U.S. patent number RE43,105 [Application Number 13/064,303] was granted by the patent office on 2012-01-17 for tracking error detection method and optical disc reproduction apparatus using the same.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Takahiro Kurokawa, Harukazu Miyamoto.
United States Patent |
RE43,105 |
Kurokawa , et al. |
January 17, 2012 |
Tracking error detection method and optical disc reproduction
apparatus using the same
Abstract
An object of the present invention is to improve a conventional
DPD method having a problem that a tracking servo becomes unstable
due to a reduction in the accuracy of detecting a phase difference
between short mark signals at an edge, in a case where the
amplitudes of the short mark signals are very small, or where a
readout signal contains large noise. To this end, the present
invention provides a method for increasing the contribution ratio
of long mark signals to generate a tracking error signal by causing
a phase difference pulse to include information on a phase
difference, and by causing the area of the pulse to be weighted
according to the length of a mark/space adjacent to a concerned
edge.
Inventors: |
Kurokawa; Takahiro (Fujisawa,
JP), Miyamoto; Harukazu (Higashimurayama,
JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
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Family
ID: |
39527018 |
Appl.
No.: |
13/064,303 |
Filed: |
March 17, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
11674230 |
Feb 13, 2007 |
7764578 |
Jul 27, 2010 |
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Foreign Application Priority Data
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Dec 15, 2006 [JP] |
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2006-338852 |
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Current U.S.
Class: |
369/44.41;
369/124.12 |
Current CPC
Class: |
G11B
7/0906 (20130101) |
Current International
Class: |
G11B
7/00 (20060101) |
Field of
Search: |
;369/44.41,124.12,44.27,44.28,44.34,44.42,44.25,44.29,124.01,124.05,120,121,122,116,59.11,59.12 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
White paper: Blu-ray Disc Format--General, Aug. 2004. cited by
other .
DVD Forum: Technical White Paper--Outline Of HD DVD formats; DVD
forum, Dec. 2005, Revised Jun. 2006. cited by other.
|
Primary Examiner: Hindi; Nabil
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP.
Claims
What is claimed is:
1. An optical disc reproduction method, comprising the steps of:
irradiating reproduction light onto an optical disc on which
information is recorded by using a plurality of marks with
different mark lengths; receiving reflected light from the optical
disc by a quadrant photo detector; generating a pulse signal by
using a first readout signal and a second readout signal each of
which is a sum signal of outputs from segment elements located
respectively at diagonal corners of the quadrant photo detector,
the pulse signal including information on the phase difference
between the first readout signal and the second readout signal, and
having at least one of a height and a width weighted according to a
mark length of a mark so that the area of the pulse signal would be
larger as the mark length of a mark adjacent to an edge of the
readout signal becomes longer; generating a tracking error signal
by adding up the pulse signals for a certain period of time; and
controlling tracking by using the tracking error signal.
2. The optical disc reproduction method according to claim 1,
wherein the pulse signal has the height proportional to the
difference between the first readout signal and the second readout
signal at signal edges, and has the width proportional to the
length of any of a mark and a space following a signal edge.
3. The optical disc reproduction method according to claim 1,
wherein the pulse signal has the width from an edge of the first
readout signal to the immediately subsequent edge of the second
readout signal, and has the height proportional to the sum of the
signal level of the second readout signal at the edge time of the
first readout signal and the signal level of the first readout
signal at the immediately preceding edge of the second readout
signal.
4. The optical disc reproduction method according to claim 1,
wherein the pulse signal has the width from an edge of the second
readout signal to the immediately subsequent edge of the first
readout signal, and has the height proportional to the difference
between the signal level of the first readout signal at the edge
time of the second readout signal, and the signal level of the
second readout signal at the immediately preceding edge time of the
first readout signal.
5. The optical disc reproduction method according to claim 1,
wherein the pulse signal has the width from a point where the
average level signal of the first readout signal and the second
readout signal crosses over the zero level, to a point where the
average level signal again crosses over the zero level next time,
and has the size proportional to the difference between the signal
levels of the first readout signal and the second readout signal at
a starting time of the pulse signal.
