Linear tracking arrangement in a photographic disc type information

Abstract

An optical system for recording information on or retrieving information from a rotating disc is disclosed herein. In this system a beam of light located on one side of the disc is directed onto and laterally across a given track of the disc. The track serves to contain the recorded information and, ignoring this information, is characterized by a predetermined light transmissivity curve in its cross-direction. An arrangement for maintaining the beam and given track in a fixed lateral position relative to one another as the disc rotates, even if the track moves laterally to a limited extent from its intended path of movement, is also disclosed herein. This position maintaining arrangement utilizes circuitry including light sensitive means, specifically two light detectors having trapezoidal light sensitive surfaces, disposed in optical alignment with the beam on an opposite side of the disc and responsive to the light passing through the disc from the beam for producing an electrical error signal corresponding in time to and substantially linearly in magnitude with limited lateral deviations of the given track from its intended position with respect to the light beam, this linear relationship resulting from the configurations of the light sensitive surfaces. The error signal is in turn utilized to vary the position of the initial beam for minimizing the lateral deviations relative to the beam.