Signal detecting system for use in optical reading apparatus

Claim 1

An optical head control signal generator comprising a lens (7) disposed in a reading light beam path caused by an optical head to be incident upon a recording medium and to be modulated by the recording medium, a photosensitive unit formed of four photosensitive elements (D 2a , D 1a , D 3a , D 4a ) disposed respectively in first, second, third and fourth quadrants divided by two perpendicular straight lines, each of said elements being rectangular in shape and having a cut-out portion formed at a respective corner thereof adjacent the intersection of said two perpendicular straight lines, whereby said photosensitive unit has a central cut-out portion that produces no output, and said photosensitive unit is responsive to only the outer peripheral portion of a diffraction pattern from the modulated reading light beam incident thereon for producing detected outputs based on said outer peripheral portion surrounding said central cut-out portion of said diffraction pattern formed on an exit pupil surface of said lens (7) by the reading light beam, and a synthesizing circuit (28) for producing a first sum output by adding the detected outputs from two photosensitive elements (D 2a , D 3a ) located in said first and third quadrants out of said four photosensitive elements (D 2a , D 1a , D 3a , D 4a ) and a second sum output by adding the detected outputs from two photosensitive elements (D 1a , D 4a ) located in said second and fourth quadrants out of said four photosensitive elements (D 2a , D 1a , D 3a , D 4a ), and for generating a signal for controlling said optical head on the basis of sum and difference signals indicative of the sum of and the difference between said first and second sum signals.