GB887532A - Improvements relating to television systems - Google Patents

Abstract

887,532. Television. BRITISH BROADCASTING CORPORATION. May 30, 1960 [May 28, 1959], No. 18277/59. Class 40(3). A television line synchronizing signal, Fig. 1, starts with a blacker-than-black pulse a, rises steeply to a peak-white reference level b, thereby providing a reference timing edge S for synchronization which occupies the full amplitude range of the video signal, and terminates with back porch c which provides a black reference level. A frame synchronizing pulse consists of a plurality, for example three, of line synchronizing signals. The signals are generated at a transmitter, Fig. 3, by means of three pulse generators A, B and C which are set in operation by line trigger pulses and trigger each other in sequence to generate respectively the pulses a, b and c. The three pulses are then added with appropriate amplitude and sign in a matrix M1 to furnish the line synchronizing signal. The frame synchronizing signal is produced in response to a frame trigger pulse which triggers a pulse generator D. The generator controls a gate L connected between the output of generator C and the input of generator A. The pulse from D lasts for approximately the duration of half a line, whereby following the production of a line signal, generators A, B and C cycle for three further signals to produce a frame signal. A second matrix M2 adds pulses a, b and c to produce line blanking pulses and also receives pulses of approximately 2¢ line periods from a generator E to produce frame blanking pulses. The synchronizing signals appearing at the output of matrix M1 are inserted into a video signal in a conventional manner with the aid of the blanking pulses appearing at the output of matrix M2. At a receiver, Fig. 4, a network "p" having an optimum signalto-noise ratio for pulse a, applies the received signal to a separator which separates pulse a. This pulse is then applied to open a gate 1 and as a result of the inherent delay in network "p" controls the gate to be transmissive for an interval including the synchronizing edge S. The received signal is applied to gate 1 through a network "q" having an optimum signal-to-noise ratio for edge S. The output of gate 1 comprising the combined line and frame pulses is applied to a normally open gate 3, a normally closed gate 2 and a network N having a delay approximately equal to the added duration of pulses a, b and c. The output of the network controls gates 3 and 2 so as to close and open them respectively. Each line pulse thus passes through gate 3, whereas the group of three pulses forming the frame signals passes through gate 2, which is opened at each pulse by the delayed preceding pulse obtained via network N.