GB649141A - Improvements relating to gain control circuits for television receiving apparatus - Google Patents

Abstract

649,141. Automatic gain controls. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 26, 1948, No. 5719. Convention date, Feb. 26, 1947. [Class 40 (v)] An A.G.C. circuit for a television receiver comprises a valve which is " gated " so as to be conductive only during the blanking period, and is characterized by responding rapidly to a decrease in signal strength but more slowly to an increase, being thereby rendered insensitive to impulsive noise. The demodulated signal is applied to an outer grid of valve 26 in such a sense that the video component is positive in nature. The valve 26 is biassed so as to be non-conductive save when positive-going impulses, which are derived from the line deflection generator 20, are applied to the inner grid; the impulses are delayed, and occur immediately after the synchronizing impulses. The current in the valve is determined by the black level in the signal; alternatively the impulses may be made simultaneous with the synchronizing impulses, the valve then responding to the peak synchronizing signal. The potential across the anode load 48 comprises negative-going pulses whose amplitude is inversely proportional to the signal strength and which are rectified by a diode 60 in conjunction with load circuit 68, 74, this arrangement forming a quick-charge slow-discharge circuit. The A.G.C. potential is obtained from the smoothed voltage across the condenser 74 by means of a reversing valve 78, which is direct-coupled to a cathode follower 86, the A.G.C. line being returned to the cathode resistance 90. The circuit is damped by means of a condenser 96 and resistance 100 connected from the cathode of valve 86 to the grid of valve 78.