| 摘要 |
PURPOSE:To execute a tracking control with high accuracy by calculating a sample holding output difference which is formed based on a detection of a reflected light by a preformat part and its just before pre-group part and detecting a track shift. CONSTITUTION:A first signal level extracting means 6 extracts an average output level of a tracking error signal when an optical spot is radiating a prescribed position of a preformat part. Also, a second signal level extracting means 7 extracts an output level of an error signal when the spot is radiating a pre-group part just before the preformat part. When a track shift is generated, a difference is generated in this output level, a track shift detecting means 8 calculates a difference of the output level, and by its difference output, an off-track quantity and its direction are detected and displayed. In such a way, an optical axis shift is monitored easily, and an alignment can be executed with high accuracy.
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