| 摘要 |
PURPOSE:To automatically decide the existence/absence of a video signal by detecting the synchronizing signal of a prescribed cycle included in the video signal and deciding the existence/absence of the video signal by the existence/ absence of the pulse output of a longer pulse duration than the cycle of the synchronizing signal based on the output of a detecting means. CONSTITUTION:The output of an one shot section 4 has a previously set constant so as to obtain an one shot output (c) of a pulse duration from the leading time of t0 to the trailing time of t2. When the one shot section 4 is triggered by the trailing of the first synchronizing signal, the output thereof is led as t0 in the figure c. When the trailing of the second synchronizing signal comes before the trailing of t2, the one shot section 4 is triggered, further the one shot output is outputted from the t1 of a c with the pulse duration based on the constant and a high level is maintained for a long period. By this repetition thereafter, while the video signal comes, the output of the one shot section 4 constantly goes to the high level. By deciding this High level by a digital processing section 5, the existence/absence of the video signal is decided and the video signal is controlled.
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