REPLICA CHECKING DEVICE

摘要

PURPOSE:To attain the automatic measurement of the drop-out produced in a replication stage by shifting an aperture lens closer to a replica in a focus servo draw-in mode and detecting a moment when a microspot light is irradiated onto a track groove or its vicinity and then closing the focus servo. CONSTITUTION:When an aperture lens 11 moves gradually toward a replica 12, the focus error signals are obtained from a rear side 12 at first and then from a track guide groove 12a respectively. In this case, the peak value is obtained at two areas for the output (b) of a differentiating circuit 22. Then two pulses of the output of acomparator 23 are supplied to a counter 24 and then delivered from a comparator 25 when the output of the comparator 23 is set at 2. While the error signal is aplified by an absolute value amplifier 26 and delivered to be inverted by an inverter 27 for shaping. An AND is obtained through an AND gate 28 between said inverted error signal and the output of the comparator 25 to obtain a signal which shows a moment when a microspot light is irradiated to the groove 12a of the replica 12. Thus an analog switch 20 is closed and then the focus servo is closed and drawn in.