PURPOSE: To secure the optimum time width of a blind sector of a waveform shaping circuit by changing the frequency of a clock signal applied to the waveform shaping circuit in response to the frequency of an input signal and also changing the delay time of the waveform shaping circuit to the clock signal.

CONSTITUTION: An input signal (a) is applied to a terminal 1 and then to a terminal D of the first FF21 among those FFs 21∼23; while a ckock signal CP1 of both CP1 and CP2 is applied to a terminal 3 and then to each terminal T of FFs 21∼23. Then an output (c) of an AND gate 6 that produced an AND of the output (b) of the terminal Q of the FF21, the output (b') of the terminal Q of the FF22 and the output (b") of the terminal Q of the FF23 is supplied to a terminal S of an FF8. While an output (d) of an AND gate 7 that produced an AND of outputs of reverse terminals Q of FFs 21∼23 respectively is supplied to a reset terminal R of the FF8. The output (e) of the terminal Q of the FF8 is delivered in the form of a signal obtained by eliminating the chattering of the signal (a) through a 3-stage delay circuit.