| 摘要 |
PURPOSE:To lead in a focus servo stably and in safety, by approaching a diaphragm lens to a recording medium from a position distant from the recording medium and by closing the focus servo at the moment the circumference of a recording surface is irradiated with a fine spot. CONSTITUTION:An error signal when supplied to a terminal B is applied to a diaphragm lens driver 9 through an amplifier 20, a phase compensating circuit 21, a switch 22, and a driving circuit 23. With the switch 22 closed, a focus servo circuit is closed. The output of the amplifier 20, on the other hand, is compared with a reference voltage Vb by a comparator 26 through a differentiating circuit 25, thereby outputting a signal (c) which exceeds the Vb. The output signal of the amplifier 20 is further passed through an absolute-value amplifier 27 and an inverter 28 to output a signal (d) which has level 0 only at zero-cross points. The signals (c) and (d) are inputted to an AND gate 29 and an FF30 and the switch 22 is closed by its output signal (h) to lead in focus servo stably and in safety. |
