GERAET ZUR WIEDERGABE VON DATEN.

摘要

An optical CD disc system uses three laser beams that operate together with photo detectors to control focussing and tracking. For focal control the 4 photo detectors (A B C D) are arranged in a group. Any diagonal offset in the detector indicates a focussing error. A separate pair of photodetectors (EF) respond to two outer beams (L2,L3) to allow tracking error detection. The output of the focus error detector elements are summed in an amplifier to generate an offset compensation signal that is supplied to a phase comparator together with the data signals. The output is processed to generate a correction.