| 摘要 |
PURPOSE:To improve color reproducibility corresponding to low illuminance by providing a switch circuit which compares a sample held voltage in a period corresponding to the optical black mask part of the output signal of an image pickup tube with a sample held voltage in a blanking period, and substitutes a DC signal in the blanking period so as to make both potentials equal to each other. CONSTITUTION:The switch circuit 13 is supplied at one input terminal (a) with a signal which has a video period and an optical black mask gate part reproduced on a DC basis and also supplied at the other input terminal (b) with a potential which is sampled and held with B and BLK pulses. Then, the potential difference between both sample held voltages is calculated by comparison and the output from the switch circuit 13 is switched so that the potentials in the period corresponding to the optical black mask part, video period, and blanking period are all equal according to the timing of B and BLK pulses supplied to the control terminal (p) of the switch circuit 13 from an input terminal 16, substituting DC pulses. Thus, a level step D between a horizontal blanking period and a horizontal scanning period is eliminated and a signal with a stable black level is obtained in all the periods.
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