Focus control system for optical read-out apparatus

Claim 1

In apparatus for reading a record carrier on which information is recorded in the form of a succession of optically readable regions disposed along a predetermined track, in combination: (a) a light source for emitting a beam of light; (b) an objective for focusing the light beam on the record carrier; (c) means for creating a relative scanning motion between the record carrier and the light beam so that the light beam follows the predetermined track on the record carrier and so is modulated by the information recorded thereon; (d) first, second, and third photodetector elements arranged to be irradiated by the information-modulated light beam and aligned in such a direction that the successive images of the optically readable regions on the record carrier travel thereover, so that the three photodetector elements produce electrical outputs having a phase relationship varying in accordance with the distance between the record carrier and the objective; (e) a time-delay circuit, connected to the second photodetector element which is disposed intermediate the first and the third photodetector elements, for delaying the output therefrom for a prescribed length of time; (f) a first phase comparator, connected to the first photodetector element and to the time-delay circuit, for producing an output representative of the phase difference between the outputs therefrom; (g) a second phase comparator, connected to the time-delay circuit and to the third photodetector element, for producing an output representative of the phase difference between the outputs therefrom; (h) a differential amplifier, connected to the first and the second phase comparators, for producing an output representative of the difference in magnitude between the outputs therefrom; and (i) means responsive to the output from the differential amplifier for controllably varying the distance between the record carrier and the objective in order to hold the light beam focused on the record carrier.