Videodisc playback system

An optical playback system for recovering from a spirally grooved disc record data recorded in the groove bottom of said disc record as a succession of depressed groove bottom areas of a given width transverse to said groove, of a given depth, and of varying lengths, alternating along the length of said groove with relatively non-depressed groove bottom areas;

• said system comprising:
• a. a source of light of a wavelength which is long relative to the shortest of said varying lengths, and which is more than four times greater than said given depth;
• b. means for focusing light from said source on said groove bottom in the form of an asymmetrical light spot having a first dimension in a first direction transverse to the length of said groove which is significantly less than said given width, and a second dimension in a second direction orthogonal to said first direction which is smaller than said first dimension;
• c. means for rotating said disc record to establish relative motion between the groove bottom and said focused light spot;
• and
• d. light detection means, disposed to receive light reflected from the grooved disc record and responsive to the diffraction of said focused light by said groove bottom areas during the occurrence of said relative motion, for developing electrical signals representative of said recorded data;
• said light detection means including (1) means for responding to the overlap of an undeviated zero diffraction order of said diffracted light with one of the plus and minus deviated first diffraction orders thereof to the relative exclusion of the overlap of said zero diffraction order with the other of the plus and minus deviated first diffraction orders;
• (2) means for responding to said overlap of said undeviated zero order with said other of said deviated first orders to the relative exclusion of said overlap of said undeviated zero order with said one of said undeviated first orders;
• and (3) means for differentially combining the outputs of the respective overlap responding means.

An optical playback system for recovering from a record data recorded in an elongated information track on said record, said information track comprising a succession of depressed areas of a given width transverse to said information track, of a given depth, and of variable lengths, alternating along the length of said information track with relatively non-depressed areas, said system comprising:

• a. means for focusing a light spot on said information track, said focused light spot having a first dimension in a direction transverse to said information track which is greater than a second longitudinal dimension thereof;
• wherein said first transverse dimension of said light spot is significantly less than said given width and said second longitudinal dimension is greater than, but less than twice, the shortest of said variable lengths;
• b. means for establishing relative motion between the information track and said focused light spot;
• and
• c. light detection means, responsive to the diffraction of said focused light by said information track during the occurrence of said relative motion, for developing electrical signals representative of said recorded data;
• said light detection means including first and second photosensitive means oriented in respective locations oppositely off-centered relative to the axis of a path for light reflected by said information track, and means for differentially combining the outputs of said first and second photosensitive means.