U.S. patent application number 09/818967 was filed with the patent office on 2001-08-16 for optical disk apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Iwata, Katsuo, Kashihara, Yutaka.
Application Number | 20010014067 09/818967 |
Document ID | / |
Family ID | 13344469 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014067 |
Kind Code |
A1 |
Iwata, Katsuo ; et
al. |
August 16, 2001 |
Optical disk apparatus
Abstract
An optical disk apparatus comprises an optical pick-up unit for
reproducing information on an optical disk, a first reproducer for
processing an information signal obtained by the pick-up unit by
one of a waveform slice method and a PRML method, and producing a
first reproduction signal, a second reproducer for detecting a
minimum value of a level of the information signal and producing a
second reproduction signal, and a switch for selecting, as a
reproduction information signal, one of the first and second
reproduction signals, in accordance with a recording density of the
optical disk to be reproduced.
Inventors: |
Iwata, Katsuo; (Tokyo,
JP) ; Kashihara, Yutaka; (Tokyo, JP) |
Correspondence
Address: |
Pillsbury Winthrop LLP
Intellectual Property Group
East Tower, Ninth Floor
1100 New York Avenue, N.W.
Washington
DC
20005-3918
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
|
Family ID: |
13344469 |
Appl. No.: |
09/818967 |
Filed: |
March 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09818967 |
Mar 28, 2001 |
|
|
|
09271270 |
Mar 17, 1999 |
|
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Current U.S.
Class: |
369/53.2 ;
369/59.13; G9B/19.022; G9B/7.018; G9B/7.029; G9B/7.039;
G9B/7.102 |
Current CPC
Class: |
G11B 7/1398 20130101;
G11B 7/139 20130101; G11B 7/007 20130101; G11B 2007/0006 20130101;
G11B 7/005 20130101; G11B 19/128 20130101; G11B 7/24085
20130101 |
Class at
Publication: |
369/53.2 ;
369/59.13 |
International
Class: |
G11B 007/005 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 1998 |
JP |
10-067422 |
Claims
1. An optical disk apparatus, comprising: a pick-up unit which
reads out information from an optical disk, the information being
recorded on the optical disk in a form of pits, and produces an
information signal by photoelectric conversion; a first reproducer
which processes the information signal by one of a waveform slice
method and a PRML method, and produces a first reproduction signal;
a second reproducer which detects a minimum value of a level of the
information signal and produces a second reproduction signal; and a
switch which selects, as a reproduction information signal, one of
the first reproduction signal produced by the first reproducer and
the second reproduction signal produced by the second reproducer,
in accordance with a recording density of the optical disk to be
reproduced.
2. The optical disk apparatus according to claim 1, wherein the
information is recorded on the optical disk by pulse width
modulation recording, and the second reproducer comprises a
differential circuit which differentiates the information signal, a
rising zero-cross detector which detects a zero-cross point of a
differential waveform, a rising detector which detects a rising of
an output pulse from the rising zero-cross detector, and a PWM
circuit which subjects an output signal from the rising detector to
pulse width modulation.
3. The optical disk apparatus according to claim 2, wherein the
second reproducer includes a correction circuit, provided at a rear
stage of the PWM circuit, for correcting a PWM signal in accordance
with record data.
4. The optical disk apparatus according to claim 3, wherein said
correction circuit comprises a unit delay element which delays a
bit signal of the PWM signal by one bit, and an OR gate which
produces a logical sum between the bit signal of the PWM signal and
the delayed bit signal.
5. The optical disk apparatus according to claim 3, wherein said
correction circuit comprises a unit delay element which delays a
bit signal of the PWM signal by one bit, and an AND gate which
produces a logical product between the bit signal of the PWM signal
and the delayed bit signal.
6. The optical disk apparatus according to claim 2, further
comprising a filter which emphasizes one of an amplitude of the
differential waveform and a peak of the reproduction waveform.
7. The optical disk apparatus according to claim 1, further
comprising a control unit which detects a location where a
reproduction signal level takes a maximum value at a pit center
portion, and controls tracking.
8. The optical disk apparatus according to claim 1, wherein the
pick-up unit includes a beam spot shape varying unit which
selectively varies a shape of a beam spot in accordance with a
location where the reproduction signal takes a minimum value.
9. The optical disk apparatus according to claim 1, wherein the
pick-up unit includes a beam spot shape varying unit which
selectively varies a shape of a beam spot when a tracking is
performed.
10. An optical disk apparatus comprising: a pick-up unit including
a light source, an optical system which focuses a light beam
emitted from the light source onto an optical disk, and a
photo-detector which detects reflection light from the disk, the
pick-up unit reproducing information recorded on the disk along a
predetermined track; and an opening limiting unit which limits an
opening of the objective lens such that numerical apertures NA1r
and NA1t in a disk radial direction and a track tangential
direction of the objective lens in a case where said optical disk
is a first disk optimized with respect to a wavelength .lambda.1 of
the light source, and numerical apertures NA2r and NA2t in the disk
radial direction and the track tangential direction of the
objective lens in a case where said optical disk is a second disk
optimized with respect to a wavelength .lambda.2
(.lambda.1<.lambda.2) of the light source, satisfy formulae: 5
0.95 1 2 NAlr NA2r 1.1 1 2 NA1r 0.95 NAlt NA2t 1.1 NAlt
11. The optical disk apparatus according to claim 10, wherein the
first disk has a track pitch of 0.42 .mu.m and a minimum pitch
length of 0.23 .mu.m, the second disk has a track pitch of 0.74
.mu.m and a minimum pitch length of 0.4 .mu.m, and said .lambda.1,
.lambda.2, NA1r, NA1t, NA2r and NA2t are, respectively,
.lambda.1=390 nm to 420 nm, .lambda.2=650 nm to 780 nm,
NA1r=NA1t=0.6, NA2r=0.33 to 0.4, and NA2t=0.54 to 0.66.
12. The optical disk apparatus according to claim 10, wherein said
opening limiting unit is formed of a plate-like member with at
least one opening, which plate-like member can be set on a light
incident path to the objective lens.
