U.S. patent number RE39,067 [Application Number 10/397,891] was granted by the patent office on 2006-04-18 for apparatus and method for optical disk reproduction.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Takuya Asano, Hiromichi Ishibashi, Takashi Kishimoto.
United States Patent |
RE39,067 |
Ishibashi , et al. |
April 18, 2006 |
**Please see images for:
( Certificate of Correction ) ** |
Apparatus and method for optical disk reproduction
Abstract
An apparatus of the present invention is an optical disk
apparatus for irradiating with a laser beam an optical disk on
which address marks are recorded at intervals based on a pulse
width modulation method and for reproducing data from a reproduced
signal based on reflected light. The apparatus includes: a pulse
signal reproduction section for producing a reproduced pulse signal
from the reproduced signal based on a threshold value; a threshold
value production section for producing the threshold value based on
the reproduced pulse signal; and a gate signal production section
for producing an address gate signal at a timing when the address
mark is reproduced.
Inventors: |
Ishibashi; Hiromichi (Ibaraki,
JP), Kishimoto; Takashi (Nara, JP), Asano;
Takuya (Saijo, JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Kadoma, JP)
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Family
ID: |
17725815 |
Appl.
No.: |
10/397,891 |
Filed: |
March 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
09175649 |
Oct 20, 1998 |
06208604 |
Mar 27, 2001 |
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Foreign Application Priority Data
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Oct 21, 1997 [JP] |
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9-288101 |
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Current U.S.
Class: |
369/59.17;
369/47.1; 369/30.01 |
Current CPC
Class: |
G11B
7/005 (20130101); G11B 7/00745 (20130101); G11B
27/3027 (20130101); G11B 27/19 (20130101); G11B
7/131 (20130101); G11B 20/10203 (20130101) |
Current International
Class: |
G11B
7/00 (20060101) |
Field of
Search: |
;369/124.01,44.25,44.34,44.35,30.01,30.04,30.07,30.27,47.1,47.15,47.22,47.27,47.31,47.47,53.1,59.1,59.13,59.17 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0757343 |
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Feb 1995 |
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EP |
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0 692 787 |
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May 1995 |
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EP |
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06592787 |
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May 1995 |
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EP |
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0 757 343 |
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Feb 1996 |
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EP |
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8-031092 |
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Feb 1996 |
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JP |
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08031092 |
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Feb 1996 |
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JP |
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9-237459 |
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Sep 1997 |
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JP |
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09237459 |
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Sep 1997 |
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JP |
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Other References
European Search Report dated Sep. 12, 1999 for Application No.
98119853.4. cited by examiner.
|
Primary Examiner: Edun; Muhammad
Attorney, Agent or Firm: Renner, Otto, Boisselle &
Sklar, LLP
Claims
What is claimed is:
1. An optical disk apparatus for irradiating with a laser beam an
optical disk on which address marks are recorded at intervals based
on a pulse width modulation method and for reproducing data from a
reproduced signal based on reflected light, the apparatus
comprising: a pulse signal reproduction section for producing a
reproduced pulse signal from the reproduced signal based .[.an.].
.Iadd.on .Iaddend.a threshold value; a gate signal production
section for producing an address gate signal at a timing when the
address mark is reproduced; and a threshold value production
section for producing the threshold value based on the reproduced
pulse signal and the address gate signal.
2. An optical disk apparatus according to claim 1, wherein the
threshold value production section produces the threshold value in
such a manner that a duty ratio of the reproduced pulse signal
reproduced by the pulse signal reproduction section becomes
substantially constant.
3. An optical disk apparatus according to claim 1, wherein the gate
signal production produces the address gate signal based on an
envelope off the reproduced pulse signal produced from the
reproduced signal.
4. An optical disk apparatus according to claim 3, wherein the gate
signal production section further comprises a section for, if
address data is detected in the reproduced pulse signal, producing
the address gate signal at a timing which is delayed by a
predetermined period of time from a timing at which the address
data is detected.
5. An optical disk apparatus according to claim 1, further
comprising a photoelectric converter including a light-receiving
portion which is divided along a tracking direction into at least
two light-receiving areas, for receiving the reflected light from
the optical disk by the light-receiving areas so as to produce at
least two reproduced signals, wherein the gate signal production
section produces a pulse signal based on a difference between the
two reproduced signals and produces the address gale signal based
on an envelope of the pulse signal.
6. An optical disk apparatus according to claim 5, wherein the
address marks include a first address mark and a second address
mark which are arranged on opposite sides of, and at a
predetermined distance from, a track center line.
7. An optical disk apparatus according to claim .[.5.].
.Iadd.6.Iaddend., wherein: a first pulse signal is produced from a
first difference signal which is produced from the first address
mark based on a first threshold value; a second pulse signal is
produced from a second difference signal which is produced from the
second address mark based on a second threshold value; and the
address gate signal is produced from an envelope of the first and
second pulse signals.
8. An optical disk apparatus according to claim .[.5.].
.Iadd.7.Iaddend., wherein: the first and second threshold values
are controlled by feedback control in such a manner that respective
duty ratios of the first pulse signal and the second pulse signal
become substantially constant; and the optical disk apparatus
further includes a section for temporarily holding the feedback
control while no address gate signal is produced.
9. An optical disk apparatus according to claim 1, further
comprising a photoelectric converter including a light-receiving
portion which is divided along a tracing direction and another
direction perpendicular to the tracking direction into at least
four light-receiving areas, for receiving the reflected light from
the optical disk by the light-receiving areas so as to produce at
least four reproduced signals, wherein the gate signal production
section produces the address gale signal based on a phase
difference between two of the reproduced signals which are output
from diagonally-located two of the light-receiving areas.
10. An optical disk apparatus according to claim 9, wherein the
address marks include a first address mark and a second address
mark which are arranged on opposite sides of, and at a
predetermined distance from, a tract center line.
