A focal servo system for an optical disc player wherein a laser beam is impinged through an object lens onto a disc having information recorded thereon such that the information is optically detected, and the laser beam reflected by the disc is sensed by a photosensor to reproduce the recorded information from the disc, comprising:
- two pairs of photo-cells interconnected in each pair and arranged in a circle at an angle of 90 degrees from each other for receiving the laser beam reflected by said disc;
- a differential amplifier connected to said two pairs of photo-cells for producing a differential signal of the output signals of said two pairs of photo-cells;
- an adder connected to said two pairs of photo-cells for producing a sum signal of the output signals of said two pairs of photo-cells;
- a phase compensation circuit connected to said differential amplifier for phase-compensating said differential signal of said differential amplifier;
- p1 a first comparator connected to said adder to receive said sum signal of said adder for comparing said sum signal with a predetermined first reference voltage and producing an output signal of a first state until said sum signal reaches said first reference voltage and producing an output signal of a second state when said sum signal is above said first reference voltage;
- an actuator disposed in a vicinity of said object lens for driving said object lens toward or away from said disc;
- a signal generator for generating a sweep signal to drive said object lens toward said disc;
- an amplifier electrically connected to said actuator and connected to said phase compensation circuit and said signal generator for supplying the phase-compensated differential signal supplied from said phase compensation circuit, to said actuator and supplying said sweep signal supplied form said signal generator, to said actuator;
- switches connected to said first comparator and actuated by the output signal of said first comparator for supplying said sweep signal from said signal generator to said amplifier when the output signal of said first comparator is in said first state and supplying said differential signal from said phase compensation circuit to said amplifier when the output signal of said first comparator is in said second state;
- a second comparator connected to said differential amplifier to receive said differential signal from said differential amplifier for comparing said differential signal with a second predetermined reference voltage and producing an output signal of a third state until said reference voltage reaches said second reference voltage and producing an output signal of a fourth state when said differential signal is above said second reference voltage;
- a logic circuit receiving the output signal of said first comparator and the output signal of said second comparator for maintaining the output signal of said first comparator supplied from said first comparator to said switches at said first state when said second comparator supplies the output signal of said third state and supplying the output signal of said first comparator to said switches when said second comparator supplies the output signal of said fourth state;
- and
- a reference voltage selection switch connected to said second comparator and said logic circuit to receive the output signal of said logic circuit for switching said second reference voltage to a third reference voltage different from said second reference voltage and maintaining the output signal of said second comparator at said fourth state when the output signal of said logic circuit is at said second state.