GB1267878A - Plural operating mode television receivers - Google Patents

Abstract

1,267,878. Television receivers; automatic gain control. RCA CORPORATION. 5 June, 1970 [11 June, 1969], No. 27256/70. Headings H3T and H4F. A television receiver can be switched to receive video signals, e.g. recorded on tape, and is also provided with a video output for the purposes of recording; it is characterized in that the automatic gain control (A.G.C.) is operative not only when receiving radio signals, but also in the reception of video signals, to stabilize the D.C. level. A colour television receiver includes ganged switches 74, 75, 80, 81, 82, 83, 84 which in the position shown are set for the reception of video signals, and in the reverse position for reception of radio signals. The receiver comprises a pentode video amplifier 56, the anode of which is coupled to chrominance processing circuits 70, and via a frequencycorrecting network to a triode luminance amplifier 57. The cathode of triode 57 feeds video signals to switch arm 74, the anode feeding sync. separator circuit 61 and the grid of an A.G.C. gate valve 60. The valve 60 is pulsed conductive by line frequency pulses applied to its anode and derived over capacitor 63 from deflection circuits 64, to produce a D.C. potential which is smoothed and applied over resistors 108, 109, 110 to control the gain of I.F. amplifier 51. Video signals to be reproduced are applied from terminal 86, via level-setting control 87 to an amplifier comprising transistors 89, 96, 105, the output of which is fed via switch 83 into the cathode of video amplifier 56: when operating in this manner, switch 84 places a positive bias, via resistor 114, on the base of the transistor 112 which is also coupled via diode 111 and line d to the output of A.G.C. valve 60. Transistor 112 thereby obtains an opposing signal-dependent bias which stabilizes the D.C. level in amplifier 89, 96, 105 which is directcoupled, and in which the base bias of transistor 89 is derived over resistor 92, through which the collector current of transistor 89 flows. Switch 84 also varies the D.C. bias on the grid of valve 56 to allow for the additional positive cathode bias when operating on video input. Switches 74, 75 apply the luminance output from the anode of amplifier 57 to the input of third video amplifier 77, via a delay line 76; additional delay is introduced by means of a second delay line 120 when receiving radio signals, to compensate for the effect of the R.F. and I.F. circuits. For the purpose of video recording an output is provided on terminal 150, being derived from the cathode of triode amplifier 57 via an amplifier comprising transistors 125, 139, 140 and 145; this amplifier includes frequency compensating circuits and also a delay line 138 which compensates for delay of the chrominance components in the R.F. and I.F. circuits. When receiving video signals, a faster capture for synchronization of the line oscillator is necessary and this is effected by switching filter timeconstants in the line oscillator circuits (Fig. 4, not shown); the switching is effected by switch 82.