| 摘要 |
PURPOSE:To obtain the video detecting circuit, which can stably demodulate a video signal by providing a gate means and a switching means, which switches the time constant of a low-pass filter, so as to switch the sensitivity of a loop composed of a VCO, phase comparator circuit and low-pass filter. CONSTITUTION:The frequency of an output video IF signal from a VIF amplifier 6 is separated and a beat signal is generated from a video detecting circuit 13. At such a time, no video signal is generated from a terminal 15, and no horizontal synchronizing signal is generated from a synchronizing separator circuit 14. Therefore, only an FBP is impressed to an AND circuit 16, the output is at an 'L' level and the output of a smoothing circuit 18 is 'L' as well. A switch 11 is opened and the time constant of an LPF 10 is reduced. When the time constant of the LPF 10 is reduced, the pull-in range of the PLL loop composed of a VCO 8, phase comparator 9 and LPF 10. Thus, the video detecting circuit, which has the wide pull-in range and can stably modulate the video signal after once pulling it in, can be obtained. |