6. An optical disc reproduction apparatus comprising: a light
source; an objective lens; an irradiation optical system for
irradiating an optical disc with light from the light source
through the objective lens; a quadrant photo detector; a detection
optical system for causing light reflected from the optical disc to
enter the quadrant photo detector through the objective lens; a
tracking error signal generating circuit for generating a tracking
error signal by using a first readout signal and a second readout
signal, each of which is a sum signal of outputs from segment
elements located respectively at diagonal corners of the quadrant
photo detector; and a lens actuator for driving the objective lens
by using the tracking error signal generated by the tracking error
signal generating circuit, wherein the tracking error signal
generating circuit generates a pulse signal by using the first
readout signal and the second readout signal, each of which is the
sum signal of outputs from segment elements located at diagonal
corners of the quadrant photo detector, the pulse signal including
information on the phase difference between the first readout
signal and the second readout signal, and having at least one of a
height and a width weighted according to a mark length of a mark so
that the area of the pulse signal would be larger as the mark
length of a mark adjacent to an edge of the readout signal becomes
longer, and wherein the tracking error signal generating circuit
generates a tracking error signal by adding up the pulse signals
thus generated for a certain period of time.
7. The optical disc reproduction apparatus according to claim 6,
wherein the tracking error signal generating circuit generates a
pulse signal having the width from an edge of the first readout
signal to the immediately subsequent edge of the second readout
signal, and having the height proportional to the sum of the signal
level of the second readout signal at the edge time of the first
readout signal and the signal level of the first readout signal at
the edge time of the second readout signal.
8. The optical disc reproduction apparatus according to claim 6,
wherein the tracking error signal generating circuit generates a
pulse signal having the width from an edge of the second readout
signal to the immediately subsequent edge of the first readout
signal, and having the height proportional to the difference
between the signal level of the first readout signal at the edge
time of the second readout signal and the signal level of the
second readout signal at the immediately preceding edge time of the
first readout signal.
9. The optical disc reproduction apparatus according to claim 6,
wherein the tracking error signal generating circuit generates a
pulse signal having the width from a point where an average level
signal of the first readout signal and the second readout signal
crosses over the zero level, to a point where the average level
signal again crosses over the zero level next time, and having the
size proportional to the difference between the signal levels of
the first readout signal and the second readout signal at a
starting time of the pulse signal.
10. An optical apparatus comprising: a light source; an objective
lens; an optical system arranged to irradiate light from the light
source onto an optical recording medium on which information
recorded using a plurality of marks with different lengths, via the
objective lens; a quadrant photo detector arranged to receive light
reflected from the optical recording medium, via the objective
lens; a tracking error signal generating circuit to generate pulse
signals by using a first readout signal and a second readout signal
each of which is a sum signal of outputs from segment elements
located respectively at diagonal corners of the quadrant photo
detector, each pulse signal including information on a phase
difference between the first readout signal and the second readout
signal, and having its height or its width weighted according to a
mark length of a mark adjacent to an edge of one of the first and
second readout signals, and to generate a tracking error signal by
adding up the pulse signals for a certain period of time; and a
lens actuator arranged to drive the objective lens relative to the
optical recording medium by using the tracking error signal.
11. The optical apparatus according to claim 10, wherein the
tracking error signal generating circuit generates a pulse signal
having the width from an edge of the first readout signal to the
immediately subsequent edge of the second readout signal, and
having the height proportional to a sum of the signal level of the
second readout signal at an edge time of the first readout signal
and the signal level of the first readout signal at an edge time of
the second readout signal.
12. The optical apparatus according to claim 10, wherein the
tracking error signal generating circuit generates a pulse signal
having the width from an edge of the second readout signal to the
immediately subsequent edge of the first readout signal, and having
the height proportional to a difference between the signal level of
the first readout signal at an edge time of the second readout
signal and the signal level of the second readout signal at the
immediately preceding edge time of the first readout signal.
