PURPOSE: To make reproduction without using any additional optical system and the like by a novel focusing method by providing an information protecting layer on a signal disc carrier.
CONSTITUTION: A protecting layer 22 is provided on a signal disc carrier 21 and signal read light 23 is entered. In order to maintain the distance between a condensing lens 24 and the signal face constant, a focus error signal is detected with a photo detector 27 and by the correction servo signal thus obtained, the focusing lens 24 is electromagnetically driven in the optical axis direction, whereby the focus error is corrected. The phase difference corresponding to the focus error is converted to an error voltage by a phase comparator 33. To this end, the output of a photo detector 26 is high-frequency amplified 29 and is limited to a fixed amplitude with a limiter 32, after which it is applied to a phase comparator 33. On the other hand, the output of the photo detector 27 is amplified 28 and is subjected to phase shifting 30 by a fixed quantity, after which it is applied to the other input terminal of the comparator 33, thence the voltage proportional to the phase difference is drawin out by an LPF 34.
KASHIHARA TOSHIAKI
HOURAI KEIICHIROU
| JPS4910057A | 1974-01-29 |
Next Patent: PRODUCTION OF OXIDE MAGNETIC THIN FILM MAGNETIC RECORDING MEDIUM