| 摘要 |
PURPOSE:To keep the loop gain of a servo loop always to a constant value by providing a gate circuit gating a signal only during a tracking signal period in a tracking servo loop and an adder circuit adding the mixing a reference signal so as to correct automatically the loop gain. CONSTITUTION:The adder circuit 7 is inserted between a tracking error detection circuit 4 in the tracking servo loop and a phase compensation circuit 5 and a reference signal generated by a reference signal generator 8 is added and mixed to an error detection signal as an external disturbance in a prescribed timing. On the other hand, a reference signal is extracted from the input side of the adder circuit 7 via a band pass filter 9, fed to a detection amplifier circuit 10, where the result is detected and amplified and then fed to a hold circuit 11 to control the gain of a gain control amplifier 3. Further, based on a pulse from a gate pulse generator 13, a gate circuit 12 is opened only during the tracking signal period to extract the tracking signal and the reference signal mixed thereto and the result is fed to the tracking error detection circuit 4. Thus, the loop gain of the tracking servo loop is kept always constant.
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