Servo data, including three types of servo patterns distinguishable by phase, are stored in concentric servo tracks on a magnetic information storage disk. Each servo track includes a plurality of only one type of pattern, with consecutive tracks arranged in a repeating sequence of the three servo pattern...http://www.google.ca/patents/US4811135?utm_source=gb-gplus-sharePatent US4811135 - Tri-phase servo pattern for providing information for positioning the transducers of a magnetic disk storage drive
