Patent ID: 6707772
Filing Date: 2004-03-16
Classification: G11B

Abstract:
A method of directing an incident optical beam to a track of information on a dynamic medium, wherein the beam reflected by the medium is picked up by an optical pickup including several photodetectors, elementary signals respectively delivered by the photodetectors are used to form two secondary signals, which, in turn, are sampled and filtered by a low-pass filter having a cutoff frequency at most equal to a quarter of a sampling frequency, the mutual phase shift of the two secondary signals being representative of the positioning error of the beam with respect to the track, the method comprising:selecting, for each secondary signal, at least one pair of samples situated outside a predetermined amplitude range located about a predetermined threshold; identifying, by interpolation from the selected samples, two transitions respectively for the two secondary signals, with respect to the predetermined threshold and corresponding to one direction of crossing of the threshold; determining a time gap between the two transitions; determining a value of the mutual phase shift based upon the time gap; and directing the optical beam based upon the value of the mutual phase shift.