Optical data disc reader - balances difference between output of pair of photodetectors sensing laser beam spot reflected from diskette

摘要

The difference between the output signals of the photodetectors (4) is determined, pref. by a differential amplifier (10). The intensity of one of the output signals is changed, pref. by a variable-gain amplifier (11) so that the difference is reduced to zero. The output signal of one photodetector element (PD2) is extended to the non-inverting input terminal of the differential amplifier via the variable gain amplifier. The output of the other photodetector element (PD1) is extended to the inverting input terminal. The differential amplifier supplies an absolute time decoder (8) and has a feedback loop (15) to the variable gain amplifier. USE/ADVANTAGE - Reading diskette with information stored in wobble groove. Reads information in absolute time even if reflected beam spot is deflected.