| 摘要 |
PURPOSE:To read information with high accuracy by shifting a lens and/or detector from a focused position by a prescribed amount accordant with the difference in heights of a prerecorded part and a write part and letting a head part focus approximately on the write part to read the information on the write part. CONSTITUTION:Ahead part 10 is displaced by piezoelectric elements 11 and 12 so that the outputs of differential amplifiers 21a and 21b are set at zero based on the outputs of comparators 22a and 22b. Thus the focus and tracking controls are given to the plane of a prerecord part 2. When these controls are through, the prescribed focus correction value is supplied to a D/A converter 24a from an arithmetic circuit 23 to focus a head part 10 approximately to a write part 3. Thus the output signal B is obtained from a light receiving area 18e in response to a pattern A of the part 3. Then the output signal C is obtained from a comparator 22c which gives waveform shaping to the signal B. The supply of the focus correction value DELTAi is cut off (D) for a short time synchronously with the rise of the output signal of the comparator 22c. Thus the tracking and focus controls are carried out on the plane of the part 2 while the supply of the value DELTAi is cut off.
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