| 摘要 |
PURPOSE:To detect the step out of focusing servo by employing constitution in which an offset is added on a signal related to a difference signal based on the output of a level comparator. CONSTITUTION:When a sum signal Y goes within the range of reference level L of the level comparator 15, the output of the comparator 15 is sent to an offset part 19, and a prescribed offset is supplied to an adder 18. Thereby, the adder 18 adds the offset on a focusing error signal X' from a divider 14, and inputs the output of the divider to a phase compensation circuit 16. An actuator 7 is driven corresponding to the offset, and a convergence lens 4 is moved, which changes the incident light quantity of a quadripartite detector 9. When level change occurs after the offset is added, the out of focusing servo is not detected erroneously by deciding the change as the one due to the reduction of the optical reflectance of an optical disk 6 even when the level of the sum signal Y is lowered. Also, when no level change occurs, the level change of the signal X' is detected, and correction by the pull-in of a lens 4 is performed. |
