| 摘要 |
PURPOSE:To decrease the time controlling a video signal level to a reference level by switching the time constant in response to the fluctuation of the video signal level. CONSTITUTION:A video signal inputted to an input terminal 1 is converted into a digital signal (b) by an analog/digital converter 8 together with an offset voltage (a). The digital signal (b) inputted to a discrimination circuit 2 is discriminated for the level and when the level is larger than the reference level, a voltage (c) is applied to lower the level of the offset voltage (a) and when the level is smaller than the reference level, a voltage (d) increasing the level of the offset voltage (a) is applied and outputted from the conversion circuit 4. On the other hand, the digital signal (b) inputted to a fluctuation discrimination circuit 3 is discriminated for the fluctuation by fluctuation discrimination circuits 13-15, and when the fluctuation is larger than the 1st threshold value, a short time constant signal (e) is fed from a time constant circuit 16 to the offset voltage (a) and when smaller than the 2nd threshold value, a long time constant signal (g) is fed from a time constant circuit 18.
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