| 摘要 |
A signal processing circuit for automatically adjusting the black level balance of each color signal of a video signal for a color video camera is disclosed. Clamp circuits clamp the blanking period of each color signal at a clamp level capable of being controlled from an external source. The level of the portion of the color difference signal waveform corresponding to the optical black mask is compared with the level of the portion thereof subjected to the blanking process by comparators and the clamp levels of the clamp circuits are controlled according to the resulting difference voltage. The level comparator circuits sample-and-hold the waveform of the color difference signal by two sample-and-hold circuits turned on by a gate pulse generated corresponding to the optical black and by a gate pulse generated corresponding to the blanking period respectively. The result of the level comparison is fed back to the clamp circuits so that the clamp level is controlled to automatically eliminate the level difference, thus adjusting the black balance to an optimum level.
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