PURPOSE:To keep a reproduced and demodulated color difference signal to an optimum amplitude at all times by detecting a voltage at the horizontal blanking period of the reproduced and demodulated color difference signal and changing the amplifier gain to bring the difference to a specified voltage. CONSTITUTION:A high voltage period of a gate pulse B is placed after the voltage change of the horizontal blanking period of the reproduced and demodulated color difference signal A and voltage detector 2, 3 detect an output power of an AGC amplifier 1 at the high voltage of the pulse B at each other horizontal scanning period. The output voltage of a dynamic amplifier 4 goes to a high or a low voltage. Thus, an output of an exclusive OR circuit 5 is a high voltage at the horizontal scanning period with a high voltage of the horizontal blanking period and a low voltage when the voltage is low and an output of an AND circuit 10 gives a gate pulse only at high voltage. Then a detection circuit 12 is activated only when the output of the circuit 10 is a high voltage, compares a voltage higher than a clamp voltage from the circuit 11 by a specified voltage difference with a voltage higher than voltages during the horizontal blanking period and changes the gain of an amplifier 1 to obtain the same voltage.
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