Vertical focus servo system for an optical disc player.

摘要

A pair of signals generated by two pairs of photo-cells (9a, 9c; 9b, 9d) is supplied to an adder (16) and a differential amplifier (14), an output signal of the adder is supplied to a first comparator (19) and an output signal of the differential amplifier is supplied to a second comparator (30). When an output signal of the adder reaches a reference voltage of the first comparator, a focus servo pull-in range is detected. Thereafter, when an output signal of the differential amplifier reaches a reference voltage of the second comparator, a stable operation condition of the focus servo system is detected and a servo system loop is closed so that a malfunction is prevented.