Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1992-09-30
1994-07-12
Dzierzynski, Paul M.
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369 13, 369100, 369116, 360 59, 360114, G11B 1110, G11B 700
Patent
active
053295129
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method for reproducing signals from an optical recording medium in which signals are read while a light beam is radiated on the recording medium. More particularly, it relates to a method for reproducing signals from the recording medium capable of reproducing the information recorded with high recording density.
BACKGROUND ART
An optical recording medium may be roughly classified into a read-only medium, such as a so-called compact disc, and a medium on which signals can be recorded, such as a magneto-optical disc. With any of these optical recording media, it is desired to improve the recording density to a higher level. It is because a data volume several to more than ten times that of digital audio signals is required when recording digital video signals and because a demand is raised for reducing the size of the recording medium, such as a disc and hence the size of a product, such as a player, even when recording digital audio signals.
Meanwhile, the recording density of recording the information on the recording medium is governed by the S/N ratio of the playback signals. In the typical conventional optical recording and reproduction, the total area of a beam spot SP, which is a radiation region of the readout beam, such as the laser beam for the optical recording medium, as shown in FIG. 1, is a playback signal region. Thus the reproducible recording density is governed by the diameter D.sub.SP of the beam spot of the readout beam.
If, for example, the diameter D.sub.SP of the beam spot SP of the readout laser beam is lesser than a pitch q of a recording pit RP, two recording pits cannot be present in the spot SP, and the playback output waveform is as shown at B in FIG. 1, so that the playback signals can be read. However, if the recording pits SP are formed at a higher density, and the diameter D.sub.SP of the beam spot SP becomes larger than the pitch q of the recording pit RP, as shown at C in FIG. 1, two or more pits may be present simultaneously in the spot SP, so that the playback output waveform becomes substantially constant as shown at D in FIG. 1. In this case, these two recording pits cannot be reproduced separately, so that reproduction becomes infeasible.
The spot diameter D.sub.SP depends on the wavelength .lambda. of the laser beam and on the numerical aperture NA. It is this spot diameter D.sub.SP that governs the pit density along the scanning direction of the read-out beam or the recording track direction, or the so-called line density, and the track density conforming to the track interval between neighboring tracks in a direction at right angles to the scanning direction of the readout beam, or the so-called track pitch. The opto-physical limits of the line density and the track density are set by the wavelength .lambda. of the readout beam source and the numerical aperture NA of the objective lens and the read-out limit of 2NA/.lambda. is generally accepted as long as the spatial frequency at the time of signal reproduction is concerned. For this reason, for achieving high density of the optical recording medium, it is necessary to diminish the wavelength .lambda. of the light source of the reproducing optical system, such as a semiconductor laser, as well as to enlarge the numerical aperture NA of the objective lens.
The present Applicant has already proposed an optical recording medium in which the recordable line recording density as well as the track density may be increased without changing the spot diameter of the readout beam spot, and a method for reproducing the optical recording medium. The optical recording medium capable of reproducing the high density information in this manner may be enumerated by a magneto-optical recording medium capable of recording information signals and a variable reflectance type optical recording medium at least capable of reproducing information signals.
The above-mentioned magneto-optical recording medium includes a magnetic layer, such as a rare earth-transition metal alloy thin film, dep
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patent: 5218581 (1993-06-01), Ohta et al.
patent: 5230930 (1993-07-01), Aratani et al.
Patent Abstracts of Japan, vol. 11, No. 389, Dec. 1987.
Fukimoto Atsushi
Ono Masumi
Udagawa Toshiki
Yasuda Kouichi
Yoshimura Shunji
Chu Kim-Kwok
Dzierzynski Paul M.
Sony Corporation
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