Patent GB-623431-A (GB)
Title: Television translating system
Inventor: LOUGHREN ARTHUR V
IPC: H04N5/53
Filed: 1946-12-24
Abstract: 623,431. Automatic gain control. HAZELTINE CORPORATION. Dec. 24, 1946, No. 37899. Convention date, Jan. 25. [Class 40 (v)] An automatic contrast control for a television system which operates in dependence on variations in the level of the synchronizing signals from a predetermined value and is thus independent of changes in the picture signal, comprises an amplifier for the signals having its load impedance in series with a rectifier, an integrating load circuit for the rectifier, and a potential supply means. Potential variations of the amplifiers' load impedence when a synchronizing signal is present tend to cause the rectifier to conduct the potential supply means controlling the rectifier so that it responds only to variations in level of the synchronizing signals and thus develops a voltage across its load circuit which is used to control the gain of the predetector stages. The Figure shows a television receiver which has a conventional circuit up to the second detector 15 and is designed to handle signals in which the synchronizing signals are represented by minimum modulation of the carrier wave. Detector 15 is D.C., coupled to the video amplifier stage 30, the anode load 34 of which is connected in series with rectifier 35, potentiometer 41 and an integrating circuit 36, 37, which has a time constant long compared with the period of the line synchronizing signals which are to be used to establish a control voltage. A potential is set up across potentiometer 41 from an A.C. source 38 which may be (a) of mains frequency ; (b) of line pulse repetition frequency, or (c) of a frequency which is high compared with (b). Its peak amplitude is adjusted to be equal to the D.C. potential of the anode of amplifier 30 in the no-signal condition. When a signal is received, the potential of the anode of amplifier 30 is driven in a negative direction by the line synchronizing signals which are peak rectified by rectifier 35 and develop a control voltage across the intergrating load circuit 36, 37. This voltage controls the gain of the H.F. amplifier, oscillator-modulator and I.F. amplifier stages 10, 13, 14 of the receiver so that the range of signals to be handled by detector 15 is confined within narrow limits irrespective of considerable variations in the level of the incoming signals to the receiver. The system may be adjusted for delayed A.C.C. by varying the level at which rectifier 35 conducts by means of potentiometer 41. When source 38 operates at line pulse repetition frequency it is synchronized and phased so that its positive peaks are coincident in time with the line synchronizing pulses of the received signal. In a modification, Fig. 3 (not shown), the A.C.C. rectifier and its associated circuit are connected in series with the cathode load impedance of the video amplifier stage. In a further modification, Fig. 4 (not shown), the integrating load circuit serves also to separate the synchronizing signals from the picture signals by the utilisation of a branch having a time constant short compared with the period of the line synchronizing signal frequency, whilst the time constant of the network as a whole is long compared with this period.