Dynamic information storage or retrieval – Binary pulse train information signal – Including sampling or a/d converting
Reexamination Certificate
2000-06-30
2004-01-06
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Binary pulse train information signal
Including sampling or a/d converting
C369S124050, C369S047170
Reexamination Certificate
active
06674706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information reproducing apparatus and an information recording medium, and more particularly relates to an information recording medium in which a shape of an information pit is changed by multiple steps in accordance with a value of digital information to be recorded and accordingly the digital information is multi-level-recorded, and an information reproducing apparatus for reproducing the digital information, which is multi-level-recorded, from the information recording medium.
2. Description of the Related Art
There is a research with regard to a technique for recording or reproducing multi-value digital information having a plurality of values in addition to “0” or “1” onto or from an information recording medium in order to record or reproduce a large amount of digital information onto or from an optical disc as an information recording medium at a high density.
As a conventional technique in this field, an information recording and reproducing method employing a so-called “pit edge multi-level recording and reproducing method” is known. This multi-level recording and reproducing method is to record the digital information by changing a position of an edge (implying both ends of an information pit of an optical disc existing in a rotation direction thereof, and hereafter implying the same) of the information pit formed at a certain cycle on a track on an information record surface of the optical disc, by multiple steps in the rotation direction in accordance with a value of the digital information to be recorded, and to reproduce the digital information recorded in this manner from the information record surface of the optical disc. For example, the information recording and reproducing method employing this pit edge multi-level recording and reproducing method is disclosed in detail in Japanese Patent Application Laid Open No. Hei.6-76303.
Here, at a time of reproduction in the pit edge multi-level recording and reproducing method, a reproduction light beam is irradiated with respect to the pit edge of the optical disc, on which the digital information is multi-level-recorded by changing the position of the pit edge by multiple steps. Then, a multiple level judgment using a plurality of preset thresholds is performed with respect to an analog detection signal obtained by receiving the reflection light from the pit. Accordingly, the digital information is reproduced.
Next, the operation at a time of reproduction in the pit edge multi-level recording and reproducing method will be described below with reference to FIG.
14
.
FIG. 14
is a diagram describing the operation in which the digital information is reproduced by irradiating a reproduction light beam to only one track at a time.
As shown on a top stage of
FIG. 14
, in the conventional pit edge multi-level recording method, an information pit row is constituted by information pits PT on a track TR at a constant cycle, and a position of a pit edge at each information pit PT is changed, for example, by three steps (in case that the digital information has a multi-level of three values) in a rotation direction of the optical disc, in accordance with the above-mentioned value of the digital information. In addition, on the top stage of
FIG. 14
, the positions that can be considered as the position of the pit edge in each information pit PT are all shown by using solid lines. However, a pit edge is actually formed at any one position among those three pit edge positions (similarly in
FIG. 15
described later).
On the other hand, when the reproduction light beam is irradiated with respect to only one track at a time to thereby reproduce the digital information, the light beam irradiated to the optical disc generates a beam spot BS having an irradiation range indicated by a circle on the top stage of FIG.
14
. Then, since this beam spot BS is moved on the track TR indicated by a solid line, it is possible to sequentially read the digital information that is multi-level-recorded in one pit edge.
At this time, one track signal indicative of a position of the information pit on the optical disc (in other words, a positional relation between a land area in which the information pit is not formed and an area in which the information pit is formed) is alternately changed between a positive area (indicative of a range of the information pit) and a negative area (indicative of a range of the land area) in accordance with a position of each pit edge, as shown on a second stage from the top stage of FIG.
14
. Also, a beam profile indicative of a change of an irradiation strength of the light beam with the beam spot BS on the optical disc as a center has the distribution as shown on a second stage from a bottom stage of FIG.
14
.
When the light beam is irradiated in the manner as shown on the top stage to the second stage from the bottom stage of
FIG. 14
, an analog detection signal detected on the basis of the reflection light has a waveform as shown on the bottom stage of FIG.
14
. At this time, a waveform of the analog detection signal corresponds to a waveform, in which the waveform indicative of the beam profile and the one track signal are multiplied by each other.
Incidentally, the bottom stage of
FIG. 14
shows a plurality of kinds of waveforms having the possibility that they may be detected as the analog detection signal in such a way that all of them overlap with each other. However, the actual analog detection signal has one waveform among those waveforms shown on the bottom stage of FIG.
14
. Moreover, the bottom stage of
FIG. 14
shows the case that a level of the analog detection signal is made higher when the information pit PT is read in (similarly in
FIG. 15
as described later).
At this time, as shown on the bottom stage of
FIG. 14
, an output analog detection signal is separated into multiple levels corresponding to the positions of the pit edge, at a timing corresponding to an average change point from the information pit to the land area or from the land area to the information pit (this phenomenon is typically referred to as an “Eye Open”). Then, the analog detection signal ise sampled at the timing corresponding to this average change point. Moreover, the sampled values are threshold-judged on the basis of the two thresholds shown on the bottom stage of FIG.
14
. Accordingly, the value of the digital information recorded in each pit edge is recovered/reproduced.
According to such a pit edge multi-level recording and reproducing method, the digital information can be recorded or reproduced at a density higher than that of the conventional case of recording or reproducing the digital information in which only “0” and “1” are used as their values.
However, in the above-mentioned conventional pit edge multi-level recording and reproducing method of example, if an angle between an optical axis of the reproduction light beam and an information record surface on which the information pit PT is formed is deviated from a right angle because of deflection or distortion of the optical disc (hereafter, this drift is referred to as a “tilt drift”), the shape of the beam spot BS formed on the information record surface of the optical disc is deformed to thereby mix a level fluctuation component into the analog detection signal at the time of the reproduction. Moreover, if the analog detection signal is threshold-judged as mentioned above in the condition that the level fluctuation component is mixed, this leads to a problem that an error is induced in the judged result.
That is, as shown in
FIG. 15
, when the tilt drift causes the shape of the beam spot BS formed, on the information record surface to be deformed (e.g., elliptically deformed in FIG.
15
), the symmetry with a line passing through the central point of the beam spot BS in the beam profile as a symmetric axis is destroyed (refer to the second stage from the bottom stage of
FIG. 14 and a
second stage from a bottom stage of FIG.
15
). Thus, the waveform o
Morgan & Lewis & Bockius, LLP
Pioneer Corporation
Tran Thang V.
Vuong Bach
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