GB979726A - Improvements in or relating to television receivers - Google Patents

Abstract

979,726. Radio-receiving circuits; automatic gain control; television. MARCONI CO. Ltd. May 29, 1962 [July 28, 1961], No. 20609/62. Headings H3Q, H4F and H4L. In a television receiver for negatively modulated signals interference with the automatic gain control and synchronizing circuits caused by noise is prevented. Video signals, in which the sync. pulses constitute the most positive part of the signal are taken from the anode of a video amplifier V 3 and applied via resistor R 14 and capacitor C 7 to the suppressor grid of a pentode V 2 . This is provided with grid leak bias by means of resistor R 15 , the suppressor/anode path acting as sync. separator and separated sync. pulses appearing on lead F in the anode circuit. Part of the signal is also taken from the anode of video amplifier V 3 via a potentiometer comprising resistors R 10 - R 13 and applied to the common grid of a double pentode V 1 A, V 1 B in which the valve V 1 A acts as a gated A.G.C. valve. Its anode is supplied with positive going line flyback pulses over capacitor C 4 , the bias on the cathode being set by resistors R 1 -R 3 and the grid bias being such that the tips of the sync. pulses on the grid cause conduction, so that there is generated on line B an A.G.C. potential in dependence upon the signal strength. The grid bias and thus the contrast is adjustable by means of a potentiometer R 12 . The video signal available at the join of resistors R 10 , R 11 is also applied to the suppressor grid of pentode V 1B via a potentiometer comprising resistors R 7 , R 8 and by-pass capacitor C 5 , which applies a suitable D.C. bias and at the same time emphasizes the higher frequency components of the video signal, and more especially sinc. and noise pulses, at the suppressor grid. The arrangement is such that with the maximum allowable signal applied to the receiver the tips of the sync. pulses are just unable to cause conduction in the valve V 1B . The more positive noise pulses, however, cause conduction and the resultant negativegoing pulses in the anode circuit are passed by a capacitor C 2 to the suppressor grid of the AGC valve V 1A and inner grid of sync. separator V 2 , whereby these valves are biased toward cut-off during noise pulses. A capacitor C 3 connected to a source of negative-going line flyback pulsses compensates for the positive-going line flyback pulses reaching the suppressor grid of the valve V 1A from the anode via the anode to suppressor grid capacity.