GB534718A - Automatic control system for television receivers - Google Patents

Abstract

534,718. Television receivers. HAZELTINE CORPORATION. Nov. 4, 1939, No. 29393. Convention date, Dec. 6, 1938. [Class 40 (iii)] A television receiver comprising a signal reproducing device such as a cathode-ray tube 17, Fig. 1, having a cathode 19 and a control electrode 21, means such as the chain of devices 12, 13, 14, 15, 16 for applying to said control electrode 21 a television modulation signal having a periodically recurring level corresponding to a predetermined shade value such as the " black-level " B Fig. 2, means connected in circuit with said cathode 19 and responsive to the cathode current only during the occurrence of said level for developing a control effect, and means for utilizing said control effect for stabilizing said signal with respect to said level. As shown, the signal is fed to the control grid 21 of the cathode-ray tube through a condenser 28 in conjunction with a resistance 29 connected to a tapping point of a resistance 38 forming a bridge circuit with a tapped battery 37 and a triode 39. The grid of this triode is connected to a condenser 36 and resistance 35 and to one electrode of a diode 34, the other electrode of which is connected to the cathode 19 of the cathode-ray tube. This cathode is moreover connected to the anode of a triode 33, the grid of which is connected to line frequency generator 31 or frame frequency generator 32 or both. In addition the resistance 29 is bridged by a diode 40. The cathode-ray tube 17 should have a substantially linear response or signal-input brightness characteristic. The operation of the circuit is as follows. The television signals, Fig. 2, when rectified, are stabilized with respect to the peaks of the synchronizing pulses S by the rectifying effect of the diode 40 on these peaks producing a grid bias in the resistance 29. The triode 33 is so biassed that :it normally passes current and becomes non-conductive during the retrace periods when it receives a synchronizing pulse from the generators 31 or 32. Therefore current is passed through diode 34 only during the retrace periods. This current is smoothed by the resistance 35 and condenser 36 having a time constant greater than the line synchronizing intervals. The voltage derived therefrom is applied to the grid of triode 39 which by the effect of its anode current on the bridge circuit 37, 38 controls the grid bias of the cathode-ray tube to stabilize the black-level.