| 摘要 |
PURPOSE:To set the cut-off frequency of a low-pass filter low sufficiently to improve characteristic of AGC, by forming the second AGC loop to make the amplitude of the beat signal fixed approximately while the synchronizing detection stage operates as a mixing stage. CONSTITUTION:Only amplitude information is selected from the output video signal of synchronizing detection stage 30 by low-pass filter 4. If the output of filter 4 is very minute or no signal exists practically, charging circuit 52 continues the charging operation, and the output of AGC filter 51 is increased to increase the gain of the radio frequency stage of tuner 1 through amplifying circuit 54 of ViF (video intermediate frequency) circuit 2. Reversely, if the output signal of filter 4 is a prescribed level or more, reverse AGC is applied by the operation start of discharging circuit 53. The output level of synchronizig detection stage 30 is detected by level detecting circuit 55, and by this output, filter 51, charging and discharging circuits 52 and 53 are controlled. |
