Focus correction system for video disc player

Claim 1

In an optical reproducing system in which a multi-turn storage track of a video record is scanned by a focused reading beam of energy to derive information stored in said track, such information including a frequency modulated carrier signal and a focus-servo pilot signal having a frequency which is low relative to that of said carrier and exhibiting the same spatial phase distribution in each turn of said track, a focus correction system comprising: a displaceable focusing lens disposed in the path of said reading beam; means, including a photoreceptor comprising first and second light responsive elements symmetrically disposed with respect to the axis of said reading beam and arranged in a spaced apart confronting relation with the gap separating said elements disposed substantially normal to that portion of said track under scansion, each said element comprising a mask portion extending across said element adjacent to and paralleling said gap for effectively shielding a predetermined area of said element from said reading beam, said elements being positioned in the far field of said reading beam so as to be responsive to the scanning of said track by said beam for developing a pair of signal components representative of the spatial pattern of said pilot signal, said components having amplitudes and a phase relation indicative of the degree and sense of the out-of-focus condition, if any, of said beam relative to said track, with one of said components effectively constituting a reference signal for the other said component; a phase shifter coupled to the output of said first element for shifting the phase of said signal component developed by said first element with respect to said signal component developed by second element an amount sufficient to establish, in the absence of an out-of-focus condition, said components as a pair of quadrature related signals; a synchronous detector responsive to said pair of quadrature related signal components for developing a focus-error correction signal having an amplitude and polarity determined by the degree and sense of out-of-focus condition; and actuating means coupled to said focusing lens and responsive to said correction signal for displacing said lens an amount and in a direction proportional to the amplitude and polarity of said correction signal to maintain said beam focused on said track.