| 摘要 |
PURPOSE:To follow an optical beam onto a track by detecting the fluctuation of optical beam outputs due to an external disturbance and a local defect distinguishingly and setting larger the servo gain in case of the external disturbance than a normal value and setting smaller in case of the local defect. CONSTITUTION:In case of the external disturbance, a control signal having a prescribed time width T is generated in synchronization with the leading point of time of a detection signal from a dropout detection circuit 32. During this time, a Q' terminal of a DFF is at a low level and no control signal is generated from an AND circuit 46. In case of the local defect, a control signal having the prescribed time with T is generated synchronously with the leading point of time of the detection signal from the dropout detection circuit 32. The terminal Q of the DFF is at a low level at the DFF during this time and no control signal is generated from an AND circuit 45. When a detection signal is not inputted from neither a level detection circuit 31 nor the dropout detection circuit 32, no control signal is generated from the AND circuits 45, 46. Thus, the optical beam is followed up even at both the external disturbance and the local defect. |
