A circuit locks onto a frequency of data encoded on a medium, taking frequency variations in the physical travel of the medium into account. The circuit includes a disk locked clock (DLC) circuit having a synthesizer operating under digital control based on a feedback signal to produce a servo reference...http://www.google.co.uk/patents/US7248549?utm_source=gb-gplus-sharePatent US7248549 - System for locking a clock onto the frequency of data recorded on a storage medium