13. The optical disk apparatus according to claim 10, wherein said
opening limiting unit is formed of a liquid crystal cell with at
least one opening, a size of which is varied by application of
voltage.
14. The optical disk apparatus according to claim 10, wherein said
opening limiting unit has at least an elliptic opening having a
short axis in the disk radial direction and a long axis in the
track tangential direction.
15. An optical disk apparatus comprising: a pick-up unit including
a light source, an objective lens which focuses a light beam
emitted from the light source onto an optical disk, and a
photo-detector which detects reflection light from the disk, the
pick-up unit reproducing information recorded on the disk along a
predetermined track; and an opening limiting unit which limits an
opening of the objective lens such that numerical apertures in a
disk radial direction and a track tangential direction of the
objective lens are made equal in a case where said optical disk is
a first disk having a predetermined recording density and optimized
with respect to a wavelength .lambda.1 of the light source, and the
numerical aperture in the disk radial direction of the objective
lens is made less than that in the track tangential direction of
the objective lens in a case where said optical disk is a second
disk having a recording density lower than the first disk and
optimized with respect to a wavelength .lambda.2
(.lambda.1<.lambda.2) of the light source.
16. The optical disk apparatus according to claim 15, wherein said
opening limiting unit is formed of a plate-like member with at
least one opening, which plate-like member can be set on a light
incident path to the objective lens.
17. The optical disk apparatus according to claim 15, wherein said
opening limiting unit is formed of a liquid crystal cell with at
least one opening, a size of which is varied by application of
voltage.
18. The optical disk apparatus according to claim 15, wherein said
opening limiting unit has at least an elliptic opening having a
short axis in the disk radial direction and a long axis in the
track tangential direction.
19. An optical disk apparatus comprising: a pick-up unit including
a light source, an objective lens which focuses a light beam
emitted from the light source onto an optical disk, and a
photo-detector which detects reflection light from the disk, the
pick-up unit reproducing information recorded on the disk along a
predetermined track; and an opening limiting unit which limits an
opening of the objective lens such that a beam spot shape on the
optical disk is the same in a disk radial direction and a track
tangential direction of the optical disk in a case where said light
source has a wavelength .lambda.1 and said optical disk is a first
disk having a predetermined recording density, and the beam spot
shape on the optical disk is large in the disk radial direction and
small in the track tangential direction in a case where said
optical disk is a second disk having a recording density lower than
the first disk and optimized with respect to a wavelength .lambda.2
(.lambda.1<.lambda.2) of the light source.
20. The optical disk apparatus according to claim 19, wherein said
opening limiting unit is formed of a plate-like member with at
least one opening, which plate-like member can be set on a light
incident path to the objective lens.
21. The optical disk apparatus according to claim 19, wherein said
opening limiting unit is formed of a liquid crystal cell with at
least one opening, a size of which is varied by application of
voltage.
22. The optical disk apparatus according to claim 19, wherein said
opening limiting unit has at least an elliptic opening having a
short axis in the disk radial direction and a long axis in the
track tangential direction.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an optical disk apparatus
using an optical disk.
[0002] In general, in an optical disk apparatus, a laser beam from
a laser diode is focused by an optical system, and an optical disk
is scanned with the focused beam. Thus, binary data recorded on the
optical disk is read.
[0003] Normally, when the focused beam is radiated on the optical
disk, if the entire'spot of the focused beam is located outside the
pits, the phase of all the reflected light is the same. Thus, no
decrease occurs in the amount of reflected light due to optical
interference. On the other hand, when a part of the spot of the
focused beam is within the pit, a phase difference occurs between
the reflected light from the inside of the pit and that from the
outside of the pit. Consequently, both reflected light components
interfere with each other and the amount of reflected light
decreases. In general, the optical disk apparatus is so designed
that the amount of reflected light becomes minimum when the center
of the focused beam spot lands on the center of the pit.
Specifically, when the entire of the focused beam spot is located
outside the pit as shown in FIG. 22, the level of the reproduced
signal is high. When a part of the focused beam spot begins to
overlap the pit, the level of the reproduced signal begins to
decrease. When the focused beam spot is located at the center of
the pit, the reproduced signal level takes a lowest value.
[0004] The density in operation of the optical disk apparatus has
increased every year. One of the techniques for achieving high
density is a technique for reducing the diameter of the focused
beam. This requires a decrease in wavelength of a laser and an
increase in NA (Numerical Aperture) of an objective lens. With the
reduction of the diameter of the laser beam, information can be
reproduced from smaller pits.
[0005] Even with the high-density optical disk apparatus wherein
the focused beam diameter is reduced, however, information needs to
be reproduced from a low-density optical disk which has already
been marketed. In this case, as is understood from the relationship
between the focused beam and the pit when information is to be
reproduced from the low-density optical disk by the high-density
optical disk apparatus, if the focused beam is located at the
center of the pit, most of the focused beam is located within the
pit as shown in FIG. 23 and the phase of most reflected light
becomes the same. At this time, a decrease in the amount of
reflected light due to interference is small. Specifically, when
the focused beam is located outside the pit, the reproduced signal
level is high. When the focused beam begins to overlap the pit, the
reproduced signal level decreases. When the focused beam is at the
center of the pit, the reproduced signal level increases once
again. This phenomenon in which the reproduced signal level
increases at the center of the pit is referred to as
"rebounding".
[0006] Because of the rebounding, the reproduced signal level
increases at the center of the pit, too, as shown in FIG. 24. Where
the reproduced waveform in this case is detected by a waveform
slice method, erroneous detection will occur even if the threshold
is set at any level. When information is reproduced from the
low-density optical disk by the high-density optical disk
apparatus, as described above, there is a problem in that an error
occurs in the signal detection result due to the rebounding.
[0007] In such an optical disk apparatus, there is a case where a
compatibility capable of reproducing information among various
types of optical disks having different recording densities such as
CD (CD-ROM, CD-R etc.), DVD RAM, high density DVD-ROM and high
density DVD-RAM of the coming generation is required.
