| 摘要 |
PURPOSE:To detect accurately an indicator area by setting a lower basic frequency of the record information in the indicator area than that in a data area. CONSTITUTION:The read signals of an indicator area (a), an address area (b) and a data area (c) are supplied to an amplifier 7 in the form of output signals (a) sent from a photodetector. The signal (a') amplified by the amplifier 7 is transmitted through a differentiating circuit 8 and inverted into a differentiated signal (b) having the peak value corresponding to the rise/fall points. This signal (b) is supplied to comparators 10-12, respectively. The above-mentioned signal (a) is changed into a waveform of the signal (b) by the circuit 8 and turned into a so-called DC free coded form. Thus the fluctuation of the DC level is eliminated. Furthermore a triangular waveform having the peak corresponding to the rise/fall of the signal (a) to the information having a low basic frequency such as that given from the area (a) is obtained with a comparatively large amplitude level. As a result, a comparatively large threshold level is obtained to the signal sent from the area (a), thereby improving the S/N. |
