A plurality of data sequences having patterns distinguishable from informational data are recorded as a servo mark portion of servo information on a disk. Upon decoding a predetermined number of these data sequences, a determination is made as to whether an accurately located servo mark has been detected....http://www.google.ca/patents/US5293276?utm_source=gb-gplus-sharePatent US5293276 - Method for generating a reference signal from unique servo mark pattern detection
