| 摘要 |
PROBLEM TO BE SOLVED: To identify whether an illicitly copied disk or not by utilizing the presence or absence of a prepit section not existing on a central line of a track or the presence or absence of the oscillation of the track. SOLUTION: Displaced bit strings 7 formed by displacing portions of the bit strings of the track 5 in a radial direction are arranged in the region of a portion of the disk 1. When a light spot 8 passes the displaced bit strings 7, the fluctuation in the width of a regenerative signal is little but a large level fluctuation occurs in a tracking error signal. The presence or absence of the displaced bit strings 7 is detected by utilizing the level fluctuation of the tracking error signal, by which the disk is discriminated. The region of a portion of the disk 51 is provided with the track 54 oscillated by the frequency higher than the frequency at the intercession point of the control gains of tracking control and the disk is discriminated by the level of the oscillation frequency component included in the tracking error signal. |
