Patent GB-777801-A (GB)
Title: Improvements in or relating to television receivers
Inventor: COPE JOHN EDWARD
IPC: H04N5/53
Filed: 1953-01-15
Abstract: 777,801. Automatic gain control. PYE, Ltd. Jan. 15, 1954 [Jan. 15, 1953; Dec. 30, 1953], Nos. 1264/53 and 36177/53. Class 40(5) In a television receiver automatic gain control (A.G.C.) is derived from the black level during the "back - porch" periods .Signal detector 1, Fig. 1, is followed by a video amplifier V2 modulating the cathode of the picture tube and also feeding the synch. separator, via a cathode-follower V3. Signals in which the synch. components are positive-going are also fed from the cathodefollower V3 to a diode V4 which is normally non-conductive. Pulses derived from the line time-base 7, via a pulse transformer 2 and time-delay network 3, are applied to the cathode of the diode V4 during the "back-porch" periods, at which the diode becomes conductive and restores the negative peaks of these pulses to the black level then present at the cathode of the valve V3. The pulses are fed to the cathode of an amplifier V5 which is biased back by a delay bias applied to its grid, from a potentiometer 5, comprising the contrast control. The pulses appearing in the anode circuit of the amplifier V5 have an amplitude dependent upon the height of the black level, are rectified by a diode V6, and after smoothing by R.C. network 6 are applied to control the gain of an I.F. amplifier V1. The grid circuit of the valve V1 is of relatively long time-constant and impulsive noise is prevented from reaching such an amplitude as to cause grid current, by means of a limiter in the form of a biased diode 9. To prevent overload of the receiver by a strong signal during the initial warming-up period, when the A.G.C. is inoperative, a diode 10 is connected between the grid of the video amplifier V2 and the cathode of the line output valve V7, so as to earth the grid of the valve V2 until such time as the line time-base becomes operative. The diode 10 also prevents the valve V2 being driven into grid current by interference pulses. In a modification, Fig. 4 (not shown), designed to prevent impulsive interference reaching the A.G.C. circuit, the diode V4 is fed from an additional cathode-follower, the grid of which is coupled to the cathode of the valve V3 via a series diode, and to earth by an R.C. circuit of relatively long timeconstant. This arrangement also prevents the pulses applied to the cathode of the diode V4 from reaching the synch. separator circuits. In the modification of Fig. 5, interference suppression is obtained by utilising grid-current limiting in the cathode-follower V9. The grid of this valve is connected 'to a tap on the load resistor of the cathode-follower V3 and its cathode resistor is shunted by a condenser C to provide a suitable time-constant for interference-suppression. A further valve V10 coupled between cathode-follower V9 and'the grid of the picture-tube' 12 forms an interference-suppression circuit of the type described in Specification 741,572. In a further embodiment, Fig. 2, the cathode follower V9 feeding the diode V4 is coupled to the cathode of video amplifier V2, an interference suppressor comprising series diode V8 with load circuit R, C returned to H.T.+ being interposed. The signal now being such that synch. pulses are negative-going, it is applied to the cathode of the diode V4, the anode of which is connected to the grid of the A.G.C. amplifier V5. The cathode bias, of this valve is variable by means of a potentiometer 5a to provide contrast control. According to Provisional Specification 1264/53, in such a circuit the cathodefollower V9 and interference suppressor V8 may be omitted, the diode V4 being directly coupled to the cathode circuit of the valve.V2. It is also envizaged that the pulses applied to the diode V4 may comprise differentiated pulses from the synch. separator circuits. Specifications 777,803, 777,804, 777.805, 777,806, 777,807 and 777,808 also are referred to.