GB805502A - Improvements in or relating to television apparatus - Google Patents

Abstract

805,502. Automatic gain control. GENERAL ELECTRIC CO. Ltd., and BOWN, G. C. S. March 5, 1957 [March 5, 1956], No. 6907/56. Class 40 (5). In a television receiver a synch. separator 7 is fed via condenser 6 from a low-impedance source comprising video stage 4. The complete video waveform (Fig. 2A, not shown) appearing across resistor 8, which is negative-going with the tips of the synch. pulses clamped at earth potential, is applied to the anode of a diode 11. Synch. pulses from the anode of the separator 7 are fed by blocking condenser 12 to an integrating circuit comprising resistors 13, 14 and condenser 15. The waveform across resistor 14, comprising a train of pulses (Fig. 2B, not shown) negative-going from earth potential, is applied to the cathode of diode 11 via a resistor 16 which renders the source impedance high. The cathode potential of diode 11 is zero, save during the first part of each " back-porch " period when the instantaneous potential across resistor 14 is more negative than the black level potential in the waveform applied to the anode. At these periods the cathode potential drops to the black level, so that the cathode potential comprises a train of pulses (Fig. 2C, not shown) of height equal to that of the black level in the video waveform. A diode 17 ensures that this pulse train is entirely negative-going. The pulses are fed to an amplifier 20 via resistor 18 which prevents loading by the amplifier input impedance. and after amplification are rectified by rectifier 25; the smoothed voltage across condenser 29 comprises the A.G.C. potential. A delay is obtained by returning the load resistor 26 to a variable potentiometer 27 across the H.T. supply, and is used as the contrast control.