GB1074072A - Keyed automatic gain control circuits - Google Patents

Abstract

1,074,072. Television A.G.C. circuits. STANDARD TELEPHONES & CABLES Ltd. Jan. 8, 1965 [Jan. 9, 1964], No. 957/65. Headings H3Q and H3T. In a keyed A.G.C. system for a television receiver the video signal is applied at 1 to a diode 3 which is biased at 4 so that only the sync. pulses pass through to charge capacitor 5. Keying pulses from the line transformer are applied through 9 to render diode 7 conductive and thus allow capacitor 5 partially to discharge through diode 6. This current pulse is proportional to the amplitude of the input video signal and charges capacitor 12 to a corresponding potential. The difference between the potentials across capacitors 5 and 12 is smoothed by 10 and 11 and provides the A.G.C. voltage at 13, 14; this may be converted to a low impedance source at 14<SP>11</SP>, 15 by an emitterfollower 16 to supply transistorized R.F. and I.F. stages. If the input signal is large or increases suddenly control may be lost due to clipping of the sync. pulses by overdriven R.F. or I.F. stages. To avoid this (Fig. 2, not shown) the bottom end of potentiometer 4 is joined to the top of diode 7 instead of to earth, and diode 6 (reversed in polarity) is joined directly to capacitor 12. The A.G.C. signal is then of opposite polarity and the transistor 16 is therefore operated in the common-emitter mode. The keying pulses may be differentiated and arranged to occur at the pedestal level of the signal to maintain the black level constant. The invention avoids the necessity for switch-on surge suppressors to eliminate intercarrier buzz during heating up as the control voltage is immediately available.