| 摘要 |
In a video and/or audio disc player wherein a laser beam is focused by an objective on a record disc for scanning same along a spiral information track thereon, a focus control is provided which comprises a converging lens disposed across the path of the information-modulated laser beam. Two or three photodetector elements are disposed on one of the conjugate planes of the converging lens the other of which substantially coincides with a focal plane of the objective. Aligned in the direction in which the successive images of pits on the record disc travel thereover, the photodetector elements produce electrical outputs having a phase relationship representative of the variable distance between record disc and objective. A focus control circuit produces an output signal corresponding to the phase relationship of the outputs from the photodetector elements, for delivery to a drive coil capable of electromagnetically moving the objective toward and away from the record disc for readjustment of the distance therebetween. |