11. An optical disk apparatus according to claim 9, wherein the
gate signal production further comprises: a section for producing a
pulse signal from a difference between two of the reproduced
signals which are output from two of the light-receiving areas
located on opposite sides of a track center line and for producing
the address gate signal from an envelope of the pulse signal; and a
section for, if address data is detected in the reproduced pulse
signal, producing the address gate signal at a timing which is
delayed by a predetermined period of time from a timing at which
the address data is detected.
12. A method for reproducing an optical disk, for irradiating with
a laser beam an optical disk on which address marks are recorded at
intervals based on a pulse width modulation method and for
reproducing data from a reproduced signal based on reflected light,
the method comprising the steps of: producing a reproduced pulse
signal from the reproduced signal based on a threshold value;
producing an address gate signal at a timing when the address mark
is reproduced; and producing the threshold value based on the
reproduced pulse signal and the address gate signal.
13. A method for reproducing an optical disk according to claim 12,
further comprising the steps of: receiving the reflected light from
the optical disk by at least two light-receiving areas divided
along a tracking direction so as to produce at least two reproduced
signals; producing a pulse signal from a difference between the two
reproduced signals; and producing the address gate signal from an
envelope of the pulse signal.
14. A method for reproducing an optical disk according to claim 12,
further comprising the steps of: receiving the reflected light from
the optical disk by at least four light-receiving areas divided
along a tracking direction and another direction perpendicular to
the tracking direction so as to produce at least four reproduced
signals; producing the address gate signal based on a phase
difference between two of the reproduced signals which are output
from diagonally-located two of the light-receiving areas.
.Iadd.15. An apparatus for producing a reproduced pulse signal from
a reproduced signal, wherein the reproduced signal is based on
reflected light of a laser beam, the laser beam is irradiated on an
optical disk on which address marks are recorded at intervals based
on a pulse width modulation method, the apparatus comprising: a
pulse signal reproduction section for producing the reproduced
pulse signal from the reproduced signal based on a threshold value;
a gate signal production section for producing an address gate
signal at a timing when the address mark is reproduced; and a
threshold value production section for producing the threshold
value based on the reproduced pulse signal and the address gate
signal..Iaddend.
.Iadd.16. The apparatus according to claim 15, wherein the
threshold value production section produces the threshold value in
such a manner that a duty ratio of the reproduced pulse signal
reproduced by the pulse signal reproduction section becomes
substantially constant..Iaddend.
.Iadd.17. The apparatus according to claim 15, wherein the gate
signal production produces the address gate signal based on an
envelope of the reproduced pulse signal produced from the
reproduced signal..Iaddend.
.Iadd.18. The apparatus according to claim 15, wherein the gate
signal production section further comprises a section for, if
address data is detected in the reproduced pulse signal, producing
the address gate signal at a timing which is delayed by a
predetermined period of time from a timing at which the address
data is detected..Iaddend.
.Iadd.19. The apparatus according to claim 15, wherein the address
marks include a first address mark and a second address mark which
are arranged on opposite sides of, and at a predetermined distance
from, a track center line..Iaddend.
.Iadd.20. The apparatus according to claim 19, wherein: a first
pulse signal is produced from a first difference signal which is
produced from the first address mark based on a first threshold
value; a second pulse signal is produced from a second difference
signal which is produced from the second address mark based on a
second threshold value; and the address gate signal is produced
from an envelope of the first and second pulse
signals..Iaddend.
.Iadd.21. The apparatus according to claim 20, wherein: the first
and second threshold values are controlled by feedback control in
such a manner that respective duty ratios of the first pulse signal
and the second pulse signal become substantially constant; and the
optical disk apparatus further includes a section for temporarily
holding the feedback control while no address gate signal is
produced..Iaddend.
.Iadd.22. A method for producing a reproduced pulse signal from a
reproduced signal, wherein the reproduced signal is based on
reflected light of a laser beam, the laser beam is irradiated on an
optical disk on which address marks are recorded at intervals based
on a pulse width modulation method, the method comprising the steps
of: producing a reproduced pulse signal from the reproduced signal
based on a threshold value; producing an address gate signal at a
timing when the address mark is reproduced; and producing the
threshold value based on the reproduced pulse signal and the
address gate signal..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk apparatus for use
with an optical disk to/from which data can be recorded and
reproduced. More particularly, the present invention relates to an
apparatus and a method for reproducing signals from any optical
disk recording medium via an optical head, which are capable of
reproducing address data with improved accuracy.
2. Description of the Related Art
The popularity of optical disk apparatuses such as CD (compact
disk) players and DVD (digital videodisk) players has been growing
considerably in recent years. A large-capacity optical disk
apparatus which allows a user to record data on an optical disk is
expected to be commercially available in the near future.
FIG. 9 is a block diagram illustrating part of a conventional
optical disk apparatus. An optical head 1001 irradiates a mark
string 1000 formed on a recording surface of an optical disk
recording medium with a laser beam and detects the reflected light
therefrom as an electric signal HF. A comparator 1002 compares the
amplitude of the signal HF with a threshold potential VTH and
outputs a reproduced pulse signal DT A charge pump 1003 charges a
capacitor 1004 while the reproduced pulse signal DT is at a high
level, and discharges the capacitor 1004 while the reproduced pulse
signal DT is at a low level. The electric potential of the
capacitor 1004 is provided to the comparator 1002 and used as the
threshold potential VTH.
The operation of the conventional optical disk apparatus having
such a structure will now be described. Binary (digital) data is
recorded on the optical disk. More specifically, a mark string
including a plurality of concave (or convex) marks is formed on the
optical disk in accordance with the binary data to be recorded. The
optical head 1001 reproduces a sinusoidal signal (not a pulse-like
digital signal) from the mark string because of inter-symbol
interference between adjacent marks. An appropriate threshold
potential value VTH is provided to produce a pulse signal. Any
amplitude in the sinusoidal signal greater than the threshold VTH
is determined as a high level amplitude, whereas any amplitude less
than the threshold VTH is determined as a low level amplitude.
