GB697821A - Control apparatus for television receivers - Google Patents

Abstract

697,821. Television. HAZELTINE CORPORATION. July 5, 1951 [July 21, 1950], No. 15989/51. Class 40 (iii). [Also in Group XL (c)] Relates to means for producing a positive control voltage proportional to the amplitude of the line synchronizing pulses in the receiver of a negative-modulation television system, the control voltage or a fraction thereof being used to bias the synchronizing pulse separator so that the amplitude of the separated synchronizing pulses is substantially independent of fluctuations in signal strength, and a similar negative control voltage being also produced for use as an A.G.C. voltage. According to the present invention, the video signal (including synchronizing pulses and any random noise pulses) is applied to a detector which is gated on by positive pulses derived from the receiver line scan generator, the positive control voltage being developed across a cathode load comprising a parallel RC circuit having a time constant greater than the line scan period. Since the detector is inoperative in the intervals between the synchronizing pulses, the control voltages are unaffected by random noise pulses occurring during such periods. As shown in Fig. 1, the detector comprises. a triode 30 with a cathode load R34, R35 and C33, the positive gating pulses being applied via terminals 29 to the anode and the positive video signal being applied to the grid via terminals 26 and R37 which serves to limit the peaks of the synchronizing and noise pulses. The amplitude-responsive synchronizing pulse separator comprises a high-impedance diode 42 and associated high impedance load R43 coupled between the grid of V30 and the junction of R34 and R35 whose relative values are such that the diode is conductive only during the recurrence of synchronizing pulses, the separated positive synchronizing pulses appearing at terminals 27. A second parallel R.C. circuit R48, C47 in series with a resistor 46 is coupled between the anode of V30 and ground to give a negative control voltage which is applied via terminals 28 to the A.G.C. line. The time constant of circuit 47, 48 is greater than the line scan period and the variable resistor 48 constitutes a contrast control. In a modification, Fig. 2 (not shown), the synchronizing pulse separator comprises a triode with its anode load coupled to terminals 27 and its grid connected to the grid of V30. A circuit, Fig. 3, not within the scope of the present invention, is also described in which the gated detector comprises a pentagrid 52 with the video and gating signals applied to the inner and outer control grids respectively, the negative A.G.C. voltage being derived by coupling the cathode of V52 directly to the grid of a triode 67 having a parallel R.C. load 60 and additional negative synchronizing pulses being produced at terminals 55 connected to the screen grids of V52. Suitable component values are given for the embodiments described and Specification 623,431, (Group XL (c)], is referred to.