[0008] Since, however, both the optimum wavelength of a light
source and the shape of a light beam spot on the optical disk vary
from optical disk type to optical disk type, it is generally
difficult to correctly reproduce information from the plural types
of optical disks by an optical disk drive using an optical head
having a single light source and a single objective lens. Moreover,
it is unfavorable to combine a plurality of light sources and a
plurality of objective lenses in order to allow information from
being reproduced from the plural types of optical disks having
different recording densities because the optical head is increased
in size and costs.
[0009] To resolve the above problem, for example, Jpn. Pat. Appln.
KOKAI publication No. 8-339572 proposes an optical disk drive whose
optical head is provided with a single light source, a single
objective lens, and an opening limitation element having a
plurality of openings of different sizes for limiting an opening of
the objective lens to allow information to be reproduced from a
plurality types of optical disks having different recording
densities. In this optical disk drive, the diameter of a light beam
spot on each of the optical disks is varied by the opening
limitation element in accordance with the size of a pit
corresponding to the recording density of an optical disk thereby
to reproduce information from the plurality of types of optical
disks having different recording densities.
[0010] On the other hand, in the conventional optical disk
apparatus, in order to enable information reproduction from a
plurality of kinds of optical disks with different recording
densities, the recording density (size of pits) of the optical
disks is merely considered and the opening size of an opening
limiting element is varied. Since the opening size of the opening
limiting element is not set in this apparatus in consideration of
the relationship among the recording density of the optical disk,
the light source wavelength and the pit depth of the optical disk,
good reproduction is not achieved in the case of information
reproduction from an optical disk which does not meet the
conditions for the light source wavelength and pit depth. For
example, the reproduced signal intensity decreases, or asymmetry of
reproduced signals increases.
BRIEF SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide an optical
disk apparatus wherein no error occurs in signal detection even if
rebounding occurs in a reproduced waveform.
[0012] Another object of the invention is to provide an optical
disk apparatus capable of exactly reproducing information from a
plurality of kinds of optical disks with different recording
densities, with a structure using a single light source and a
single objective lens.
[0013] The present invention provides an optical disk apparatus for
reproduction, comprising a pick-up unit which reads information
from an optical disk, the information being recorded on the optical
disk in a form of pits, and produces an information signal by
photoelectric conversion, a first reproducer which processes the
information signal by one of a waveform slice method and a PRML
method, and produces a reproduction signal, a second reproducer
which detects a minimum value of a level of the information signal
and producing a reproduction signal, and a switch which selects, as
a reproduction information signal, one of the reproduction signal
produced by the first reproducer and the reproduction signal
produced by the second reproducer, in accordance with a recording
density of the optical disk to be reproduced.
[0014] With the above structure, information can be reproduced from
a low-density optical disk even with use of a high-density optical
disk apparatus wherein a focused beam size is reduced.
[0015] This invention also provides an optical disk apparatus
comprising a pick-up unit including a light source, an objective
lens which focuses a light beam emitted from the light source onto
an optical disk, and a photo-detector which detects reflection
light from the disk, the pick-up unit reproducing information
recorded on the disk along a predetermined track, and an opening
limiting unit which limits an opening of the objective lens such
that numerical apertures NA1r and NA1t in a disk radial direction
and a track tangential direction of the objective lens in a case
where the optical disk is a first disk optimized with respect to a
wavelength .lambda.1 of the light source, and numerical apertures
NA2r and NA2t in the disk radial direction and the track tangential
direction of the objective lens in a case where the optical disk is
a second disk optimized with respect to a wavelength .lambda.2
(.lambda.1<.lambda.2) of the light source, satisfy formulae: 1
0.95 1 2 NA1r NA2r 1.1 1 2 NAlr 0.95 NAlt NA2t 1.1 NAlt
[0016] The present invention also provides an optical disk
apparatus comprising a pick-up unit including a light source, an
objective lens which focuses a light beam emitted from the light
source onto an optical disk, and a photo-detector which detects
reflection light from the disk, the pick-up unit reproducing
information recorded on the disk along a predetermined track, and
an opening limiting unit which limits an opening of the objective
lens such that numerical apertures in a disk radial direction and a
track tangential direction of the objective lens are made equal in
a case where the optical disk is a first disk having a
predetermined recording density and optimized with respect to a
wavelength .lambda.1 of the light source, and the numerical
aperture in the disk radial direction of the objective lens is made
less than that in the track tangential direction of the objective
lens in a case where the optical disk is a second disk having a
recording density lower than the first disk and optimized with
respect to a wavelength .lambda.2 (.lambda.1<.lambda.2) of the
light source.
[0017] Furthermore, this invention provides an optical disk
apparatus comprising a pick-up unit including a light source, an
objective lens which focuses a light beam emitted from the light
source onto an optical disk, and a photo-detector which detects
reflection light from the disk, the pick-up unit reproducing
information recorded on the disk along a predetermined track, and
an opening limiting unit which limits an opening of the objective
lens such that a beam spot shape on the optical disk is the same in
a disk radial direction and a track tangential direction of the
optical disk in a case where the light source has a wavelength
.lambda.1 and the optical disk is a first disk having a
predetermined recording density, and the beam spot shape on the
optical disk is large in the disk radial direction and small in the
track tangential direction in a case where the optical disk is a
second disk having a recording density lower than the first disk
and optimized with respect to a wavelength .lambda.2
(.lambda.1<.lambda.2) of the light source.
