| 摘要 |
PURPOSE:To obtain a video image same in shape as of an object by driving a constant current circuit with a peak voltage of a video signal, using a potential difference between the peak voltage and a prescribed voltage generated across a variable resistor inserted in series with the constant current circuit as a threshold voltage so as to move vertically equivalently the threshold voltage against the amplitude fluctuation of the video signal. CONSTITUTION:A peak voltage detecting circuit 13 detects and hold the peak voltage Vp of a video amplified signal being an output of a video amplifier 1. The video signal 2 is amplified into a desired amplitude by the video amplifier 1, a video amplified signal 3 is obtained, which is fed to one input of a video comprator 5. Further, a threshold voltage 14 fed to the other input of the comparator 5 is a potential Ex which is obtained by applying the peak voltage Vp obtained from the detection circuit 13 to a variable threshold value generating circuit 15 and subtracting a voltage Et=RXI formed from a resistance value R of the variable resistor 7 of the circuit 15 and a constant current I of the constant current circuit 12 from the peak voltage Vp. Thus, the threshold voltage 14 is moved vertically equivalently against the amplitude fluction of the video amplified signal 3.
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