| 摘要 |
An output from a left half light-receiving portion C1, C4 of a main-beam photodiode 5 of an optical pickup and an output from a right half light-receiving portion C2, C3 of a main-beam photodiode 5 of an optical pickup are normalized, and the difference between the normalized outputs is calculated.(C1+C4)/ alpha *(C1+C4)-(C2+C3)/ alpha *(C2+C3)in which alpha is a constant. Accordingly, even in the case where the objective lens 1 is shifted to the inside or outside of a track, a wobble signal can be extracted securely because the DC offset components of the difference signals in the on-track main laser beam MB are cancelled out.
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