| 摘要 |
PURPOSE:To secure a constant level detection condition with simple circuit constitution and to improve read margin by switching plural prescribed lapse of times from the time of detecting a peak so that they come in inverse proportion to the frequency of a read analog signal. CONSTITUTION:A read analog signal (a) from an optical head 1 is supplied to a differentiating circuit 3, and a signal (b) generated by converting the peak point of the signal (a) to a zero crossing point is transmitted to a zero crossing comparator circuit 4 and a level detection circuit 5. In the circuit 4, the positive peak point of the signal (a) is detected as a polarity inversion of a peak signal (c) from '0' to '1', and the signal (c) from the circuit 5 and a positive level signal (d) are transmitted to a variable delay circuit 8. A selector circuit 11 selects a peak signal whose delay time is T1 or a positive level signal (g) whose delay time T2 is larger than the T1, and the signals (f), (g) and a negative level signal (e) are transmitted to a coincidence decision circuit 12. And, the time T1 is made coincident with a time TA which is the time from the zero crossing point of the signal (b) to its negative peak point, and the time T2 is made coincident with a time TB which is the time from the positive peak point to the negative peak point of the signal (b). A switching with timing in inverse proportion to the diameter of a disk is executed by the circuit 11.
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