[0018] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0020] FIG. 1 schematically shows a structure of an optical disk
apparatus according to an embodiment of the present invention;
[0021] FIG. 2 is a block diagram showing a second reproducer used
in the optical disk apparatus shown in FIG. 1;
[0022] FIG. 3 is an operational waveform diagram in a case where
the present invention is applied;
[0023] FIG. 4 is an operational waveform diagram in a case where
the second reproducer including a correction circuit is used;
[0024] FIG. 5 is a block diagram of the second reproducer including
the correction circuit;
[0025] FIG. 6 is a circuit diagram of a correction circuit which
increases a continuous length of code bits "1" by one pit;
[0026] FIG. 7 is a circuit diagram of a correction circuit which
increases a continuous length of code bits "0" by one pit;
[0027] FIG. 8 shows a reproduced waveform in a case where there is
a tracking off-set;
[0028] FIG. 9 shows a first correction example of a very small
portion due to deformation of a spot shape;
[0029] FIG. 10 shows a second correction example of a very small
portion due to deformation of a spot shape;
[0030] FIG. 11 shows an example wherein tracking cannot be
performed with a normal spot shape;
[0031] FIG. 12 shows improvement of tracking performance due to
deformation of a spot shape;
[0032] FIG. 13 schematically shows a structure of an optical disk
apparatus according to a second embodiment of the invention;
[0033] FIGS. 14A, 14B and 14C show specific structures of the
opening limiting element used in the present invention;
[0034] FIG. 15 is a view for explaining the definitions of a
modulation degree and asymmetry of reproduced signals from the
optical disk;
[0035] FIG. 16 shows a reproduced signal waveform in a case where
information is reproduced from a first disk by a first optical disk
apparatus;
[0036] FIG. 17 shows a reproduced signal waveform in a case where
information is reproduced from a second disk by a second optical
disk apparatus;
[0037] FIG. 18 shows a reproduced signal waveform in a case where
information is reproduced from the second disk by the first optical
disk apparatus;
[0038] FIG. 19 shows a reproduced signal waveform in a case where
information is reproduced from the second disk by the first optical
disk apparatus with use of an elliptic opening having a numerical
aperture of 0.36 in a radial direction of the disk and a numerical
aperture of 0.36 in a tangential direction of the track;
[0039] FIG. 20 is a view for describing the optical disk apparatus
according to the another embodiment of the invention and
specifically shows an NA dependency characteristics of asymmetry of
reproduced signals from the optical disk;
[0040] FIG. 21 shows a reproduced signal waveform in a case where
information is reproduced from the second disk by the first optical
disk apparatus with use of an elliptic opening having a numerical
aperture of 0.36 in a radial direction of the disk and a numerical
aperture of 0.6 in a tangential direction of the track;
[0041] FIG. 22 shows a conventional state of the light beam focused
on the optical disk;
[0042] FIG. 23 shows a state of the focused light beam when
rebounding occurs; and
[0043] FIG. 24 shows an operational waveform diagram in a case
where the rebound occurs.
DETAILED DESCRIPTION OF THE INVENTION
[0044] An embodiment of the present invention will now be described
with reference to the accompanying drawings.
[0045] According to an optical disk apparatus shown in FIG. 1, a
read signal (READ signal) is input to a laser diode driver (LD
driver) 2 of a pick-up unit to drive a laser diode 3, whereby
information recorded on an optical disk 1 is read out.
Specifically, if the laser diode 3 is driven, a laser beam is
output from the laser diode 3 and is focused on the optical disk 1
through a collimate lens 4, a beam splitter 5 and an objective lens
6. Reflected light from a pit stream on the optical disk 1 is
reflected by the beam splitter 5 and made incident on a focusing
lens 7. The focusing lens 7 focuses the reflected light on a
photo-detector 8.
[0046] The photo-detector 8 outputs an electric signal, which
corresponds to the incident reflected light, to a first reproducer
10 via an amplifier 9 of a signal processing unit. The first
reproducer 10 processes the amplified signal by means of a wave
slice method or a PRML (Partial Response Maximum Likelihood)
method, thereby producing a reproduced signal.
[0047] In this invention, a second reproducer 11 is provided in
parallel to the first reproducer 10 in rear of the amplifier 9. A
switch 12 is provided in rear of the first and second reproducers.
The switch 12 is operated to select one of the outputs from the
first and second reproducers 10 and 11 in accordance with the kind
of the reproduced optical disk. For example, the optical
characteristics of the optical disk, e.g. CD or DVD (particularly
high density DVD), are determined on the basis of an output signal
from the photo-detector 8, and the switch 12 is operated in
accordance with the determination result. Specifically, it is
determined whether the reproduced signal is being normally
reproduced, and if the reproduced signal is detected as noise, the
switch 12 is changed over. This operation of the switch is effected
by a controller provided on the disk apparatus, although not
shown.
[0048] In the above structure, if the output of the amplifier 9,
i.e. the amplified reproduced signal, is supplied to the second
reproducer 11, a minimum value of the reproduced signal level is
detected by the second reproducer 11 and one of the outputs of the
first and second reproducers 10 and 11 is selected by the switch 12
in accordance with the kind of the reproduced optical disk.
[0049] The structure and operation of the second reproducer of the
present invention will now be described with reference to FIGS. 2
and 3.
[0050] As is shown in FIG. 2, the second reproducer 11 comprises a
differential circuit 13, a rising zero-cross detector 14, a rising
detector 15 and a PWM (Pulse Width Modulation) circuit 16.
[0051] In the second reproducer 11 with the above structure, if the
reproduced signal is input to the differential circuit 13, the
waveform of the reproduced signal is differentiated. The
differentiated waveform, as shown in FIG. 3, zero-crosses at
extreme values of the reproduced waveform. If the differentiated
waveform is input to the rising zero-cross detector 14, the rising
zero-cross detector 14 detects points where the polarity of the
differentiated waveform changes from negative to positive, and
outputs detection pulses to the rising detector 15. The rising
detector 15 detects the rising edge of the pulse and sets a code
bit "1" at the detection point and a code bit "0" at other points.
If the output signal of the rising detector 15 is input to the PWM
circuit 16, the PWM circuit 16 subjects the output signal of the
rising circuit 15 to PWM and thus produces decode data.
