| 摘要 |
PURPOSE:To control the oscillation of a VCO by providing a video signal presence detecting means and generating a pulse train with a closed loop short-circuiting an input and an output terminal of a delay means when no video signal exists, and using the pulse train as a reference. CONSTITUTION:A video signal presence detection circuit 41 brings an output signal S1 when no video signal exists, a negative pulse P1 is inputted to a 1H delay line 46 and an input and an output terminal of the delay line 46 are short- circuited by switches 44, 45. Then the pulse P1 is circulated in the closed loop and a 1H period pulse train S3 is obtained. The pulse train S3 is fed to a synchronizing pulse separation circuit 22, the pulse P1 is separated from the pulse train S3 and it is outputted as the positive pulse P2. The pulse train S4 of the pulse P2 thus obtained is given to the phase comparator circuit 25 as a pulse train of the horizontal synchronizing pulse and the oscillation of a VCO circuit 23 is controlled based on the pulse P2. |
