| 摘要 |
PURPOSE:To improve the CN ratio without generating a truncation noise by using a phase locked loop so as to demodulate a video signal subject to frequency modulation and changing the width of noise band of a demodulation circuit in response to the frequency component included in a video signal delayed by a delay circuit giving a delay of one horizontal scanning period. CONSTITUTION:A video signal detected by a demodulation circuit 2 is fed from a loop filter 6 to a video amplifier circuit 7 and fed to a band width variable circuit 9 via a 1H delay line circuit 8. A band width variable circuit 9 changes over properly three kinds of low pass filter circuits with different pass band width in response to a frequency component included in the video signal before one horizontal scanning period and when only a low frequency component exists, the band width as the tracking filter is narrowerd by narrowering the band width of the loop filter 6 so as to narrower the noise band width so as to improve the threshold value. |