[0052] The bit stream after PWM does not necessarily coincide with
the record data. For example, as shown in FIG. 4, if the bit stream
after PWM is compared with the record data, coded bit "0" appears
at the rear end of a train of successive coded bits "1". Reversely,
coded bit "1" may appear at the rear end of a train of successive
coded bits "0" (not shown). It can be estimated from the
specifications of the optical disk to be reproduced what difference
arises between the stream after PWM and the record data. If a
correction circuit 17 is provided at the rear stage of the PWM
circuit 16, as shown in FIG. 5, decoding can be correctly
performed.
[0053] For example, when the bit stream after PWM conversion is
compared with the record data, if the coded bit "0" appears at the
rear end of the train of successive coded bits "1", the correction
circuit 17 comprising a delay circuit 18 and an OR circuit 19, as
shown in FIG. 6, is provided at the rear stage of the PWM circuit
16 to perform data correction. Thus, decoded data coinciding with
the record data can be obtained. Specifically, if coded bit "1" is
input to the delay circuit 18 and input to the OR gate 19 with a
delay of a one-bit period, coded bit "1" is output from the OR gate
even if the next PWM coded bit becomes "0". Accordingly, corrected
data corresponding to the record data is obtained.
[0054] On the other hand, when the bit stream after PWM conversion
is compared with the record data, if the coded bit "1" appears at
the rear end of the train of successive coded bits "0", the
correction circuit 17 comprising a delay circuit 17 and an AND
circuit 20, as shown in FIG. 7, is provided at the rear stage of
the PWM circuit 16 to perform data correction. Thus, decoded data
coinciding with the record data can be obtained. Specifically, if
coded bit "0" is input to the delay circuit 17 and input to the AND
gate 20 with a delay of a one-bit period, coded bit "0" is output
from the AND gate even if the next PWMs coded bit becomes "1".
Accordingly, corrected data corresponding to the record data is
obtained.
[0055] In the above embodiment, a filter for emphasizing an
amplitude of a differential waveform may be provided in a case
where the rebound is small and the amplitude of the differential
waveform obtained from the differential circuit 13 of the second
reproducer 11 is small. Thereby, a signal with a small rebound can
be reproduced with high precision.
[0056] FIG. 8 shows a variation in a reproduced waveform due to
off-track when a rebound is occurring. The rebound amount obtained
when the laser beam spot scans a first scan line 1 extending
through the center of a pit 21, that is, a reproduction signal
level 22, is greater than the rebound amount obtained when the spot
scans a second scan line 2 extending off the center of the pit 21,
that is, a reproduced signal level 23 corresponding to the rebound
amount due to off-track. Specifically, the rebound amount due to
off-track is less than that due to on-track. The focused beam can
thus scan the track center if a tracking control is performed so
that the reproduced signal level may take a maximum value at the
center of the pit on the basis of the reproduced waveform. This
tracking control can be achieved by delivering the reproduced
signal due to on-track to the tracking control circuit of the
optical disk apparatus.
[0057] When the signal detection is effected by detecting the
minimum value of the reproduced signal, the location of the minimum
value is not necessarily at the optimal point. For example, as
shown in FIG. 9, the minimum value of the reproduced signal
obtained with a normal spot (substantially circular) is displaced
outside the optimal point. In this case, by changing the shape of
the spot to a vertically elongated elliptic shape, the location of
the minimum value is shifted to the inside and made closer to the
optimal point. Inversely, in FIG. 10, the minimum value of the
reproduced signal obtained with the normal spot (substantially
circular) is displaced inside the optimal point. In this case, by
changing the shape of the spot to a horizontally elongated elliptic
shape, the location of the minimum value is shifted to the outside
and made closer to the optimal point.
[0058] As has been described above, when the location of the
minimum value of the reproduced signal is displaced from the
optimal point, the shape of the spot is changed so that the
location of the minimum value may be closer to the optimal
point.
[0059] In a system wherein the tracking is made to increase the
rebound amount to a maximum, there is a case where optimal tracking
cannot be performed due to the relationship between the pit and the
spot. For example, in a case illustrated in FIG. 11, all the beam
spot resides on a bottom portion of the pit near the center of the
pit, irrespective of the scan lines 1 and 2. In this case, the
rebound amount of the reproduced signal is equal when the spot
scans the scan lines 1 and 2.
[0060] In order to solve this problem, the spot shape is changed to
a vertically elongated elliptic shape, as shown in FIG. 12. By
changing the spot shape to the vertically elongated elliptic shape,
all the spot enters the bottom portion of the pit when the spot
scans the scan line 1 and part of the spot overlaps a wall portion
of the spot when the spot scans the second scan line 2. That is,
the difference in rebound amount takes a maximum value only when
the spot scans the scan line 1, and exact tracking can be
effected.
[0061] The mechanism for changing the beam spot shape, as described
above, can be realized by inserting an opening limiting element
between the objective lens and the beam splitter as described
hereinafter.
[0062] As has been described above, according to the present
invention, the location of a negative extreme value is detected
from the reproduced waveform with the rebound. The coded bit at the
detected location is determined to be "1" and the coded bit at
other locations is determined to be "0". Subsequently, the result
of determination is subjected to PWM so that no error may occur in
signal detection even when the rebound occurs in the reproduced
waveform.
[0063] An optical disk apparatus according to a second embodiment
of the invention will now be described with reference to FIG. 13.
In FIG. 13, an optical disk 110 is, for example, a read-only
optical disk and comprises a transparent substrate having pit
streams representing record information, a reflection layer or a
recording layer (hereinafter referred'to as "reflection/recording
layer") 111 formed on the transparent substrate, and a protection
layer formed on the reflection/recording layer 111. Where the
reflection/recording layer 111 is a recording layer, it may be
formed of any material capable of recording information with
radiation of a light beam. For example, a phase change medium layer
or a photo-magnetic layer may be used as the reflection/recording
layer 111.
