| 摘要 |
PURPOSE:To improve supervisory ability and to securely execute the supervision of maintenance by generating a detection signal when a video signal which grasps only dark parts is impressed in a magnetic recording and reproducing device or when the video signal is not impressed in the above-mentioned device. CONSTITUTION:When the video signal is inputted from a TV camera 1 in a VTR 2, its pedestal level is set to 0V in a clamping circuit 5 so as to make it the positive polarity input signal of a comparator 6. The comparator 6 outputs an L level signal when the TV camera 1 grasps only thick dark pictures and it outputs an H level signal when usual video signals are outputted. Meanwhile, if input video signals do not exist, namely in case of no signals, the output from a detection circuit 9 becomes the L level signal and transmitted to an output circuit 10 as a video existence decision signal. The output circuit 10 outputs the H level signal to an information means 11 so as to inform the occurrence of abnormality in the case of a dark signal and in the case of no signal.
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