Concentric spacing of virtual data tracks using run-out compensation

The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to compensation for errors in servo systems. Disclosed are a method and apparatus for compensating for written-in repeatable run-out in a disc drive. A written-in repeatable run-out compensation value is stored in a servo field residing on a track. The compensation value is subtracted from a corresponding servo value to obtain a compensated servo value. The head is positioned relative to the track based upon the compensated servo value. Also disclosed is a method for determining the written-in repeatable run-out compensation values wherein the head is positioned over the intended center of the track so that the head follows an essentially circular path relative to the disc surface. For each revolution of the disc, a servo value indicating the position of the head relative to the track at the servo field is determined. The servo values are averaged over a number of revolutions in order to extract a non-repeatable portion of the run-out.