| 摘要 |
When a light beam is irradiated to an information track, on which a pre-pit carrying pre-information has been formed, and reflected light of the light beam is received at a light receiving portion, which is divided into two parts by a division line at least optically parallel with the information track, a first read signal and a second read signal generated based on the outputs of electrical signals from the light receiving portion are input to a push-pull circuit. Then the difference between the amplitude of the first read signal and the amplitude of the second read signal is computed to bring both amplitudes close to each other based on the difference and consequently make them coincide (corrected). The computation on the difference between the first and second read signals whose amplitudes coincide generates a difference signal to detect a pre-pit on the basis of the difference signal.
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