Video disc read back scanner

Claim 1

An improved tracking system for use in an optical disc playback system for recovering information from a selected one of a plurality of information tracks provided on a reflective optical disc, the average reflectivity of any one of the information tracks being different from the reflectivity of the disc between the tracks, said playback system including: an optical system for directing a beam of radiant energy onto a reflective disc and for directing the radiant energy reflected by the disc to a detector; and beam steering means for moving the beam of radiant energy radially relative to the disc being read to follow a selected one of the information tracks, said improved tracking system controlling the beam steering means such that the beam impinges a predetermined edge portion of the track being read, said tracking system comprising: a photocell detector arranged to receive all of the radiant energy of a beam reflected by the disc being read and to output a recovered signal proportional thereto; comparator means for comparing the level of said recovered signal with a reference level and for generating an error signal having a first characteristic when the recovered signal indicates that the beam is impinging more than said predetermined edge portion of the track being followed, and having a second characteristic when the recovered signal indicates that the beam is impinging less than said predetermined edge portion; and, wherein further, said characteristic of the error signal is the polarity; and the amplitude of the error signal is proportional to the difference between the recovered signal level and the reference level.