| 摘要 |
PURPOSE:To fix loop gain without deteriorating self-loop characteristics by detecting the resonance value level of the loop characteristics and inputting the detected signal into a VCA formed in a loop. CONSTITUTION:A difference signal corrected at its phase by a phase compensating circuit 12 is amplified by a driving amplifier 13 and a pickup 2 is controlled at its driving in its diameter direction so that the levels of both tracking signals are equal. Since the braking ratio of closed loop characteristics appears as the resonance value level of the closed loop characteristics, a change in the loop gain can be detected by detecting said level. The resonance value level is extracted by a band filter 22, detected by a detector 23 and compared with a reference voltage 24 by a voltage comparator 25. An output from the comparator 25 is inputted to a variable amplifier (VCA) 21 and the gain of the VCA 21 is controlled so that the resonance value level is fixed. Consequently, the braking ratio can be fixed and the loop gain can be also fixed.
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