| 摘要 |
<p>1389202 Television; automatic gain control RCA CORPORATION 1 May 1972 [3 May 1971] 12667/71 Headings H4F and H3T Video demodulation, automatic gain control voltage generation, horizontal synchronizing pulse separation, and vertical synchronizing pulse separation are all effected by the circuit shown in Fig. 1, by taking into account the mutual dependency that exists between the amplitude of the synchronizing pulses in the television receiver and the AGC voltage developed, and also the fact that the amplitudes of the black level and gain control voltages at the video demodulator are substantially equal. The IF composite video signal across the secondary of the IF output transformer 34 is demodulated by rectifier 32 having a load circuit 38, 40, but during horizontal blanking periods transistor 10 is driven to saturation by horizontal flyback pulses applied at terminal 12 causing the anode of rectifier 32 to be coupled to ground which places in parallel with load circuit 38, 40 a further load circuit 42, 44 of smaller impedance, i.e. the essential load of the demodulator is provided by 42, 44 during the horizontal flyback periods. Thus the polarity of the rectifier 32 is effectively reversed so that the video component of the composite signal is derived at output terminal 14, this component being clamped to ground, and the horizontal synchronizing pulse component at output terminal 24. An AGC voltage is also developed at output 24 and vertical synchronizing pulses are derived at output 20 with the AGC voltage amplitude. Specifically capacitor 42 is charged by the horizontal synchronizing pulses to the AGC voltage, resistor 44 determining the time constant of the charging operation such that the pedestal of the AGC voltage is substantially equal to the black level amplitude of the IF envelope. The AGC voltage is added to the video output at terminal 14 so that the black level of the output video signal is fixed. In colour television receivers the horizontal pulses applied to terminal 12 are narrowed to avoid blanking the burst interval.</p> |
