A tracking system for positioning a light spot onto a magneto-optic disc having a beam guide track preformatted thereon, includes a main light beam and two auxiliary light beams, and a circuit for switching between a low frequency component of a tracking error signal, detected by respective auxiliary...http://www.google.com.au/patents/US4787076?utm_source=gb-gplus-sharePatent US4787076 - Optical disc tracking system with switching of tracking error signals at boundary between track guide and track address
