GB775673A - Improvements relating to automatic gain control circuits for television receivers - Google Patents

Abstract

775,673. Automatic gain control. ELECTRIC & MUSICAL INDUSTRIES, Ltd. April 26, 1955 [May 6, 1954], No. 13194/54. Class 40(5) In a television A.G.C. circuit a video amplifier 1 feeds video and synch signals, with the latter positive going, to a synch separator 12. These signals are also fed via condenser 15 and resistor 16 to a D.C. restoring diode 17, to which are also fed amplified and inverted synch signals from the anode of the separator 12, via condenser 19 and resistor 20. The resistors 16, 20 are so proportioned that the amplified and inverted synch pulses cancel the synch pulses in the combined signals; the resultant waveform at the anode of the diode 17 has the black level as its most positive component (Fig. 3(c), not shown). Hence the diode 17 clamps the waveform at its anode at the black level, so that the waveform across resistor 18 comprises positive synch pulses and negative video signals. This waveform is applied to the grid of an A.G.C. valve 21. The anode of the valve 21 is connected to the ground line by a resistor 25 and via condenser 27 to a source of positive going pulses, comprising the line output stage. The cathode is given a positive potential by means of a potentiometer comprising resistors 22, 24 connected between the ground line and the screen of the video amplifier 1. The arrangement is such that the valve 21 is rendered conductive by the pulses on its anode, to an extent determined by the amplitude of the positive-going synch pulses at its grid. The resultant potential across the resistor 25 is smoothed by R, C, filter 26, 27a and constitutes the A.G.C. potential. The A.G.C. potential is greatly increased if the valve 1 is overloaded, since the cathode potential of the 'valve 21 is thereby reduced. Embodiments are also described in which the A.G.C. valve 21 is energized by a steady potential, instead of pulses. In one, Fig. 3 (not shown), the resistors 18, 22 and the cathode of the diode 17 are returned to a source of negative potential, which may comprise a smoothing circuit across the grid circuit of the line output valve. In another, Fig. 4 (not shown) the anode resistor, of the valve 21 is connected to the H.T. + line; in this case the pulses appearing in its anode circuit are coupled by a condenser to a further diode rectifier, the output of which constitutes the A.G.C. potential. Specifications 484,202 [Group XL] and 774,113 are referred to.