| 摘要 |
PURPOSE:To improve a detecting reliability of an SM (sector mark) signal, and also to improve a reliability of a sector jump by providing a sensor circuit for writing an SM mark in the same position as the SM signal and detecting independently its SM mark, and counting the number of sectors by OR of both detecting signals. CONSTITUTION:When a sector of one sector before, the instance, a sector 63 is in a sector jump operating position, an SM marker 100 is marked by a width for covering enough a corresponding SM signal 101 extending from the trailing edge of its position to the leading edge of the next sector 0', therefore, an emitted light of a light emitting circuit 1 irradiates the marker 100 before the SM signal 101 of the next sector. Its reflected light is photodetected by a circuit 2, its waveform is shaped by an SM mark detecting circuit 8, and it is counted up by an SM counter 7 through an OR circuit 6. An output signal of the detecting circuit 8 is brought to waveform shaping enough even if a noise is generated by the circuit 2, and thereafter, converted to a digital signal. Also, an output pulse width of the detecting signal 8 covers enough the pulse width of an SM signal detecting circuit 5, therefore, even if the SM mark 100 or the SM signal 101 cannot be detected, the SM counter 7 executes counting exactly.
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