Apparatus for reading a record carrier in which information, for example video and/or audio information, is recorded in at least one track

Claim 1

In an apparatus for reading a record carrier in which information is recorded in at least one optically interogatable longitudinally moving track, a focus detection system comprising a source of radiation providing a read beam of radiation, at least two optical detection elements spaced along the longitudinal direction of said track for converting optically modulated light into electrical signals, a lens system means in the path of said read beam for focussing said read beam to a radiation spot on said track that is substantially the same size as the smallest recorded detail on said record carrier whereby said information on said track modulates said read beam and for converging the radiation beam modulated by said track towards said two optical detection elements, whereby said optical detection elements receive radiation modulated by different portions of said track when said read beam is improperly focussed, and a phase detector connected to said at least two optical detection elements for measuring the phase difference between said electrical signals from said optical detection elements, said phase difference indicating the extent of the improper focus of said read beam on said track.