| 摘要 |
PURPOSE:To attain more stable focusing servo by extracting a difference signal of detected outputs to generate a focusing error signal thereby preventing instability of the focusing servo due to external disturbance based on a flaw of the surface of an information recording medium and due to an information signal. CONSTITUTION:Output signals 1a-1d corresponding to photodetecting planes 19a-19d of a photodetector 18 are led to adders 20a, 20b, where arithmetic of (Ia+Ic).(Ib+Id) is performed and the output signal (Ia+Ic).(Ib+Id) from the adders 20a, 20b is fed respectively to envelope detectors 21a, 21b. The detectors 21a, 21b have a time constant nearly equal to a time constant corresponding to the upper limit of the focusing servo. A signal whose band is limited by the envelope detectors 21a, 21b is fed to a subtractor 22, where a difference signal being (Ia+Ic)-(Ib+Id) is calculated to generate a focusing error signal If. The error signal If is fed to a drive mechanism 24 moving an objective lens 4 in the optical direction via a drive circuit 23 to attain focusing servo. |
