Videodisc playback system

Abstract

Recovery from a record of data recorded in an elongated information track, comprising depressed areas of a given width, a given depth, and a variable length, alternating along the length of the track with relatively non-depressed areas, is effected by an optical playback system. The playback system employs means for focusing light from a laser source on the information track as a light spot having a (half-intensity) width which is less than the given width of the depressed track areas. Light detection means, responsive to light diffracted by the track as relative motion is established between the track and the focused light spot, include: (1) means responding to the overlap of the undeviated zero diffraction order of the diffracted light and the plus first diffraction order thereof, and (2) separate means responding to the overlap of the undeviated zero order and the minus first order. Differential combination of the outputs of the respective overlap responding means provides an output signal representative of the recorded data. Tracking error indications are also derived from the diffracted light. In one embodiment of the playback system, the focused light spot is asymmetrical, having a lateral dimension greater than its longitudinal dimension along the track. Preferred relationships between incident light wavelength and depression depths are disclosed for transmissive and reflective record playback systems.