| 摘要 |
PURPOSE:To shorten the lock-in time even with a large shift amount of a head by switching the phase gain of the tracking servo system in response to the result of detection of a signal of the 3rd or the 4th frequency difference in a 4- frequency pilot system CONSTITUTION:A detection circuit 23 detects a level of a signal of the 3rd frequency difference gamma from a BPF21 and a detection circuit 24 detects the level of a signal of a frequency difference delta from a BPF22 and they are outputted to a differential amplifier 25. The differential amplifier 25 outputs a deviation signal SA in response to the level difference of the gamma and delta to a hold circuit 26 and an error signal SE is outputted. The signal is fed to a differential amplifier 32 via a gain switching circuit 27 and a phase filter 31. When the error signal SE outputted from the hold circuit 26 goes to an H level, the switch 28 of the gain switching circuit 27 is switched from (a) side to (b) side. A tracking error signal T'E is outputted via a resistor 30 and even if the shift amount of the head being in the relation of R2<<R1 is large, the lock-in time is short and the locking of the servo system is attained quickly, where gamma=f3-f1, delta=f4-f20.
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