[0064] When information is to be reproduced from the optical disk
110, the disk 110 is rotated by a spindle motor 112 driven by a
motor driver 113 and a laser diode (LD) 115 is driven by an LD
driver 114 to emit a light beam. The light beam emitted from the
semiconductor laser 115 is converted to a collimated light flux
through a collimator lens 116 and the collimated light flux is made
incident on the objective lens 119 through a beam splitter (half
prism) 117 and an opening limiting element 118 (to be described
later in detail). The beam is then focused on the
reflection/recording layer 111 of optical disk 110 by the objective
lens 119, and a small beam spot is formed on the
reflection/recording layer 111.
[0065] The reflected light from the reflection/recording layer 111
of disk 110 is returned through the objective lens 119 and opening
limiting element 118 in a direction reverse to the direction of the
incident beam on the disk 110. The reflection light is guided to a
focusing lens 120 via the beam splitter 117 and then focused on a
photo-detector 121 by the focusing lens 120. The photo-detector 121
is a plural-segment split photodetector having a light-receiving
surface divided into plural segments, e.g. two segments or four
segments (i.e. two- or four-segment split photo-detector). An
output from the photo-detector 121 is input to an arithmetic
circuit 122.
[0066] The arithmetic circuit 122 subjects the output from the
photo-detector 121 to addition/subtraction operations, thereby
producing a reproduction signal corresponding to the information
recorded on the optical disk 110, a focus error signal, and a
tracking error signal. Of these signals, the reproduction signal is
delivered to a signal processor 123. The focus error signal and
tracking error signal are delivered to a focus servo system and a
tracking servo system, both not shown.
[0067] The signal processor 123 subjects the reproduction signal
input from the arithmetic circuit 122 to processing such as
equalization, binarization, demodulation and decoding, thus
producing reproduction data. In addition, the signal processor 123
may have a function of determining the kind (track pitch, i.e.
recording density) of the optical disk 110 on the basis of track
pitch data included in physical format data reproduced from a
read-in area of the optical disk 110, and outputting a
determination result, as will be described later.
[0068] The opening limiting element 118 will now be described.
[0069] The opening limiting element 118 is an element for limiting
the opening of the objective lens 119 and has a plurality of
openings with different sizes, in particular, dimensions in the
radial direction of the optical disk 110 (hereinafter referred to
as "the disk radial direction"). These openings are switched by an
opening switch circuit 124 shown in FIG. 13. The opening switch
circuit 124 switches the opening by controlling the opening
limiting element 118, for example, in accordance with a
discrimination result from a disk discriminator 125 for determining
the kind of the optical disk 110, or a discrimination result 126 of
the signal processing by the signal processor 123.
[0070] FIGS. 14A, 14B and 14C show various examples of the
structure of the opening limiting element 118.
[0071] An opening limiting element, as shown in FIG. 14A, has a
rectangular light shield plate 131 in which a circular opening 132
and an elliptic opening 133 are formed. The elliptic opening 133
has a short axis in the disk radial direction and a long axis in a
track tangential direction on the optical disk 110. This opening
limiting element is parallel-moved by the opening switch circuit
124 in a direction of double-headed arrow a, and thus one of the
openings 132 and 133 is selectively put on the light incidence path
to the objective lens 119 in FIG. 13.
[0072] An opening limiting element, as shown in FIG. 14B, has a
sectorial light shield plate 141 in which a circular opening 142
and an elliptic opening 143 are formed. The elliptic opening 143
has a short axis in the disk radial direction and a long axis in
the track tangential direction. This opening limiting element is
rotated by the opening switch circuit 124 about a rotational axis
140 in a direction of double-headed arrow b, and thus one of the
openings 142 and 143 is selectively put on the light incidence path
to the objective lens 119 in FIG. 13.
[0073] An opening limiting element, as shown in FIG. 14C, is formed
of a liquid crystal cell 151. An elliptic opening 153 having a
short axis in the disk radial direction and a long axis in the
track tangential direction is formed in a central portion of the
liquid crystal cell 151. The cell 151 is controlled by the
turn-on/off of application of voltage to the cell 151 or the
magnitude of applied voltage. Specifically, when no voltage is
applied to the liquid crystal cell 151, incident light is passed
through the opening 153. When voltage is applied, a circular
opening 152 is formed, as indicated by a broken line, and incident
light is passed through the circular opening 152 which includes the
opening 153. In this case, the control of voltage to the liquid
crystal cell 151 is performed by the opening switch circuit
124.
[0074] In the following description, the openings 132, 142 and 152
shown in FIGS. 14A, 14B and 14C are generally referred to as
circular opening A, and the openings 133, 143 and 153 as elliptic
openings B. The circular opening A may be the same as the opening
of the objective lens 119 and in this case the circular opening A
is not needed.
[0075] In the optical disk apparatus having the above structure,
the beam spot size on the reflection/recording layer 111 of optical
disk 110 is inversely proportional to the numerical aperture (NA)
of the objective lens 119 and proportional to the wavelength of the
light beam, i.e. wavelength .lambda. of the semiconductor laser 115
or the light source. Specifically, if the wavelength .lambda. and
the numeral aperture NA are determined, the shortest pit length
representing information on the reflection/recording layer 111 and
the optimal value of the track pitch are determined. Accordingly,
the optical disk 110 is generally optimized with respect to the
wavelength .lambda. and the numeral aperture NA.
[0076] Since the optical disk 110 is optimized with respect to the
wavelength .lambda. and the numeral aperture NA, as mentioned
above, an optical disk apparatus can basically reproduce
information from only the optical disk matching with the wavelength
.lambda. and the numeral aperture NA of this apparatus. In the
present embodiment, however, information can be reproduced from
various kinds of optical disks if the opening of the objective lens
110 is changed by the opening limiting element 118 in accordance
with the kind of the optical disk 110.
[0077] The details of the opening limiting function of the opening
limiting element 118 in the optical disk apparatus according to the
present embodiment and the advantages obtained by the opening
limiting function will now be described.
[0078] The degree of modulation M and asymmetry A of the
reproduction signal from the optical disk 110 which is Output from
the arithmetic circuit 122 are defined as follows.
[0079] When the maximum level and minimum level of a reproduction
signal Smin of repeat signals of shortest pits are IminH and IminL
and the maximum level and minimum level of a reproduction signal
Smax of repeat signals of longest pits are ImaxH and ImaxL in FIG.
