Abstract: In a system for writing a plurality of information tracks between adjacent portions of a reference track on a rotatable medium, advantage is taken of the fact that laser beam dithering a cross an edge of a reference track produces signals which are analyzable to determine much more exact radial positions. A pattern of bits is written on the medium with a known spacing between bits. This may be at the time of disk manufacture or later. As these bits are being read, a clock is started along with a bit counter. At this time, a precise determination is made of the radial position at which the read bits are present. From these values, angular and/or linear velocities are computed and stored on the medium. Collected over time, this information is useful for determining aging characteristics of the drive mechanism for the medium.