| 摘要 |
PURPOSE:To keep the loop gain at a fixed level by controlling the variable gain with use of an error signal obtained by comparing the detecting signal that detected the average light quantity of the light beam exclusive for tracking with a reference signal. CONSTITUTION:The voltage value EN corresponding to the average level of the reflected light quantity of the subspot light is delivered through a circuit 42 and supplied to an amplifier 32 and then to an arithmetic circuit 33 after k-multiplication. The circuit 33 outputs the voltage value (k.EN-ED), i.e., the difference voltage from the voltage value ED equivalent to the average level of the reflected light quantity of a primary beam. Then said difference voltage is compared with the reference voltage by a comparator 34 and a difference signal between both voltage values is delivered from the comparator 34. This difference signal is converted into an error signal by an absolute value circuit 36 and supplied to a gain controller 37 for control of the gain of a variable gain amplifier 38. Thus the amplitude is controlled for a tracking error detecting signal T which is supplied from a comparator 23. Thus the gain of a tracking control loop is kept at a fixed level.
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