| 摘要 |
PURPOSE:To execute a focus servo (FS) in a short period with a simple constitution by supplying a bias current and after an objective lens is displaced for a constant quantity, restarting the FS, when the FS is started and the pull-in of the FS is not executed. CONSTITUTION:A switch 51-1 is turned on by a control circuit 56 and FS is started. When the position of an objective lens 7 is in a normal part I within + or -l, the output of an absolute value circuit 55 when a prescribed period passes after the FS is started is smaller than a prescribed reference value, the output of a comparator 57 becomes L, and therefore, the FS is continued as the fact that the FS is pulled in. On the other hand, when the initial value of the objective lens 7 is at a regular part II, there is a case in which the pull-in of the FS is not carried out. At such a time, the switch 51-1 is once turned off, a switch 51-2 is turned on, the constant bias current of one side polarity is supplied to a focus coil by a reference voltage +V, the objective lens 7 is displaced for a prescribed quantity so that an optical card 1 can be positioned in +l, the switch 51-2 is turned off and the ordinary FS is executed. |
