Patent GB-910116-A (GB)
Title: Circuit arrangements for automatic gain control
IPC: H04N7/01
Filed: 1960-06-09
Abstract: 910,116. Automatic gain control. FERNSEH G.m.b.H. June 9, 1960 [June 9, 1959; March 2, 1960], No. 20302/60. Class 40 (5). [Also in Group XL (b)] In a television standards conversion system in which the initial and final standards are of different frame frequencies (e.g. U.S. 60 frames/ sec. and European 50 frames/sec.) disturbing fluctuations in brightness of the final signal occur at the difference frequency. To prevent this a quick-acting automatic gain control (A.G.C.) is applied to the converted signals. The system (Fig. 1, not shown) comprises a signal amplifier and image tube followed by an optical system transferring the image to a pick-up tube, the head amplifier of which is followed by an A.G.C. amplifier. In order to be able to derive A.G.C. in accordance with the peak white amplitude of the signal peak white pulses W<SP>1</SP>, Fig. 2, are added in the signal amplifier to the signal before conversion, occupying 4% of the line period and occurring during the latter part of the blanking period which itself occupies 18% of the line period. The peak white pulses are arranged to comprise a bright line at the left-hand edge of the picture appearing on the image tube. The A.G.C. amplifier, Fig. 3, comprises a buffer stage 22, a gain controlled stage 31 and a cathode-follower output stage 32. Signals from the output of stage 32 are applied to one triode of a double triode gate 15, to the other triode of which are applied gating pulses G of such duration and timing as to include the reference peak white pulses W<SP>1</SP> in the converted television signal. The series of gating pulses G is interrupted by framefrequency pulses with a duration of 3% of the frame frequency, since the reference pulses W<SP>1</SP> are absent during these periods. The output of the gate circuit 15 is applied to an amplifier 42, the control grid of which is clamped during the blanking period by means of a clamp circuit comprising double diode 43 and semiconductor diodes 44, 45 which is driven by clamping pulses D, E. The bias on valve 42 is such that its output comprises peak white pulses of fluctuating amplitude coincident in time with the gating pulses G. This output forms one input to a clamp circuit 18, the other input of which comprises constant amplitude comparison pulses F of similar duration and phase. The output of clamp circuit 18 comprises pulses whose amplitude is proportional to the peak white level which are applied to the grid of a cathode follower 20, the grid potential of which is clamped during each blanking period by pulses A (Fig. 2) occurring during that period and applied to the grid over condenser 19. The pulses from the cathode of the cathode follower 20 are applied to a clamp circuit 11, the second input of which comprises pulses C of constant amplitude occurring during the blanking period: the output from this clamp circuit controls the gain of variable gain stage 31 by charging up condenser 23. To overcome the displacement in the black level in the signal, produced by the shifts in the working point of the valve 31, the signal from this valve has its black level stabilized by means of a clamp circuit 13 driven by constantamplitude pulses A and C occurring during the blanking interval. The clamp circuit 13 may alternatively be arranged to continue in operation after the clamp circuit 11 has ceased to be effective, to provide a more constant black level potential than otherwise would be attained (Figs. 9, 10, not shown). If the pick-up tube is of the videcon type, there appears a lift in the signal level corresponding to picture black during the blanking level, due to the standing current in this tube: the influence of this is compensated for by feeding impulses of 11% line duration and occurring during the blanking periods to the input of the gain control system via resistor 57 which may be adjusted to provide the desired compensation. The Specification also describes modified clamping circuits (Figs. 4-8, not shown) which may be used in other applications of the A.G.C. circuit in which the direct current component of the inpulses forming the control signal must be transmitted, e.g. when the brightness fluctuations are not periodic, and also for use where the phases of the control voltage impulses B and comparison pulses C differ.