Patent ID: 7643388

Claim:
The optical disc apparatus comprising: an optical pickup to emit a light beam to an optical disc; a moving mechanism to move the optical pickup over the optical disc so that a light spot of the light beam crosses tracks formed on the optical disc; a tracking error detector to detect tracking error signals carried by a light beam reflected from the optical disc, each tracking error signal being detected when the light spot crosses a corresponding track among the tracks formed on the optical disc; a track-crossing pulse generator to generate track crossing pulses based on the tracking error signals, each track crossing pulse being generated for the corresponding track; a period measurer to measure a half period of each track crossing pulse; a defect detector to determine whether a defect occurs to each track crossing pulse, based on a latest half period measured at present and an anterior half period measured one half-period before the latest half period; an average-period generator to generate an average half period of the latest and the anterior half periods when determined that no defect occurs whereas to generate a corrected average half period based on the latest half period and a plurality of anterior half periods measured before the latest half period when determined that the defect occurs; a reference period generator to generate a reference half period that indicates a predetermined speed for the optical pickup to move over the optical disc; and a servo mechanism to control the moving mechanism to move the optical pickup at the predetermined speed based on the reference half period and the average half period or the corrected average half period; wherein the defect detector detects a first abnormal state in which a half period of at least one of the track crossing pulses is out of a specific range of error variation or a second abnormal state in which the track crossing pulses suffer drop-outs; and wherein the defect detector detects neither the first nor the second abnormal state when a relationship (1) only is satisfied, detects the first abnormal state when a relationship (2) only is satisfied, and detects the second abnormal state when both of the relationships (1) and (2) are not satisfied, where the relationships (1) and (2) are expressed as (Tz−ΔT)<T<(Tz+ΔT) and T<(2·Tz−ΔT), respectively, in which T and Tz indicate the latest half period and the anterior half period, respectively, and ΔT indicates the specific range of error variation.