements in or relating to television transmission systems and the like - Google Patents

Abstract

515,474. Wireless signalling ; television. BEATTY, W. A. May 4, 1938, No. 13258. [Classes 40 (iii) and 40 (v)] Vision and synchronizing signals are transmitted as modulations of a carrier frequency within different amplitude ranges and the carrier frequency is varied from one value during vision signals to another value during synchronizing signals. The vision signal may be transmitted as an asymmetrical sideband and the change in carrier frequency during synchronizing may be used to indicate correct tuning at the receiver. The change in amplitude during synchronizing periods may be small, e.g., from 85 per cent. to 90 per cent. modulation, since it is accompanied by a frequency change. A small change in amplitude may be used for the line frequency while the frame frequency may be transmitted as a large change in carrier amplitude without further frequency change. Frequency modulation may be effected by a saturated diode or pentode valve in parallel with an oscillation circuit. The Figure shows an arrangement in which crystal oscillators 27, 28 feed a driving stage 31 through baffer stages 29, 30 which may be differentially controlled by the synchronizing pulses or by pulses derived from a saturated diode. Two stage variable impedances may be used to give two stages of frequency control for use in three colour or in stereoscopic television. At the receiver an auxiliary selector circuit may be provided to give outputs varying in accordance with the steps of the frequency modulation. The system may be used to increase the definition and a receiver is described for 875 lines which is also capable of receiving 441 lines transmitted by a double sideband system. The peak values of the synchronizing signals may be used for gain control.