| 摘要 |
PURPOSE:To realize the focus control with reduced error caused by the temperature change or the like by giving a periodic fluctuation to the space between the recording medium and the beam focusing device and then utilizing the fluctuation of the output level of the reproduced light detector caused by the fluctuation of the above mentioned space to the focus control. CONSTITUTION:The output of oscillator 40 and differential amplifier 30 are compounded, and the current is flown to moving coil 32. As a result, beam focusing device 14 vibrates the disk surface in the vertical direction to the surface of disk 1 and in the readable range, and thus the focusing state of reproducing beam 4 varies on disk 1. The high frequency (RF) output level obtained from optical detector 16 and according to the presence or absence of the bit on the disk surface varies by the focus state. Thus a phase comparison is given through phase comparator 45 between the envelope signal of the RF output and the output of oscillator 40, and correction circuit 46 controls the level of the detection signal in accordance with the output of comparator 45. Thus the focus control signal is controlled. |