As illustrated in the left-hand side of FIG. 10, when the threshold
potential VTH is relatively low, the reproduced pulse signal DT
becomes wide on the high level side and narrow on the low level
side. As a result, the capacitor 1004 is charged more than it is
discharged, thereby increasing the threshold potential VTH. Thus,
the threshold potential VTH is controlled so that the average
amount of current charged into the capacitor 1004 substantially
equals the average amount of current discharged therefrom. In other
words, the threshold potential VTH is controlled so that the
average length of the "H" period of the reproduced pulse signal DT
(a period during which the signal DT is at the high level) and the
average length of the "L" period thereof (a period during which the
signal DT is sit the low level) are equal or at least closer to
each other. The ratio between the "H" period and the "L" period is
referred to as "the duty ratio" of the signal DT.
Such an optical disk apparatus may be advantageously used with
recording media such as CDs and DVDs, where data is recorded based
on the PWM (pulse width modulation) method. The PWM method is a
recording method suitable in high density recording applications,
where the length of a recording mark varies in accordance with the
data to be recorded. When reproducing binary data based on the PWM
method, however, even a slight shift in the threshold value VTH may
cause an error in the pulse length of the reproduced pulse signal
DT, thereby resulting in a reproduction error.
In view of this, feedback control may be constantly performed for
the threshold value VTH so that the duty ratio of the reproduced
pulse signal is substantially constant as described above, thereby
reproducing data without an error (see Japanese Laid-open
Publication No. 63-201957).
However, such a method assumes that data is recorded continuously
without interruption. When PWM data segments (data segments which
are recorded based on the PWM method) exist at intervals on the
optical disk, the threshold value VTH follows (varies in accordance
with) noise when an optical head sans over an area with no recorded
data.
As described above, the conventional method is used for reproducing
data from read-only media such as CDs and DVDs, where data is
continuously recorded based on the PWM method across the entire
surface of the disk. However, an optical disk and an optical disk
apparatus which allow a user to record data on the disk ate
expected to be commercially available in the near future. While
several different recording formats have been proposed, such a
recordable optical disk typically includes address areas and data
areas which are arranged alternately at predetermined intervals. In
the data area, a film (e.g., a phase change material film or a
magneto-optical recording film) to which data can be recorded by
laser heat is provided. In the address area, address data has been
recorded as concave or convex marks. Since a recordable optical
disk is also desired to have a higher recording density, the PWM
method should be applied to the address area as well as to the data
area. However, such a "recordable" medium may have a data area with
no recorded data. In such a case, only the address area has PWM
concave or convex marks (marks recorded based on the PWM method).
When this recordable optical disk is reproduced by the conventional
method, the unrecorded area is reproduced as a long low-level
signal. In response, the feedback control system tries to decrease
the threshold value VTH as low as possible. Thus, the threshold
value VTH follows the noise, and the optical disk apparatus
generates undesired signals by digitizing the noise. As a result,
it is not possible to identify the correct address.
SUMMARY OF THE INVENTION
According to one aspect of this invention, an optical disk
apparatus is provided for irradiating with a laser beam an optical
disk on which address marks are recorded at intervals based on a
pulse width modulation method and for reproducing data from a
reproduced signal based on reflected light. The apparatus includes:
a pulse signal reproduction section for producing a reproduced
pulse signal from the reproduced signal based on a threshold value;
a threshold value production section for producing the threshold
value based on the reproduced pulse signal; and a gate signal
production section for producing an address gate signal at a timing
when the address mark is reproduced.
In one embodiment of the invention, the threshold value production
section produces the threshold value in such a manner that a duty
ratio of the reproduced pulse signal reproduced by the pulse signal
reproduction section becomes substantially constant.
In one embodiment or the invention, the gate signal production
produces the address gate signal based on an envelope of the
reproduced pulse signal produced from the reproduced signal.
In one embodiment: of the invention, the gate signal production
section further includes a section for, if address data is detected
in the reproduced pulse signal, producing the address gate signal
at a timing which is delayed by a predetermined period of time from
a timing at which the address data is detected.
In one embodiment of the invention, the optical disk apparatus
further includes a photoelectric converter including a
light-receiving portion which is divided along a tracking direction
into at least two light-receiving areas, for receiving the
reflected light from the optical disk by the light-receiving areas
so as to produce at least two reproduced signals. The gate signal
production section produces a pulse signal based on a difference
between the two reproduced signals and produces the address gate
signal based on an envelope of the pulse signal.
In one embodiment of the invention, the address marks include a
fast address mark and a second address mark which are arranged on
opposite sides of, and at a predetermined distance from, a track
center line.
In one embodiment off the invention, a first pulse signal is
produced from a first difference signal which is produced from the
first address mark based on a first threshold value. A second pulse
signal is produced from a second difference signal which is
produced from the second address mark based on a second threshold
value. The address gate signal is produced from an envelope of the
first and second pulse signals.
In one embodiment of the invention, the first and second threshold
values are controlled by feedback control in such a manner that
respective duty ratios of the first pulse signal and the second
pulse signal become substantially constant. The optical disk
apparatus further includes a section for temporarily holding the
feedback control while no address gate signal is produced.
In one embodiment of the invention, the optical disk apparatus
further includes a photoelectric converter including a
light-receiving portion which is divided along a tracking direction
and another direction perpendicular to the tracking direction into
at least four light-receiving areas, for receiving the reflected
light from the optical disk by the light-receiving areas so as to
produce at least four reproduced signals. The gate signal
production section produces the address gate signal based on a
phase difference between two of the reproduced signals which are
output from diagonally-located two of the light-receiving
areas.
