| 摘要 |
PURPOSE:To obtain an invariably stable servo-loop gain without being affected by the kind of a medium and variance in characteristics of an optical head by re-setting the amplitude of a tracking error signal automatically before a tracking servo operates. CONSTITUTION:Reflected light from an information recording medium 1 is led out as two signals by a photodetector 6 and the tracking error signal 16 and its relative signal 17 are obtained by a differential amplifier 7 and a summing amplifier 8. A tracking error signal 18 outputted by a multiplier 9 is inputted to a control circuit and compared with a reference level to perform counting-up or counting-down operation, and the counted value is D/A-converted to generate a reference signal 21. The signal 21 is processed by a differential amplifier 11 together with the output of a multiplier 10 and the gains of multipliers 9 and 10 are set with an arithmetic output 20. Consequently, the signal 18 has a gain controlled with the signal 21, both signals 18 and 21 have the gain of proportional relation, and the loop gain of a servo-loop is held at an invariably constant value.
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