| 摘要 |
A method and apparatus for detecting a focussing position wherein a difference between photoelectric outputs from two photodetectors is amplified by a differential amplifier to detect a deviation of a focal point of a reader/writer head in the reading/writing operation for an optical recording medium. Prior to the amplification of the difference between the photoelectric outputs from the photodetectors, the output voltage from one of the photodetectors is changed to be input to the differential amplifier and the changing ratio is set so that the output from the differential amplifier will be zero when focalized. For this purpose, a voltage changing device for changing the photoelectric output from the photodetector is provided between an output of the photodetector and an input of the differential amplifier.
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