GB709038A - Automatic-gain control apparatus for television receivers - Google Patents

Abstract

709,038. Automatic gain control. HAZELTINE CORPORATION. March 13, 1952 [May 9, 1951], No. 6555/52. Class 40 (5) In a television receiver employing keyed automatic gain control, the demodulated video signals, including synchronization signals, are applied to the control grid of a valve 30 which is periodically gated on by the application to its anode circuit of pulses developed in the line-scanning circuits of the receiver, the potential developed across a condenser 41, in series with the anode circuit of the valve and in parallel with a level setting diode 42, as a result of the periodic pulses of current flowing through the valve being employed to control the gain of the radio frequency and intermediate-frequency stages of the receiver. Under conditions of normal operation pulses are delivered by line-scanning amplifier 23 in synchronism with the line synchronizing pulses and a continuous train of pulses of current is passed by the triode 30 to charge condenser 41 negatively in accordance with its mean value. The output from the condenser is used, after smoothing by the integrating circuit 44, 46 to effect A.G.C. Under abnormal conditions however when the video-frequency amplifier is overloaded thereby reducing the effective amplitude of the synchronizing pulses and permitting incorrect operation of the scanning circuits, the pulses from the line scanning amplifier 23 will not be coincident with the line synchronizing pulses in the video signal applied to the control grid of the triode 30 and pulses of current will be passed by the triode in intermittent groups corresponding to the beat-frequency between the two sets of pulses. Consequently a varying potential is developed across the condenser 41, which in the absence of diode 42 would be of smaller amplitude than the A.G.C. signal developed when the circuit is operating correctly and so would be unable to prevent overloading in the video amplifier and restore normal operation. However, by virtue of diode 42, the most positive portion of the A.G.C. signal is reduced to earth potential so that the mean level of the signal has a sufficiently large negative value to restore normal operation. Figs. 3 and 4 (not shown) each show a circuit wherein the triode is replaced by a pentode and the cathode circuit is biased by a potential-dividing network connected to the screen-grid, the condenser 33 being omitted. The circuit of Fig. 3 also comprises a seriesresonant circuit tuned to the line frequency byepassing the condenser 41 and a switch for alternatively returning the resistor 43 to earth instead of to the positive HT potential. Specification 697,821 is referred to.