In one embodiment of the invention, the address marks include a
first address mark and a second address mark which are arranged on
opposite sides of, and at a predetermined distance from, a track
center line.
In one embodiment of the invention, the gate signal production
further includes: a section for producing a pulse signal from a
difference between two of the reproduced signals which are output
from two of the light-receiving areas located on opposite sides of
a track center line and for producing the address gate signal from
an envelope of the pulse signal; and a section for, if address data
is detected in the reproduced pulse signal, producing the address
gate signal at a timing which is delayed by a predetermined period
of time from a timing at which the address data is detected.
According to another aspect of this invention, a method is provided
for reproducing an optical disk, for irradiating with a laser beam
an optical disk on which address marks are recorded at intervals
based on a pulse width modulation method and for reproducing data
from a reproduced signal based on reflected light. The method
includes the steps of: producing a reproduced pulse signal from the
reproduced signal based on a threshold value; producing the
threshold value based on the reproduced pulse signal; and producing
an address gate signal at a timing when the address mark is
reproduced.
In one embodiment of the invention, the method further includes the
steps of receiving the reflected light from the optical disc by at
least two light-receiving areas divided along a tracking direction
so as to produce at least two reproduced signals; producing a pulse
signal from a difference between the two reproduced signals; and
producing the address gate signal from an envelope of the pulse
signal.
In one embodiment of the invention, the method further includes the
steps of receiving the reflected light from the optical disk by at
least four light-receiving areas divided along a tracking direction
and another direction perpendicular to the tracking direction so as
to produce at least four reproduced signals; producing the address
gate signal based on a phase difference between two of the
reproduced signals which are output from diagonally-located two of
the light-receiving areas.
Thus, the invention described herein makes possible the advantages
of (1) providing an optical disk apparatus capable of selling and
maintaining an appropriate threshold value even when reproducing a
recordable optical disk which includes address areas and data areas
arranged alternately at predetermined intervals; and (2) providing
such an optical disk reproduction method.
These and other advantages of the present invention will become
apparent to those skilled in the art upon reading and understanding
the following detailed description with reference to the
accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating an optical disk apparatus
according to Example 1 of the present invention;
FIG. 2 is a timing diagram illustrating an operation of the optical
disk apparatus according to Example 1 of the present invention;
FIG. 3 is a block diagram illustrating an optical disk apparatus
according to a variation of Example 1;
FIG. 4 is a block diagram illustrating an optical disk apparatus
according to Example 2 of the present invention;
FIG. 5 is a timing diagram illustrating an operation of the optical
disk apparatus according to Example 2 of the present invention;
FIG. 6 is a block diagram illustrating an optical disk apparatus
according to Example 3 of the present invention;
FIG. 7 is a timing diagram illustrating an operation of the optical
disk apparatus according to Example 3 of the present invention;
FIG. 8 is a diagram illustrating an operation of the optical disk
apparatus according to Example 3 of the present invention;
FIG. 9 is a block diagram illustrating a conventional optical disk
apparatus; and
FIG. 10 is a timing diagram illustrating an operation of the
conventional optical disk apparatus of FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples 1 to 3 of the present invention will now be described with
reference to the accompanying figures.
According to Example 1, it is possible to detect the address area
and reproduce the address data therefrom so as to set a threshold
value, which is held only in an unrecorded area, thereby
successively and accurately reproducing PWM address data segments
which exist at intervals.
According to Example 2, even when the PWM address data marks are
shifted by a predetermined distance from a track center line, it is
possible to detect the address area and reproduce the address data
therefrom so as to set a threshold value, which is held only,
thereby successively and accurately reproducing PWM address data
segments which exist at intervals.
According to Example 3, even when the PWM address data marks are
shifted by a predetermined distance from a track center line, it is
possible to detect the address area, and the detection of the
address data is free from disturbance caused by a tracking
offset.
EXAMPLE 1
Example 1 of the present invention will now be described in detail.
FIG. 1 is a block diagram illustrating an optical disk apparatus
according to Example 1. An address area 100 is provided on the
recording surface of an optical disk recording medium. Address data
is recorded in the address area 100 as an address mark string
(e.g., presence and absence of a concave (or convex) mark). An area
provided between two adjacent address areas 100 is used as a data
area 101. In the present example, it is assumed that no data is
recorded in the data area 101.
An optical head 1 irradiates the recording surface of the optical
disk with a laser beam, converts the reflected light therefrom into
an electric signal HF, and outputs the electric signal HF A
comparator 2 compares the amplitude of the signal HF with a
threshold potential VTH and outputs a reproduced pulse signal ADR.
A charge pump 3 charges a capacitor 4 while the reproduced pulse
signal ADR is at a high level, and discharges the capacitor 4 while
the reproduced pulse signal ADR is at a low level. The electric
potential of the capacitor 4 is provided to the comparator 2 and
used as the threshold potential VTH. Thus, the comparator 2, the
charge pump 3 and the capacitor 4 together form a loop for
generating the threshold value VTH while performing feedback
control of the threshold value VTH.
The operation of the optical disk apparatus having such a structure
will now be described. The optical bead 1 reproduces the string
including concave (or convex) address marks which have been
recorded on the optical disk as the signal HR The signal UP has a
sinusoidal waveform due to the inter-symbol interference occurring
between adjacent marks. Therefore, an appropriate feedback control
is performed for the threshold value VTH so that any amplitude
greater than the threshold VTH is determined as a high level
amplitude, whereas any amplitude less than the threshold VTH is
determined as a low level amplitude, thus obtaining binary address
data.
