Eccentricity correction during optical head accessing of a desired track

DETAILED DESCRIPTION The object of the present invention is to provide an access method rapidly seeking for a desired track from a number of tracks formed on a rotating record medium, and an inprovement on a macro-seek control in which coarse positioning of a head is speed-controlled by using a scale coarser than a track pitch.
The effect of any eccentricity is minimized even though an external scale which is independent from the eccentricity is used, when the head is positioned to a target position (target scale point).
In this manner, the access time can be reduced.
If the optical head is macro-positioned to a vicinity of the target track by the external scale, and then the optical spot is pulled into the track to read out an address recorded on the track to determine a difference from the target track, and the light spot is micro-moved track by track, the distance of the micro-movement varies in accordance with an extent of the track eccentricity and the total access time is increased.
In the present invention, the condition or status of the eccentricity is previously detected and the distance of the movement in the macro-seek is corrected depending on the eccentricity so that the distance of the movement in the micro-seek is reduced and the access time is reduced.
In accordance with another feature of the present invention, when the speed of the head is low, a scale having a much finer pitch than the external scale pitch is used so that the head speed, from a high speed to a low speed, can be precisely detected.