GB742942A - Automatic gain control in television apparatus - Google Patents

Abstract

742,942. Automatic gain control. MURPHY RADIO, Ltd. Feb. 26, 1954 [Feb. 28, 1953], No. 5696/53. Class 40 (5). In a television A.G.C. circuit for positivelymodulated signals, video signals in which the sync. signals comprise peak positive are applied from a video stage to the grid of a cathode follower 3. A.C. coupling is employed, the valve 3 acting as a D.C. restorer by virtue of grid current and being arranged to be cut off for signals more negative than the black level. The adjustment may be made by means of an anode resistance 17. D.C. coupling to this valve may alternatively be used. The cathode follower 3 is cathode coupled to an amplifier 4 comprising a triode-connected pentode, the anode of which is fed with gating impulses, derived from the line or frame generators via a transformer 9. A delay network 8 is interposed so that the valve is only capable of conduction during the # back porch " period following the sync. pulses. A smoothed A.G.C. voltage is obtained across resistance 14. The signal level at which conduction occurs in the valve 4 is determined by the grid potential, derived from a variable potentiometer 12 which is utilized as contrast control. In an alternative arrangement, Fig. 2 (not shown), the gating pulses are applied to the grid, instead of the anode of the valve 4 via a potentiometer comprising the contrast control. In this case the anode is connected via a load resistor to the H.T. line and the negativegoing pulses which appear in the anode circuit are applied via a blocking condenser to a diode rectifier, the smoothed output of which constitutes the A.G.C. voltage.