| 摘要 |
PURPOSE:To ensure the discrimination with a white level by controlling a discharge time constant of a charge/discharge circuit charging and discharging a peak value of an output of an image sensor based on its output even when a wide black picture signal is inputted. CONSTITUTION:A differential amplifier circuit 2 takes a difference between a white level being an output of a charge/discharge circuit 1 and a picture signal and amplifies the difference to a required level. A comparator circuit 3 compares the difference output with a slice level and when the difference exceeds the slice level, a low output is outputted and when the difference is lower than the slice level, a high output is outputted. While an output of the comparator circuit 3 is high, a switch circuit 4 decreases the discharge time constant of the charge/discharge circuit 1 and when the output is low conversely, the discharge time constant is increased.
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