GB1290091A - - Google Patents

Abstract

1290091 Automatic gain control RCA CORPORATION 2 March 1970 [3 March 1969] 9873/70 Heading H3T In a television A.G.C. arrangement in which the gain control voltage is derived from the sync. pulses, means are provided for inhibiting the derivation of gain control voltages from accompanying impulsive noise. Video signals in which sync. pulses comprises the most negative component are derived from video amplifier 20 over a cascade emitter-follower 21a, 21b and are applied to the base of transistor 34 over resistors 60, 62. In the absence of a video signal transistor 34 is biased to saturation and its collector-emitter path biases a second transistor 38 to cut-off. The collector-emitter path of transistor 38 shunts resistor 42 in a potentiometer network 42, 44 from which the first I.F. amplifier 12 is biased over resistor 48, and in the absence of video signal no effective shunting occurs. The base of transistor 38 is impulsed with line flyback pulses from source 50 via levelsetting resistor 53, Zener diode 52 and resistors 36, 54, and when the shunt on its base-emitter circuit formed by transistor 34 is removed by the impulsing of the latter by the negativegoing sync. pulses, transistor 38 becomes conductive, shunting resistor 42 and so reducing the base bias of I.F. amplifier 12. The bandwidth of the A.G.C. circuit is determined by capacitor 56 connected between collector and base of transistor 34. Noise impulses derived over resistor 60 are differentiated by differentiating circuit 88, 86 and are applied over transistors 74, 76 to the base of transistor 78 which acts as a clamp, preventing the application of a line flyback pulse from source 50 to the base of transistor 38 for the duration of a noise pulse.