GB935975A - Direct current reinserting circuit - Google Patents

Abstract

935,975. Television. STANDARD TELEPHONES & CABLES Ltd. Dec. 24, 1959 [Dec. 30, 1958], No. 43886/59. Class 40 (3). In a direct current reinsertion circuit for a television system a D.C. signal dependent on the back porch signal level of the video signal is derived across the cathode circuit 54, 55 of a triode 22 which receives the video signal 30 on its anode and is pulsed into conduction during the back porch intervals 29 by keying pulses 50 which are applied to the grid, the keying pulses being derived from an amplifier 44 to which synchronizing pulses 28 separated at 25 and differentiated at 39, 40 are applied via a rectifier 42 which forms part of a circuit 40, 42, 49 to bias the amplifier to saturation whereby an output 50 is produced only in response to the negative differentiation components which occur in the back porch intervals. The D.C. signal from the cathode circuit 54, 55 is resistively coupled to an electrode (the grid, as shown) of the cathode-ray tube 16 in the case of a receiver or an amplifier in the case of a transmitter. The synchronizing pulses 28 are separated by a noise-immune separator circuit which receives video inputs of both positive and negative polarity. Picture mean brightness is adjusted by potentiometer 58 which controls the anode voltage of triode 22. Switches 51 and 52, together with a grounded grid amplifier 61 which is fed from a cathode circuit 7 of the video amplifier 5, are included to permit operation with original television signals corresponding to either positive or negative modulation. Specification 836,217 is referred to.