15, the degree of modulation M and asymmetry A of reproduction
signals are defined by 2 M = I min H - I min L I max H - I max L (
1 ) A = ( I max H - I max L ) - ( I min H + I min L ) 2 ( I max H -
I max L ) ( 2 )
[0080] In general, in the optical disk apparatus, if the degree of
modulation of a reproduction signal from the optical disk is M=0.2
or more and the asymmetry thereof is A=-0.05 to 0.15, it is
considered that information can be correctly reproduced from the
reproduction signal. Calculation results of the degree of
modulation M and asymmetry A of reproduction signals obtained with
various combinations of the optical disk apparatus and optical
disks will be shown below and it is examined whether such
combinations meet the above conditions.
[0081] One example of the combinations of the optical disk
apparatus and optical disks may comprise an optical disk apparatus
("first optical disk apparatus") having a light source with
wavelength .lambda.1=410 rim and having an objective lens with
numeral aperture NA=0.6, which will possibly be used as a
short-wavelength light source in future, and an optical disk
("first disk") with a track pitch=0.42 .mu.m and a shortest pit
length=0.23 .mu.m, which will prospectively be used as a so-called
high-density DVD.
[0082] FIG. 16 shows calculation results obtained with this
combination, that is, calculation results of the repeat
reproduction signal of a shortest pit (3T) and the repeat
reproduction signal of a longest pit (14T) which were reproduced
from the first disk by the first optical disk apparatus.
[0083] Another example of the combinations of the optical disk
apparatus and optical disks may comprise an optical disk apparatus
("second optical disk apparatus") having a light source with
wavelength .lambda.2=650 nm and having an objective lens with
numeral aperture NA=0.6, which is currently used as a modern
DVD-ROM system or DVD-RAM system, and an optical disk ("second
disk") with a track pitch=0.74 .mu.m and a shortest pit length=0.4
.mu.m.
[0084] FIG. 17 shows calculation results obtained with this
combination, that is, calculation results of the repeat
reproduction signal of a shortest pit (3T) and the repeat
reproduction signal of a longest pit (14T) which were reproduced
from the second disk by the second optical disk apparatus.
[0085] TABLE 1 shows in (CASE 1) and (CASE 5) the degree of
modulation M and asymmetry A of the repeat reproduction signal
obtained from the first disk by the first optical disk apparatus
and the repeat reproduction signal obtained from the second disk by
the second optical disk apparatus.
1TABLE 1 Optical disk apparatus 1st disk apparatus 2nd disk
apparatus Light source wavelength 410 nm 650 nm Optical disk 1st
disk 2nd disk 2nd disk 2nd disk 2nd disk NA (Radial direction/
(0.6/0.6) (0.6/0.6) (0.36/0.36) (0.36/0.6) (0.6/0.6) tangent
direction) Modulation factor 0.22 0.78 0.26 0.75 0.30 Asymmetry
0.082 0.207 0.189 0.140 0.017 (Case 1) (Case 2) (Case 3) (Case 4)
(Case 5)
[0086] The degree of modulation M and asymmetry A in (CASE 1) and
(CASE 5) satisfy the above conditions, M=0.2 or more and A=-0.05 to
0.15. It is thus considered that exact information reproduction can
be performed.
[0087] FIG. 18 shows calculation results of the repeat reproduction
signal of a shortest pit (3T) and the repeat reproduction signal of
a longest pit (14T) which were reproduced from the second disk by
the first Optical disk apparatus. The degree of modulation M and
asymmetry A of the reproduction signal in this case are shown in
(CASE 1) in TABLE 1. Since the degree of modulation M and asymmetry
A fail to satisfy the above conditions, A=-0.05 to 0.15, good
information reproduction cannot be performed. The reason appears to
be that the relationship among the wavelength .lambda.1 of the
light source, the beam spot size on the reflection/recording layer
of the optical disk, and the shape of pits is not proper.
[0088] In the present embodiment, the condition of asymmetry is not
satisfied. However, there may be a case where the condition of a
modulation factor is not satisfied, depending upon the combination
between an optical disk apparatus and an optical disk or a case
where the conditions of both the modulation factor and the
asymmetry are not satisfied.
[0089] FIG. 19 shows calculation results of the repeat reproduction
signal of a shortest pit (3T) and the repeat reproduction signal of
a longest pit (14T) which were reproduced from the second disk,
which is the optical disk 100, with use of the first optical disk
apparatus wherein the numerical aperture NA of the objective lens
was set at 0.36 so that the beam spot size may become equal to that
in the second optical disk apparatus. In this case, the degree of
modulation M and asymmetry A of the reproduction signal are shown
in (CASE 3) in TABLE 1 and fail to satisfy the above condition,
A=-0.05 to 0.15. Thus, good information reproduction cannot be
performed.
[0090] However, if the opening of the objective lens 119 is limited
from circular opening A to elliptic opening B by the opening
limiting element 118 in the optical disk apparatus (first optical
disk apparatus) according to the present embodiment shown in FIG.
13, the beam spot shape on the reflection/recording layer 111 can
be optimized. Thus, the reproduction signal capable of exactly
reproducing information from the second disk can be obtained.
[0091] FIG. 20 shows calculation results of the asymmetry A of the
reproduction signal reproduced from the second disk with use of the
optical disk apparatus (first optical disk apparatus) shown in FIG.
13 wherein the numerical aperture NA in the disk radial direction
and the numerical aperture NA in the track tangential direction of
the objective lens 119 were varied. It is understood from FIG. 20
that a hatched region, where the NA in the disk radial direction is
0.396 or less and the NA in the track tangential direction is 0.57
or more, satisfies the condition, A=-0.05 to 0.15. In other words,
it is understood that where information is reproduced from the
second disk by the first optical disk apparatus, the asymmetry is
improved as the NA in the disk radial direction is decreased and
the NA in the track tangential direction is increased.
