GB968854A - Improvements in or relating to arrangements for automatic gain control in television receivers - Google Patents

Abstract

968,854. Automatic gain control. PHILIPS ELECTRICAL INDUSTRIES Ltd. Dec. 8, 1960 [Dec. 11, 1959], No. 42282/60. Heading H3Q. . In a television receiver contrast is controlled by variation of the grid bias of an I.F. stage 4 by means of a cathode circuit potentiometer 14 and automatic gain control (A.G.C.) is obtained from a stage 6 subsequent to the video 'detector, 5. To obviate interaction between the contrast control and A.G.C., the actual gain control voltage is derived from a potentiometer Rl, R2 connected between the stage 6 and the slider of the potentiometer 14; a diode 11 prevents the gain control voltage going positive. The resistance R1 may alternatively be connected to the cathode end of the potentiometer 14 (Fig. 3, not shown). The A.G.C. may be of the meanlevel type and derived from the grid of a synch. separator (Fig. 4, not shown). To prevent saturation of the final video amplifier 10, Fig. 5, when the signal is unmodulated, a voltage from the frame scanning generator is applied to the anode of a diode 12, which is normally biased to cut off by the voltage applied to its cathode from the anode of the final video amplifier 10. When the latter saturates its anode voltage falls, lowering the bias on diode 12 and permitting pulses to pass via condenser C5 to the A.G.C. circuit, where they are rectified by diode 11 to provide a negative bias on the A.G.C. line, so preventing overloading.