| 摘要 |
PURPOSE:To lead in accurately the focus servo on the surface of a recording medium even in case of the input of an erroneous signal by outputting a detection output to close the servo loop when two pulses of a comparator appear in one cycle. CONSTITUTION:The difference of the output of a focus sensor 4, which detects whether the distance between a focus actuator 1 and a disc is proper or not, and the surface oscillation of the disc accompanied with rotation is inputted as an error signal A to a driver 2 and a control circuit 7. The signal A inputted to the circuit 7 is inputted to one input terminal of a comparator 12 of an incorporated leading-in detecting circuit and is compared with threshold voltages VTH and VTH' in the other input terminal. If two pulses appear in the output waveform while the actuator 1 is moved back and forth once, leading-in detection outputs E and G are outputted to a switch 6 through a counting circuit 13 to close the servo loop accurately. |
