| 摘要 |
PURPOSE:To supplement the uncertainty of the moving operation of an optical beam due to the omitting detection and erroneous detection of movement starting information, by providing a specific optical beam movement starting signal in addition to a signal which reads and prepares the movement starting signal of the optical beam. CONSTITUTION:When an external mark detector 8 detects an external mark 7, a gate counter circuit 9 starts the counting operation. An optical head 3 reads reproduced signals while a gate signal (2) is outputted and, when a mark detecting circuit 5 detects information a''-c'' instructing the movement start of an optical beam, the AND conditions of an AND circuit 10 coincide with each other and a prescribed pulse (4) is outputted. When the information a''-c'' cannot be detected, the condition that the output signal (4) of the AND circuit 10 cannot be detected is outputted to another AND circuit 14 from a gate detection latching circuit 11. Then, through the AND calculation of the AND circuit 14, the output signal (2)' of a pulse fall detecting circuit 13 is outputted to an optical beam control circuit 6 through an OR circuit 15 as a supplementary optical beam movement starting signal (5)' and the optical beam is shifted by one track by means of the optical beam control circuit 6.
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