GB816725A - Improvements in or relating to automatic gain control circuit arrangements for television receivers - Google Patents

Abstract

816,725. Automatic gain control. MULLARD Ltd. Aug. 26, 1957, No. 26837/57. Class 40 (5). In a television A.G.C. circuit of the " sync. cancellation " type, in which a peak rectifier charges a condenser during the residual " porch " periods, an additional charge is added during the " back porch" periods which is sufficient to prevent the voltage across the condenser decaying below the " porch " level during the line scanning time. Inverted demodulated video signals, i.e. having the sync pulses positive-going, are applied via a series resistor R1 to the grid of a cathode-follower peak rectifier V1A which charges condenser C3, the sync. pulses in the waveform being cancelled out by inverted sync. pulses of greater amplitude which are derived from the sync. separator (not shown) via condenser C2 and resistor R2. The charge on condenser C3 is proportional to the height of the black level and decays during the line periods, due to leak resistor R3; it is prevented from falling below the black level by adding to the " back-porch " signals at the grid of the valve V1A positive-going pulses of constant amplitude which are derived by differentiation of the sync. pulses. The differentiation is performed by condenser C1 and resistor R1. The A.G.C. voltage across condenser C3 is amplified in a grid-controlled rectifier V1B, being applied to its cathode, while positivegoing line flyback pulses are applied to the anode via condenser C5: the A.G.C. voltage is derived from the anode -via smoothing network R4, C6. The grid of the valve V1B is given a positive bias via resistance network R5, R6, R7, the potentiometer R6 being used as contrast control. The arrangement may also be used for negatively-modulated television signals by interchanging the grid and cathode connections to the valve V1B. Specification 484,202, [Group XL], is referred to.