[0092] When the objective lens 119 is mass-produced, it is
generally difficult to produce lenses with the numerical aperture
NA=0.66. It is thus desired that the upper limit of the NA in the
track tangential direction be set at 0.66. In addition, if the NA
in the disk radial direction is decreased, the beam spot size in
the disk radial direction increases and leak from adjacent tracks
increases. It is thus desired that when the NA in the disk radial
direction of the objective lens 119 is limited by the opening
limiting element 118, the lower limit of the NA be set at 0.35 in
order to make the beam spot size substantially equal to that in the
case of reproducing information from the second disk by the second
optical disk apparatus.
[0093] In brief, when information is reproduced from the second
disk by the first optical disk apparatus, the numerical apertures
NA2r and NA2t in the disk radial direction and track tangential
direction, with which the reproduction signal capable of exactly
reproducing information, are given by
0.35.ltoreq.NA2r.ltoreq.0.396 (3)
0.57.ltoreq.NA2t.ltoreq.0.66 (4)
[0094] FIG. 21 shows calculation results of the reproduction signal
of a shortest pit (3T) and the repeat reproduction signal of a
longest pit (14T) which were reproduced from the second disk, which
is the optical disk 100, ith use of the optical disk apparatus
(first optical disk apparatus) shown in FIG. 13 wherein the
elliptic opening B with numerical aperture NA=0.36 in the disk
radial direction and numerical aperture NA=0.6 in the track
tangential direction is set on the light incidence path to the
objective lens 119. In this case, the degree of modulation M and
asymmetry A of the reproduction signal are shown in (CASE 4) in
TABLE 1 and satisfy the conditions, M=0.2 or more and A=-0.05 to
0.15. It is understood from these results that exact information
reproduction can be performed by setting the beam spot shape with
use of the proper opening limited by the opening limiting element
118.
[0095] Specifically, the influence of asymmetry due to the pit
depth can be reduced by increasing the beam spot size in the disk
radial direction and thus increasing the amount of beam radiation
on an area other than the pit portion. Besides, since the optical
resolution is enhanced by the reduction in beam spot size in the
track tangential direction, the amplitude ratio (modulation factor)
of a signal with highest density to a signal with lowest density
can be increased and the influence of asymmetry can be reduced.
[0096] Moreover, since the beam spot size is proportional to the
wavelength and inversely proportional to the numerical aperture NA,
formulae (3) and (4) can be rewritten to the following formulae (5)
and (6) by using the wavelength (.lambda.) of the light source and
the numerical aperture (NA) of the objective lens: 3 NAlr 410 650
0.95 NA2r NA1r 410 650 1.1 ( 5 ) NAlt 0.95 NA2t NAlt 1.1 where NAlr
410 650 0.36 , NAlt 0.6 ( 6 )
[0097] In brief, by establishing the relationships of formulae (5)
and (6) by means of the opening limiting element 118, the
reproduction signal capable of exactly reproducing information can
be obtained from the second disk with use of the first optical disk
apparatus.
[0098] The above description may be summarized as follows.
[0099] In the optical disk apparatus according to the present
invention, the wavelength .lambda.1 of the semiconductor laser 115
is constant and the opening of the objective lens 119 can be
switched by the opening limiting element 118 between the circular
opening A with numerical aperture NA1r in the disk radial direction
and numerical aperture NA1t in the track tangential direction and
the elliptic opening B with numerical aperture NA2r in the disk
radial direction and numerical aperture NA2t in the track
tangential direction. Suppose that the degree of modulation and
asymmetry of the reproduced signal, which is obtained from the
first disk by the first optical disk apparatus having the light
source with wavelength .lambda.1 and the objective lens with
numerical aperture NA1r in the disk radial direction and numerical
aperture NA1t in the track tangential direction, satisfy the
above-mentioned conditions, M=0.2 or more and A=-0.05 to 0.15, and
that the degree of modulation and asymmetry of the reproduced
signal, which is obtained from the second disk by the second
optical disk apparatus satisfy the same conditions. At this time,
if the wavelength and numerical aperture (NA) meet the following
relationship, 4 0.95 1 2 NA1r NA2r 1.1 1 2 NA1r ( 7 ) 0.95 NAlt
NA2t 1.1 NAlt ( 8 )
[0100] a reproduction signal capable of exactly reproducing
information can be obtained from each of the first and second disks
by selecting, in the optical disk apparatus of this embodiment, the
circular opening A in the case of reproducing information from the
first disk or the elliptic opening B in the case of reproducing
information from the second disk.
[0101] In fact, if the numerical apertures NA1r and NA1t are set to
be equal to the numerical aperture NA of the objective lens 119 in
the disk radial direction and track tangential direction
(NA=NA1r=NA1t), the opening limiting element 118 may not have the
circular opening A, as mentioned above. When information is to be
reproduced from the first disk, the opening limiting element 118
may be set in such a state as to pass all the beam. When
information is to be reproduced from the second disk, the elliptic
opening B may be set on the light path to the objective lens.
[0102] The optical disk apparatus capable of reproducing
information from two kinds of optical disks (first and second
disks) with different recording densities has been described. The
present invention, however, is applicable to reproduction of
information from three or more kinds of optical disks wherein the
pit length (in particular, minimum pit length), track pitch, etc.
are optimized with respect to different light source wavelengths.
In this case, it should suffice to use the opening limiting element
119 capable of switching three or more sets of numerical apertures
in the disk radial direction and track tangential direction,
including the numerical aperture of the objective lens 119
itself.
[0103] The opening limiting element for limiting the opening of the
objective lens switches the opening in accordance with the optical
disk to be reproduced, thereby optimizing the beam spot shape.
Therefore, information can be exactly reproduced from plural kinds
of optical disks with different recording densities.
[0104] Specifically, the optical disk apparatus with the light
source wavelength of, e.g. 410 nm, can exactly reproduce
information from either a relatively high-density optical disk
optimized to match with this wavelength or a relatively low-density
optical disk optimized at light source wavelength of 650 nm.
[0105] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *