Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2000-06-29
2004-07-06
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
Reexamination Certificate
active
06760299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disc on which information is recorded in advance by means of pits having recessed and protruded shapes formed partially or entirely over a recording surface. More specifically, the present invention provides a technique for increasing recording capacity, preventing reduction in the capacity of recording main information by utilizing the increased capacity for recording additional information and enabling effective use of the recorded additional information. More specifically, the present invention relates to an optical disc of which pit depth represents information, an optical disc device for reproduction of the same, and the method of reproduction.
2. Description of the Background Art
A conventional optical disc is for binary recording, in which binary data correspond to presence/absence of pits.
When density of the disc is to be increased, the size of the pit is made smaller, and a laser beam spot for reading the same is also made smaller. Besides, multi-value recording in which one pit represents multi-valued data is also an effective means to attain higher density.
Japanese Patent Laying-Open No. 58-215735, for example, proposes an optical disc enabling recording of multi-valued data by changing pit depth in a plurality of stages so that quantity of light reflected therefrom changes in multiple steps. In this method, however, it is difficult to determine the level of the reflected light quantity, resulting in increased errors in the reproduced data. Japanese Patent Laying-Open No. 5-205276, alternatively, discloses a method of reproducing multi-valued recorded data by combining the level of the reflected light quantity and a push-pull signal level.
In an optical disc on which information is recorded by forming recessed and protruded pits in advance, as currently represented by a CD (Compact Disc) or a DVD (Digital Versatile Disc), the information is recorded in accordance with the method referred to as pit length recording, in which the information is mainly represented by presence/absence of a pit and the length of the pit.
FIGS. 1A
to
1
C are illustrations related to reproduction of a conventional optical disc on which information is recorded in accordance with the pit length recording method.
Referring to
FIG. 1A
, when a light beam
1101
directed from a pick up, not shown, onto the optical disc comes to a pit
131
, the quantity of reflected light varies as shown in
FIG. 1B
because of interference and diffraction phenomenon of the light caused by the pit. When the reflected light is condensed on a photodetector, taken out as an electric signal and binarized by the comparison with a prescribed reference voltage, a reproduction data such as shown in
FIG. 1C
is obtained. By detecting presence/absence as well as the length of pit
131
based on the data, it becomes possible to reproduce the information. This is the principal of reproducing information from the conventional optical disc on which recording is done by the pit length recording method.
A CD and a DVD are much different in capacity. The difference derives from the difference in density of tracks, which are string of pits, as well as the difference in size of the pits formed on the discs. Further, optical wavelength and NA (Numerical Aperture) of the objective lens used are also different. More specifically, for a CD, NA of the objective lens is about 0.4 and the laser wavelength is about 780 nm to about 830 nm, whereas for a DVD, NA is 0.6 and the laser wavelength is 650 nm. The difference in the optical system results in the difference of the light beam.
The information recorded on the optical disc may be classified into the information eventually required by the user, that is, main information, and additional information for enabling efficient reproduction of the main information and for improving reliability of the main information.
Sound, image and characters may be the main information. The additional information may include index information, navigation information or the like for efficient reproduction, superimposed characters for a motion picture, subvoice, error correction code for improving reliability of the main information and address information representing a position on a disc. Recently, information for preventing unauthorized copying and a technique referred to as an electronic water mark, embedding information for protection within the main information have been studied, as techniques for preventing unauthorized reproduction of the information on the optical disc, and these may also be considered as additional information.
Existence of such additional information tends to increase the ratio of the additional information to the total capacity of the optical disc. With the limited recording capacity of the optical disc, increase of the additional information means decrease of the main information. To solve this problem, it is necessary to increase the recording capacity by improving recording density of the optical disc.
In order to increase the surface recording density of the optical disc, however, it is necessary to form smaller pits with high density, and to change optical system of a pickup for reproduction to generate smaller optical beam spot, from the reasons as described with reference to the difference in capacities of the CD and the DVD.
Japanese Patent Laying-Open No. 11-66607, for example, discloses a related technique. In this technique, as shown in
FIG. 1
or
FIG. 3
of this laid-open application, general main information is recorded as high density pits P
0
for the CD, and large and low density pits P
1
enclosing a number of pits P
0
are recorded superposed.
The information of the high density pits (main information) is reproduced by an optical beam spot having the diameter of about 2 &mgr;m directed from a CD reading optical head HO shown in
FIG. 2
of this application, while the information of low density (additional information) is reproduced by an optical beam having the diameter of about 500 &mgr;m directed from a low density reading optical head H
1
having a larger spot diameter.
In the method disclosed by Japanese Patent Laying-Open No. 5-205276, it is necessary to provide a beam spot for obtaining a push-pull signal, separate from the beam spot to obtain the reflected quantity of light, namely, two beam spots are necessary. When it is to be implemented by only one beam spot, it becomes necessary to offset the beam spot from the center of the track, or to cause relative positional deviation between the beam and the center of the track, by wobbling the track.
Such approach does not ensure stable tracking control, and therefore it is prone to deviation from the track or error in reproduction.
The technique disclosed by Japanese Patent Laying-Open No. 11-66607requires two different types of optical heads (pickups). This leads to increased manufacturing cost and a larger scale device. Further, control of an optical head (pickup) for reproducing the additional information is also necessary. As the additional information is formed as large pits, the amount of recordable additional information is limited, and therefore decrease in the recording capacity of the main information caused by recording of the additional information cannot be avoided.
Various tracking servo techniques for positioning an optical beam on a pit string have been proposed for an optical disc reproducing device for reproducing an optical disc on which information is recorded by pits having recessed and protruded shapes formed in advance on a disc surface. Japanese Patent Laying-Open No. 58-150145 discloses an example.
FIG. 2
is a block diagram of the tracking servo in accordance with a phase difference (time difference) method disclosed by Japanese Patent Laying-Open No. 58-150145.
According to the phase difference (time difference) method, the reflected light beam from the optical disc is received by a photodetector having four elements divided along the radial direction of the optical disc and a
Nakajima Junsaku
Nomura Masaru
Takeuchi Hitoshi
Conlin David G.
Edwards & Angell LLP
Sharp Kabushiki Kaisha
Tran Thang V.
Tucker David A.
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