| 摘要 |
A focus servo control system is a system for selectively applying a light beam to a disk on which data is recorded on plurally layered recording surfaces and setting the focus position of the light beam on a selected recording surface of the two-layered recording surfaces based on a signal read from reflected light from the selected recording surface of the plurally layered recording surfaces of the disk and includes a focus error detector for detecting a positional difference between the focus position of the light beam and the selected recording surface of the plurally layered recording surfaces, a sample-hold circuit for sampling and holding an output of the focus error detector, a focus position correcting circuit for correcting the focus position of the light beam based on one of the output of the focus error detector and an output of the sample-hold circuit, a data signal level detector for detecting the signal level relating to data read out from the disk, a comparator for comparing a signal level detected by the data signal level detector with a predetermined level to output the result of comparison, and a selector for selecting one of the output of the focus error detector and the output of the sample-hold circuit as an input signal of the focus position correcting circuit based on the output of the comparator.
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