| 摘要 |
PURPOSE:To suppress variation in the degree of optical modulation even there is thermal variation or variation in power supply voltage by monitoring the synchronous foremost end level of a video signal, and providing a feedback loop to make the monitored potential be in a prescribed constant level. CONSTITUTION:A drive signal (b) from an LED drive circuit 4 is intensified its low-frequency component by a deemphasis circuit 6 and returned to its original gain, further, is deprived of its whiskers and is made a signal (c) to be supplied to a synchronous foremost end level monitor circuit 7, where the synchronous foremost end potential of the signal (c) is monitored. Thereafter the signal (c) is fed back through a clamping control circuit 8 on which a reference potential Vref is set and a video amplifier 5 so that a video signal comes in the prescribed constant level corresponding to the reference voltage Vref. In case the DC level of the signal (b) fluctures because of thermal variation and power supply voltage flucture, the foremost end level of a horizontal synchronizing signal is moved comporting with the variation. In such a way, the clamping control is executed so that a video signal comes in the prescribed constant level.
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