| 摘要 |
PURPOSE:To improve the efficiency of A/D conversion by detecting an envelope of a digitized FM video signal, controlling the amplitude of the FM video signal before digitizing in response to the detection level to make the amplitude of the FM video signal inputted to the A/D converter constant. CONSTITUTION:An AGC amplifier 3 is provided to a pre-stage of an A/D converter 4 to detect the envelope of the digitized FM video signal and the mean value of the amplitude of the FM video signal is made constant by controlling the gain of the AGC amplifier 3 based on the mean value of the detection output. That is, since an output signal of a square sum circuit 72 is the square component of the amplitude of the digitized FM video signal, the envelope of the digitized FM video signal is detected from the output signal. The output signal of a square sum circuit 72 becomes a control signal of the AGC amplifier 3 via an integration circuit 8 and a D/A converter 9 to make the mean value of the amplitude of the FM video signals constant by controlling the gain of the amplifier 3.
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