13. The optical apparatus according to claim 10, wherein the
tracking error signal generating circuit generates a pulse signal
having the width from a point where an average level signal of the
first readout signal and the second readout signal crosses over a
zero level, to a point where the average level signal again crosses
over the zero level next time, and having the size proportional to
a difference between the signal levels of the first readout signal
and the second readout signal at a starting time of the pulse
signal.
14. The optical apparatus according to claim 10, wherein the pulse
signal has the height proportional to a difference between the
first readout signal and the second readout signal at signal edges,
and has the width proportional to the length of any of a mark and a
space following a signal edge.
Description
CLAIM OF PRIORITY
The present application claims priority from Japanese application
JP 2006-338852 filed on Dec. 15, 2006, the content of which is
hereby incorporated by reference into this application.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tracking error detection method
allowing a stable tracking servo to be achieved in an optical disc
reproduction apparatus even during reproduction of data on a high
density disc or during high-speed reproduction, and relates to an
optical disc reproduction apparatus using the method.
2. Description of the Prior Art
An optical disc known as a CD (Compact Disc) and a DVD (Digital
Versatile Disc) is an information recording medium which is used
for recording and reproducing information by means of an optical
spot formed by concentrating laser light on the information
recording surface of the disc. A BD (Blu-ray Disc) or an HD DVD is
one of discs that have recently put into practical use as a high
density optical disc. The Blu-ray Disc is described in details in
"White Paper: Blu-ray Disc Format-General," and the HD DVD is
described in details in "DVD Forum Gijyutsu Hakusho--HD DVD Format
no Gaiyou--(DVD Forum Technology White Paper--Outline of HD DVD
Format--)."
In a case of a reproduction-only optical disc (ROM: Read Only
Memory), information is recorded in the form of fine
convexoconcaves (pits) formed on the information recording surface,
and the information is reproduced by detecting the change in the
reflectance of light caused by the presence and absence of pits.
The pits are aligned spirally or concentrically, and thus form a
recording track.
When information is reproduced, it is necessary to cause the
optical spot to follow a recording track precisely (perform
tracking). The tracking is performed by causing optical means to
detect the amount of deviation (a tracking error) of the optical
sport from the center of a recording track, and by driving an
objective lens in a radial direction of the disc so that the
tracking error would be zero.
There are a 3-spot method, a DPD (Differential Phase Detection)
method and the like as a tracking error detection method for
reproduction-only optical discs. The DPD method is often used in
recent driver apparatuses in order to handle a plurality of types
of disc standards, since the DPD method can eliminate the influence
of the difference between track pitches.
A fundamental principle of the tracking error detection using the
conventional DPD method will be described by using FIG. 1. FIG. 1
shows states of pits, optical spots, and light intensity
distribution on a quadrant photo detector when information in a
reproduction-only disc is reproduced. When an optical spot is on a
pit, the 0th-order diffracted light and the .+-.1st-order
diffracted light among all the orders of diffracted lights
generated by the pit interfere with each other, and this
interference generates areas (interference areas) with the small
light intensity on the quadrant photo detector.
In this respect, assuming that segment elements of the quadrant
photo detector are A, B, C and D, an examination is given on the
sums (I.sub.A+I.sub.C) and (I.sub.B+I.sub.D) each of which is the
sum of the output currents from two segment elements located at
diagonal corners, respectively. In a case where the optical spot
passes exactly through the center of the pit column, no phase
difference occurs between (I.sub.A+I.sub.C) and (I.sub.B+I.sub.D).
On the other hand, when the optical spot is deviated from the
centerline of the pit column (off-track), a phase difference occurs
between (I.sub.A+I.sub.C) and (I.sub.B+I.sub.D). This is because
the above-described interference areas are generated as a pair at
the respective diagonal corners of the quadrant photo detector,
when the optical spot passes on an edge of the pit (a front or rear
end of the pit in the direction of optical spot's movement). As
shown in FIG. 1, the sign of phase difference varies according to
off-track directions, and the size of phase difference is
approximately proportional to the off-track amount.
