PURPOSE:To make it possible to detect a mark position regardless of noises, by performing impulse-response operation of the difference signal or the added signal of two signals from a mark without a differential circuit, and obtaining the position of the mark and the width of the mark based on the peak value. CONSTITUTION:An impulse-response operating circuit 11 receives the difference signal from an operating circuit 4 and performs impulse response operation. A maximum-value/minimum-value detecting circuit 12 receives the output of the impulse-response operating circuit 11 and detects the maximum value and the minimum value. In place of a differential circuit, the impulse-response operation is performed, and the central position of a mark and the peak of the signal are emphasized based on the detected signal. Therefore, the position of the mark and the width of the mark can be obtained based on the output of the maximum-value/minimum-value detecting circuit 12. Thus the mark position can be detected regardless of noises.
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