GB697820A - Automatic-control apparatus for television receivers - Google Patents

Abstract

697,820. Television. HAZELTINE CORPORATION. July 5, 1951 [July 21, 1950], No. 15988/51. Class 40 (iii). Relates to means for automatically stabilizing the black level of the image reproduced at the receiver of a negative modulation television system in which the synchronizing pulses are superimposed on black level " pedestals ". According to the present invention a final control voltage D, Fig. 2, proportional to the amplitude B of the line synchronizing pulses is developed across the cathode load 32, Fig. 1, of a detector 30 to which the positive video signal is applied (via terminals 26) and which is gated on by positive pulses (applied via terminals 29) derived from the receiver line scan generator and a fraction E of the control voltage appearing across R35 is applied to a resistance network 65 to give a brightness control voltage whose amplitude G changes by the same amount as the amplitude C of the black level pedestals when the video signal fluctuates. The positive video signals are also applied to the cathode of the cathode-ray tube (not shown) so that if the signal fades the brightness of the image tends to increase but this increase is balanced out by the decreased brightness control voltage which is applied to the grid of the cathode-ray tube via terminal 72. Since the detector is inoperative in the intervals between the synchronizing pulses, the brightness control voltage is unaffected by radmon noise pulses during such intervals. The detector 30 is similar to that described with reference to Fig. 1 in Specifications 697,819, 697,821 except that the cathode is connected to a positive voltage source +B through a resistor 60 so that the cathode potential D, Fig. 2, is more positive than the peaks B of the synchronizing pulses applied to its grid. Network 65 comprises a potentiometer 71 with its slider 70 connected via high resistances 67 and 68 to the junction of resistors 34 and 35 in network 32, the junction of resistors 67 and 68 being connected to terminal 72. Potentiometer 70 constitutes a manual brightness control and variable resistor 35 constitutes a contrast control. As described in Specification 697,821, a fraction of the control voltage 'developed across network 32 is applied to bias a synchronizing pulse seperator 50 and a negative A.G.C. voltage is developed across a network 45. Fig. 3 (not shown), illustrates a modification with the synchronizing pulse seperator omitted in which the ungrounded end of potentiometer 71 is connected to a negative voltage source and resistor 67 is connected to the cathode of V30, resistors 34 and 35 being replaced by a single resistor. Suitable component values are given for the embodiments described, and Specification 534,718 also is referred to.