| 摘要 |
PURPOSE:To effectively detect the presence of an input video signal irrespective of the presence of noise by providing a discrimination circuit for the presence of the signal, and detecting the output voltage in a sample hold circuit in a feedback type clamp circuit. CONSTITUTION:In the input (f) of a differential amplifier 4, the reference value of the voltage at a natural base part corresponding to the base part of a horizontal synchronizing signal pulse is inputted to a minus terminal to amplify the voltage of the difference between the input (e) and the input (f). By applying this output to the input (b) of a differential amplifier 2, the clamp circuit is formed and the voltage at the base part of the horizontal synchronizing pulse of the output of the amplifier 2 is converged to the voltage value at the natural base part and becomes constant, after a fixed time determined by the time constant of the feedback type clamp circuit passes. Accordingly, the output voltage of the sample hold circuit 5 branches, is applied to the discrimination circuit 6 for the presence and the absence of the signal, when there is the detection voltage in a fixed detection range, centering the value of the natural base part, it is discriminated that the input video signal is present.
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