| 摘要 |
PURPOSE:To attain a stable focus servo draw-in action regardless of the characteristics of a compensating circuit by using a control means to change the compensating characteristics of a phase compensating circuit for focus servo system in a focus draw-in mode. CONSTITUTION:The detection signal S1 of an optical pickup 8 is supplied to the arithmetic circuits 10 and 11 in a focus draw-in mode. The circuit 10 calculates an error signal S2 from the signal S1 and delivers it to a focus servo draw-in sequence circuit 1. While the circuit 11 calculates a focus sum signal S3 from the signal S1 and delivers it to a comparator 13. The signal S3 is compared with the reference voltage signal S4 through the comparator 13. Then the pulse signal S5 which has an H level in a just-focus mode and an L level in an under-focus mode respectively is delivered to a monostable multivibrator 14. The vibrator 14 delivers the signal S6 to a switch circuit 5 of a phase compensating circuit 3. Here the signal S6 has an H level when the H and L levels of the signal S5 are repeated and then an L level when the signal S5 has an H or L level in its stable mode. The ON/OFF of the circuit 5 is controlled according to the level of the signal S6 and the phase delay compensation 4 of the circuit 3 is invalidated in a focus draw-in mode. Then the compensation 4 can be validated in an automatic focusing mode.
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