| 摘要 |
PURPOSE:To exactly detect a frame synchronizing signal even in a transient state immediately after the starting of reception or the like and to normally operate clamping thereafter, by clamping after switching the clamping potential. CONSTITUTION:A clamping level switching circuit 10 selects one of the three clamping potentials Vo, V+, VA by using an output A from a frame synchronizing signal detection circuit 8 and an output B from a timer circuit 11 that shifts alternately between '1' and '0' at a frame period longer than a certain time, and supplies the selected output to a clamping circuit 2. If the black level of an input video signal is erroneously clamped in a normal clamping potential Vo in the circuit 2, the frame synchronizing signal detection output A comes to '0', and if the output B of the timer circuit 11 is at '1', the lower clamping potential V<-> is supplied to the circuit 2 and the frame synchronizing signal detection output A comes to '1' representing the presence of detection, and further, a clamping pulse CP at a normal timing is generated by a clamping pulse generation circuit 9. As a result, thereafter, the detection of frame synchronzing signal and the clamping action are normally achieved.
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