When the threshold potential VTH is relatively low, the reproduced
pulse signal ADR becomes wide on the high level side and narrow on
the low level side. As a result, the capacitor 4 is charged more
than it is discharged, thereby increasing the threshold potential
VTH. Conversely, when the threshold potential VTH is relatively
high, the capacitor 4 is discharged more than it is charged,
thereby decreasing the threshold potential VTH. Thus, the threshold
potential VTH is controlled so that the average amount of current
charged into the capacitor 4 substantially equals the average
amount of current discharged therefrom. In other words, the
threshold potential VTH is controlled so that the average length of
the "H" period of the reproduced pulse signal ADR (a period during
which the signal ADR is at the high level) and the average length
of the "L" period thereof (a period during which the signal ADR is
at the low level) are equal or at least closer to each other.
Referring to FIG. 2, when the optical head 1 scans the unrecorded
data area 101 where no data has been recorded in the initial stage
of the feedback loop, the threshold potential VTH controlled by the
feedback loop follows a base level (a potential level of the
reproduced signal HF when there is no recorded signal). Even when
there is "no recorded signal", there exists a small amount of
noise. Therefore, the threshold value VTH is controlled so that the
duty ratio of the pulse signal becomes substantially constant. When
the optical head 1 then scans the address area 100, the threshold
value VTH is increased so that the duty ratio of the reproduced
pulse signal ADR in the address area 100 becomes substantially
constant.
Without the present invention, when the feedback control is
continued after scanning the address area 100, the threshold value
VTH follows the base level (the "no signal" level) again as the
optical head 1 enters the next unrecorded data area 101. If the
address area 100 is provided with sufficient length, the behavior
of the threshold value VTH in the unrecorded data area 101 may not
cause a significant problem. However, to have such an increased
length for the address area 100, the length (and thus the recording
capacity) of data area 101 is sacrificed because the total capacity
of the disk is fixed. Therefore, the length of the address area 100
should be minimized.
When the address area 100 is too short, however, the laser beam
passes over the address area 100 before the threshold value VTH
reaches a desired level. Although it is possible to avoid such a
problem by increasing the speed at which the threshold value VTH
varies (e.g., by decreasing the capacitance of the capacitor 4, or
by increasing the current output of the charge pump 3), it would
still be impossible to distinguish data obtained by digitizing
address data from data obtained by digitizing noise.
In view of this, the optical disk apparatus according to Example 1
is provided with a section for appropriately holding the threshold
value VTH while scanning the unrecorded data area 101 based on an
address gate signal ADRG. More specifically, the feedback loop is
opened by opening a switch 6 while the address gate signal ADRG is
at a low level (e.g., while scanning the unrecorded data area 101).
Thus, the threshold value VTH at the time when the address gate
signal ADRG goes low is maintained. The feedback control of the
threshold value VTH resumes when the address gate signal ADRG goes
high at the beginning of the next address area 100. Thus, feedback
control (such that the duty ratio becomes substantially constant)
is performed at intervals only while scanning the address areas
100. Accordingly, it is possible to obtain the reproduced pulse
signal ADR which has been digitized with the optimal threshold
value VTH. The reproduced pulse signal ADR is then provided to an
address decoder, thereby accurately reproducing the address data
segments.
An exemplary method for producing the address gate signal ADRG will
now be described. Since address data segments are typically
provided periodically, once an address data segment is detected, it
is possible to estimate the time when the next address segment
should be reproduced and to produce the address gate signal ADRG at
the estimated time. Referring to FIG. 1, as soon as an address mark
detection section 7 (including, for example, a pattern match
detection circuit) detects an address identification flag in the
reproduced pulse signal ADR, the address mark detection section 7
generates a trigger pulse to activate a timer counter 8. Upon
receiving the trigger pulse, the timer counter 8 starts counting
clock pulses from a quartz oscillator 81. After a predetermined
time interval, the timer counter 8 generates a gate signal, which
is provided to the switch 6 as the address gate signal ADRG. The
time interval can he uniquely determined by the address interval
defined by the optical disk format and the linear velocity of the
optical disk.
While the above-described operation assumes that an address mark
can be detected without the address gate signal ADRG, there is no
address gate signal ADRG provided when detecting the first address
mark after the start-up of the apparatus or after a track jump.
Detecting an address without using the address gate signal ADRG but
only using the reproduced pulse signal ADR may not be successful in
some situations. For example, one of the pulse signal strings
obtained by digitizing noise may happen to have a data pattern that
is identical to that of an address mark, whereby subsequent
operations may be performed at erroneous timings based on this
erroneously-detected address.
In view of this, in the present example, the fast address mark is
detected using a re-triggerable monostable multi-vibrator 3, not
the address mark detection section 7, thereby ensuring detection of
the first address mark.
Referring to FIG. 1, the re-triggerable monostable multi-vibrator 3
holds a fixed high (or low) level output for a certain period of
time (a "holding period") once the monostable multi-vibrator 5
receives a pulse edge. If the monostable multi-vibrator 5 receives
another pulse edge during the holding period, the monostable
multi-vibrator 5 begins a new holding period. As a result, the
obtained signal substantially represents an envelope of the
reproduced pulse signal ADR, as illustrated in FIG. 2. While
scanning an address area 100, pulse edges of the reproduced pulse
signal ADR are successively provided to the monostable
multi-vibrator 5. Therefore, while scanning an address area 100, by
setting the holding period to be longer than the possible longest
address mark, it is ensured that a pulse edge is input before a
holding period ends, thereby maintaining the output of the
monostable multi-vibrator 5 at the high level.
The obtained signal can be used as the address gate signal ADRG to
allow the threshold value VTH to be appropriately varied and held
without using the address mark detection section 7, thereby
accurately reproducing the address data. In this method, however,
spike-like noise may occur in an unrecorded area, and an erroneous
address gate signal may be generated therefrom. In view of this,
the monostable multi-vibrator 5 is used to generate the address
gate signal ADRG, only before the first address mark is detected.
Once an address mark is detected, the operation mode is switched to
the mode where the timer counter 8 is employed. Referring to FIG.
1, a switch 9 receives an instruction from a controller 10 and
switches the mode of detecting the address gate signal ADRG.
As described above, according to the present example, address data
segments can be accurately reproduced from an optical disk on which
PWM data segments exist at intervals.
In the present example, a single comparator (the comparator 2) is
used to generate a pulse signal to be provided to the monostable
multi-vibrator 5 for address gate generation and to generate a
pulse signal to be provided to the address decoder. However,
separate comparators may alternatively be provided without
departing from the spirit of the present invention.
FIG. 3 illustrates a variation of Example 1. Referring to FIG. 3, a
comparator 202, a charge pump 203 and a capacitor 204 together form
a separate threshold value control circuit which products a
reproduced pulse signal ADR used for address reproduction. Since
the threshold value control circuit does not have a function to
hold the threshold value VTH, a pulse signal may be generated by
digitizing noise as described above. However, this an be eliminated
by using an AND gate 205 to which the address gate signal ADRG is
provided through one of the terminals thereof.
An advantage of this variation is that the respective threshold
value control circuits for gate generation and for address
reproduction an be independently optimized. It is typically
desirable that the threshold value VTH varies at a relatively high
speed for address reproduction and at a relatively low speed for
gate generation which involves the holding operation.
EXAMPLE 2
Example 2 of the present invention will now be described in detail.
FIG. 4 is a block diagram illustrating an optical disk apparatus
according to Example 2. Referring to FIG. 4, in the address area
100 on the optical disk address mark strings 100a and 100b are
formed on opposite sides of, and a particular distance from, the
tract center line. The track center line corresponds to the center
of a mark recorded in the data area 101, and is an ideal track for
the laser beam from an optical head to scan while recording and
reproducing marks to/from the data area 101. An optical head 11
includes a photosensor which is divided into two photosensor
elements 11a and 11b along the track center line. A differential
amplifier 12 is used to obtain a difference signal based on
respective outputs from the photosensor elements 11a and 11b.
Comparators 13 and 14 produce binary pulse signals ADRP and ADRN
from the difference signal based on threshold values VTHP and VTHP,
respectively. An OR gate 15 (an addition section) logically adds
the binary pulse signals ADRP and ADRN to provide the logical sum
as the reproduced pulse signal ADR. The charge pump 3, the
capacitor 4, the monostable multi-vibrator 5 and the switch 6,
respectively, function in the same manner as those illustrated in
FIG. 1. A differential output amplifier 16 provides a positive
output and a negative output to the comparators 13 and 14 as the
threshold values VTHP and VTHN, respectively.
The reason for separating the address mark strings 100a and 100b
from each other, as illustrated in FIG. 4, is as follows. When the
track density of the data area is increased, a read error rate is
also increased due to crosstalk noise between adjacent tracks.
However, a read error rate to a certain degree can be tolerated
since data is recorded in the data area with an error correction
code attached thereto. However, such an error correction code is
not typically attached to address data because as soon as an
optical disk drive recognizes the address data in an address area,
the drive has to start recording or reproducing data to/from the
data area following the address area, and therefore there is no
time for the optical disk drive to perform an error correction
process for the address data. Thus, in order to suppress a
eliminate the influence of the crosstalk noise, address marks are
typically arranged at a track pitch which is twice as great as that
used in the data area, while the address marks are shifted from the
track center line by 1/2 track pitch so that the address marks an
be equally reproduced along any one of the tracks on which the
marks are present.
When the laser beam from the optical head 11 scans along the track
center line off the optical disk, the laser beam is diffracted by
the address mark strings 100a and 100b. More specifically, the
address mark strings 100a and 100b cause optical interference which
results in a diffraction pattern on the photosensor elements 11a
and 11b, The address mark strings 100a and 100b are detected based
on the brightness of the diffraction pattern. Consequently, the
address mark strings 100a and 100b can be considered as being
detected by the photosensor elements 11a and 11b, respectively. A
signal DHF is obtained as the difference between the respective
outputs from the photosensor elements 11a and 11b, Thus, the
address represented by the address mark strings 100a and 100b can
be detected as the signal DHF. As illustrated in FIG. 5, the signal
DHF goes high and low with respect to the base level in accordance
with the address mark strings 100a and 100b.
As described above, the threshold values VTHP and VTHN used by the
comparators 13 and 14, respectively, are provided as the positive
and negative outputs from the differential output amplifier 16.
Thus, the threshold values VTHP and VTHN respectively vary
symmetrically with respect to the base potential, as illustrated by
broken line A in FIG. 5 in accordance with an input signal (the
potential at the terminal of the capacitor 4). The comparator 13
generates a pulse signal ADRP which is at a high level while the
signal DHF is over the threshold value VTHP The comparator 14
generates a pulse signal ADRN which is at a high level while the
signal DHF is below the threshold value VTHN. The pulse signals
ADRP and ADRN are logically added together to obtain the reproduced
pulse signal ADR. Therefore, this detection system is substantially
equivalent to that illustrated in FIG. 1 (which uses only one
comparator) for the charge pump 3 and the capacitor 4 of the
feedback control section. Thus, feedback control is performed so
that the pulse duty ratio of the reproduced pulse signal ADR
(=ADRP+ADRN) becomes substantially constant. As in Example 1, the
reproduced pulse signal ADR is then provided to an address decoder,
thereby accurately reproducing the address data segments.
Thus, it is possible to perform the threshold value VTH feedback
control at intervals in the same manner as described in Example 1
by operating the switch 6 using the output of the monostable
multi-vibrator 5 as the address gate signal ADRG. Also as described
in Example 1, after the first address mark is detected by the
address mark detection section 7, the switch 9 is operated by the
controlled 10 so that a pulse signal generated by the timer counter
8 is used as the address gale signal ADRD.
As described above, according to the present example, the address
data can be reproduced from a pair of address mark strings 100a and
100b (which are shifted from each other with respect to the track
center line) as accurately as when the address mark strings are
arranged along the track center line.
EXAMPLE 3
Example 3 of the present invention will now be described in detail.
FIG. 6 is a block diagram illustrating an optical disk apparatus
according to Example 3. Referring to FIG. 6, in the address area
100 on the optical disk, the address mark strings 100a and 100b are
formed on opposite sides of, and a particular distance from, the
track center line, in the same manner as illustrated in FIG. 3. An
optical head 21 includes a photosensor which is divided into four
photosensor elements 21a, 21b, 21c and 21d along the track center
line and a direction perpendicular thereto. The photosensor
elements 21a, 21b, 21c and 21d output signals HF21a, HF21b, HF21c
and HF21d, respectively. An addition amplifier 22 computes and
outputs HF21a+HF21b, and another addition amplifier 23 computes and
outputs HF21c-HF21d. The output sum signals HF21a+HF21b and
HF21c+HF21d are substantially equivalent to the signals HFA and HFB
in FIG. 4. An addition amplifier 24 computes and outputs
HF21a+HF21c, and another addition amplifier 25 computes and outputs
HF21b+HF21d. The differential amplifier 12, the comparators 13 and
14, the OR gate 15, the differential output amplifier 16, the
charge pump 3, the capacitor 4 and the switch 6 function in the
same manner as those illustrated in FIG. 3. Thus, the feedback
control system for the threshold values VTHP and VTHN also
functions in the same manner as that of Example 2.
A distinctive feature of the present example is the use of an
additional detection section, in addition to the monostable
multi-vibrator 5, for holding the threshold value feedback control.
Comparators 26 and 27 digitize the sum signals HF21a+HF21c and
HF21b+HF21d, respectively. An EXOR gate 28 detects a phase
difference between respective pulse signals output from the
comparators 26 and 27. Using the photosensor elements 21a, 21b, 21c
and 21d and the addition amplifiers 24 and 25, a received light
beam is appropriately divided into two portions, from which first
and second reproduced signals are produced, respectively. The
comparators 26 and 27 and the EXOR gate 28 detect a relative phase
difference between the first and second reproduced signals. A
determination section 29 determines whether the phase difference is
greater than a predetermined value (VO), and outputs a pulse signal
HLD based on the determination. The output signal HID is used to
operate the switch 6.
The operation of the optical disk apparatus having such a structure
will now be described. In an actual optical disk drive, a tracking
control is performed so as to accurately scan a laser beam along
the data segments recorded in the data areas 101. Referring to FIG.
6, a tracking groove 100c has been provided in an optical disk, and
a tracking error signal TE is obtained using the tracking groove
100c. A tracking actuator 21e is controlled so that the tracking
error signal TE is kept substantially at a target value. The output
of the differential amplifier 12 can be used directly as the
tracking error signal TE. When the laser beam is shifted from the
tracking groove 100c, the reflected light becomes asymmetric
between the photosensor elements 21a and 21b and the photosensor
elements 21c and 21d, thereby generating a difference between the
respective detected signal amplitudes, which is used as the tracing
error signal. In particular, the obtained difference is provided
back to the tracking actuator 21e of the optical head 21 via a
drive amplifier 30 so as to perform the feedback tracking control.
However, since the address mark strings 100a and 100b are shifted
from the track center line, the address mark strings 100a and 100b
create tracking disturbance. When the laser beam scans the address
area 100, the tracking feedback system functions to shift the laser
beam toward the address mark string 100a first, and then toward
address mark string 100b. Since the response of the tracking
actuator 21e is not so fast, the actual shitting of the laser beam
toward the address mark strings 100a and 100b occurs with some
delay. As a result, in the beginning of the data area 101, the
laser beam is shifted from the track center line, and then starts
winding about the tracking groove until the laser beam starts
running stably along the tracking center line (as indicated by
broken line D in FIG. 7). Accordingly, the difference signal DHF
also winds widely for a while and then stares converging to the
base level.
When the difference signal DHF winds widely and goes beyond the
threshold value VTHP or VTHN, the monostable multi-vibrator 5 is
activated, as indicated by "A" in FIG. 7. With the structure
illustrated in FIG. 4, the switch 6 would be operated, thereby
starting an erroneous threshold value control, as indicated by the
broken line B in FIG. 7. In view of this, the optical disk
apparatus of the present example uses the following means to
effectively identify the address mark strings 100a and 100b which
are shifted from the track center tine.
First, the addition amplifiers 24 and 25 output the diagonal sum
signals HF21a+HF21c and HF21b+HF21d, respectively. The signals
HF21a+HF21c and HF21b+HF21d are converted into pulse signals by the
comparators 26 and 27, respectively, and then provided to the EXOR
gate 28. As will be discussed later, the comparators 26 and 27 and
the EXOR gate 28 together form a type of phase comparison section.
The determination section 29 produces a pulse signal ADRHLD which
is at a high level when a phase difference EXOUT is greater than a
predetermined value. The switch 6 is operated by the pulse signal
ADRHLD.
It is known that when the laser beam scans along the concave (or
convex) marks provided in a non-continuous, broken pattern on an
optical disk, there is generally some relative phase difference
between the diagonal sum signals HF21a+HF21c and HF21b+HF21d in
accordance with the amount by which the scanning track is shifted
from the track center line (see, for example, Japanese Publication
for Opposition No. 5-80053). This is believed to be due to optical
diffraction caused by an edge of each mark along the tangential
direction of the disk. Since the address mark strings 100a and 100b
are shifted from the track center line, when the laser beam scans
approximately along the track center line, the diagonal sum signals
HF21a+HF21c and HF21b+HF71d have a phase difference with respect to
each other, as illustrated in FIG. 7.
This will be briefly discussed with reference to FIG. 8. Assuming
that the laser beam scans substantially along the track center
line, the address mark string 100a is projected onto the
photosensor elements 21a and 21b. As the laser beams moves on, the
projected image moves from the photosensor element 21b toward the
photosensor element 21a. Therefore, the diagonal sum signal
HF21a+HF21c for the pair of the photosensor elements 21a and 21c,
which are hatched in FIG. 8, has a phase lag with respect to the
other diagonal sum signal HF21b+HF21d for the other pair of the
photosensor elements 21b and 21d, as illustrated in FIG. 8.
Similarly, the address mark string 100b is projected onto the
photosensor elements 21c and 21d. Then, the diagonal sum signal
HF21a+HF21c has a phase lead with respect to the other diagonal sum
signal HF21b+HF21d, as illustrated in FIG. 8. Thus, it is possible
to distinguish the address area 100 from other areas by detecting
the phase difference between the diagonal sum, signals and
comparing the phase difference with a predetermined value. In the
present example, the diagonal sums signals are converted into pulse
signals EXIN1 and EXIN2 by the comparators 26 and 27 and then input
to the EXOR gate 28. The EXOR gate 28 outputs the pulse signal
EXOUT having a width in accordance with the phase difference,
regardless of whether it is a phase lead or a phase lag. The
determination section 29 smoothes the phase difference pulse signal
EXOUT using a low pass filter, and produces the pulse signal ADRHLD
which is at a high level while the value of the smoothed signal
represents is greater than a predetermined value VO.
Since the winding of the difference signal DHF due to the vibration
of the objective lens actuator is caused by a tracking groove,
which is formed continuous manner unlike the recording marks, such
winding does not cause a phase difference. Even if a slight phase
difference is generated, it is not recognized by the comparators 26
and 27. Therefore, the outputs EXIN1 and EXIN2 are not influenced
by the tracking groove. Thus, the EXOR gate 28 outputs the pulse
signal EXOUT which is purely based on the address mark strings 100a
and 100b. In other words, the address mark strings 100a and 100b
are detected by detecting the above-described phase difference,
thereby avoiding an erroneous detection due to such winding.
By using the phase difference between the diagonal sum signals as
described above, it is possible to distinguish the address area 100
from other areas without being influenced by the tracking control
residue. However, according to this method, the pulse signal
representing the phase difference has to be smoothed through the
low pass filter, thereby generating a detection delay (see ADRSHLD
in FIG. 7). Particularly; when the falling edge is delayed
(indicated by "C" in FIG. 7), the gate is opened in an arcs other
than the address area 100, the threshold value varies toward the
base level before starting a holding operation. Although the amount
of delay can be reduced by increasing the cut-off frequency of the
low pass filter, the smoothing efficiency is then reduced, whereby
the optical disk apparatus may malfunction due to a ripple
component. In view of this, in the present example, the delay of
the signal is eliminated by employing the output of the monostable
multi-vibrator 5 (which functions in the same manner as described
in the preceding examples) and an AND gate 91. As in the preceding
examples, after the first address mark is detected by the address
mark detection section 7, a switch 92 is operated by the controller
10 so that a pulse signal generated by the timer counter 8 is used
as the address gate signal ADRG.
Although it is assumed in the above-described examples that the
data area 101 (a tracking groove) is unrecorded, recording marks
may be recorded in the data area 101 in practice. It is believed
that such recording marks would not influence the operation of the
above-described examples. A significant phase difference is
generated between the diagonal sum signals only when the laser beam
scans over concave (or convex) marks formed on the recording
surface of the optical disk. A very slight or no phase difference
is generated when the laser beam scans over flat marks in the data
area 101 which are formed by varying the refractive index of a
portion of the film material by heating the recording film (e.g., a
phase change film) with a laser beam. The phase difference
generated by address mark strings and the phase difference
generated by data recording marks can be distinguished from each
other by setting an appropriate threshold value VO. Moreover, the
marks recorded substantially along the track center line are
equally projected onto the photosensor elements 21a and 21b and the
photosensor elements 21c and 21d. Therefore, these signal
components are cancelled out by the differential calculation
performed by the differential amplifier 12, thereby preventing the
recording mark components from being mixed in the difference signal
DHF. As a result, the address gate signal ADRG produced by the
monostable multi-vibrator 5 is not influenced.
As described above, the optical disk apparatus of the present
example is provided with a section for detecting the phase
difference between the diagonal sum signals based on the output
signals from the group of photosensor elements. Therefore, it is
possible to accurately detect the address data even when there is
some tracking winding.
In Examples 2 and 3, a single set of comparators (the comparators
13 and 14) is used to generate a pulse signal to be provided to the
monostable multi-vibrator 5 for address gale generation and to
generate a pulse signal to be provided to the address decoder.
However, without departing from the spirit of the present
invention, separate sets of comparators may alternatively, be
provided. As in the variation of Example 1, a separate comparator
can be separately provided for generating a pulse signal to be
provided to the address decoder, where a noise component can be
removed by an AND gate.
As described above, it is possible to detect the address area and
reproduce the address data therefrom so as to set a threshold
value, which is held only in an unrecorded area, thereby
successively and accurately reproducing PWM address data segments
which exist at intervals.
Moreover, even when the PWM address data marks are shifted by a
predetermined distance from a tract center line, it is possible to
detect the address area and reproduce the address data therefrom so
as to set a threshold value, which is held only, thereby
successively and accurately reproducing PWM address data segments
which exist at intervals.
Furthermore, even when the PWM address data marks are shifted by a
predetermined distance from a track center line, it is possible to
detect the address area, and the detection of the address data is
free from disturbance caused by a tracking offset.
Various other modifications will be apparent to and can be readily
made by those skilled in the art without departing from the scope
and spirit of this invention. Accordingly, it is not intended that
the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *