| 摘要 |
An optical disc drive (I) comprises an optical pickup ( 3 ) with a 4-quadrant photo-detector ( 20 ); an objective lens ( 14 ) mounted displaceably with respect to the photo-detector ( 20 ); an optical displacement actuator ( 30 ) for displacing the objective lens ( 14 ); and a servo controller ( 70 ) for controlling the focus actuator ( 30 ). The controller ( 70 ) receives respective detector signals (S 1 , S 2 , S 3 , S 4 ) from the photo-detector ( 20 ). The controller ( 70 ) is adapted to process these detector signals to produce a focal error signal (FE) which is zero if these signals all have identical magnitude. The controller ( 70 ) is further adapted to subtract/add an offset parameter (?<SUB>off</SUB>) from/to said focal error signal (FE), and to provide the difference/sum as a control output signal.
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