| 摘要 |
A defect detecting circuit (36) generates a defect detection signal (/DEFECT) when the level of a sum signal (SUM1, SUM2) obtained from an optical sensor in an optical pickup (12) is lower than a predetermined level, and a gate circuit (34) responds to the defect detection signal (/DEFECT) to set the level of a fine clock mark signal (FCM1, FCM2) to zero, that is a difference signal obtained from the optical sensor in the optical pickup (12). A fine clock mark is thus distinguished from any defect such as scratch and a correct fine clock mark signal is accordingly generated. Alternatively, the value of a sum signal (SUM), which indicates the total amount of reflected light output from the optical pickup (12), is compared with a predetermined threshold. When the period in which the value of the sum signal (SUM) is smaller than the threshold lasts for a predetermined threshold time period, it is determined that focus of an objective lens (128) deviates from an optical disk (10) and accordingly focus servo control is stopped. The threshold is defined by gradually reducing a drive signal (DV) while detecting peak and bottom values of the sum signal (SUM) which is generated as the objective lens (128) is moved in the direction of its optical axis, and then setting the threshold between the detected peak and bottom values. In this way, defocus of the objective lens (128) is surely detected. |