Hereinafter, an example of the tracking error signal generating
method using the conventional DPD method will be described by using
a block diagram of FIG. 2 and a diagram of signal waves of FIG. 3.
The diagonal sum signals (I.sub.A+I.sub.C) and (I.sub.B+I.sub.D)
are amplified by the respective amplifiers, and thus become A1 and
A2, respectively. The amplitudes of short mark signals of the A1
and A2 are relatively amplified by the respective equalizers.
Subsequently, the signals are binarized by the respective level
comparators, and thus become signals B1 and B2, respectively. The
signals B1 and B2 are inputted to a phase comparator, and the phase
comparator detects the phase difference between edges of the two
signals (an edge is a point where each of the signals crosses over
the zero level). The phase comparator outputs a phase difference
pulse to C1 when the phase of the signal B1 precedes the phase of
the signal B2. On the other hand, the phase comparator outputs a
phase difference pulse to C2 when the phase of the signal B2
precedes the phase of the signal B1. The heights of the phase
difference pulses are constant, and the widths thereof are equal to
the absolute value of the phase difference between the signals B1
and B2. The pulse signals of each of C1 and C2 are integrated with
a predetermined time constant by the respective low-pass filters
(LPF), and the difference between the signals thus obtained becomes
a tracking error signal.
SUMMARY OF THE INVENTION
A problem in tracking error detection using the conventional DPD
method will be pointed out here. FIG. 4 is a schematic diagram of a
readout signal of a reproduction-only optical disc. In this
example, pits aligned in the order of 8T marks, 2T spaces, 2T marks
and 8T spaces are reproduced. Here, T denotes a channel bit cycle.
In a case of a long mark readout signal such as 8T, the gradient is
large at the edge. In contrast, in a case of a short mark readout
signal such as 2T, the gradient is small at the edge, since the
signal amplitude is reduced due to insufficient optical resolution.
In particular, in a case of a high density optical disc such as BD
(Blu-ray Disc) and HD DVD which have been recently put into
practical use, the gradient of a signal is small at the edge
because the amplitude of a short mark signal is very small. For
this reason, the influence of noise, variation in a signal level
and the like results in a reduction in the accuracy of phase
difference detection, and the reduction in the accuracy causes
tracking servo to be unstable.
Moreover, in a case where data is reproduced at a high speed, the
amplitude of a short mark signal with high frequency is decreased
due to the limit of a signal transmission band of an electric
circuit in a driver apparatus, and this decrease brings about the
same problem as that in a case of the reproduction of data on a
high density optical disc.
Any type of main optical discs (CD, DVD, BD and HD DVD), which have
been put into practical use, employs a mark edge recording method.
In this method, in order to increase recording capacity by
enhancing the efficiency in data recording, data is modulated so
that the existence ratio of signals with shorter mark length would
be larger than that of signals with longer mark length, and then
the data thus modulated is recorded. In other words, the ratio of
the number of edges of short mark signals to the total number of
edges of a readout signal is high.
Accordingly, in the case of the tracking error detection method
using the conventional DPD method, the contribution ratio of edges
of short mark signals to generate a tracking error signal is high,
and there is a problem that the tracking servo becomes unstable in
reproducing data on a high density optical disc or reproducing at a
high speed for the aforementioned reasons.
Against this background, an object of the present invention is to
provide a tracking error detection method and an optical disc
reproduction apparatus using the method, by using the method it
being possible to reduce the influence of unstable short mark
signals at edges, thereby achieving a stable tracking servo.
In the present invention, a tracking error signal is generated by
using the following method. In the present invention, specifically,
for the purpose of reducing the contribution ratio of short mark
signals at edges to generate a tracking error signal, and of
increasing the contribution ratio of long mark signals at edges,
the height or width of a phase difference pulse outputted in a
process for generating a DPD tracking error signal is weighted
according to the length of a mark/space adjacent to a concerned
edge. In other words, the height or the width of a phase difference
pulse is made larger as the length of a mark/space adjacent to a
concerned edge is longer. In the DPD method, a tracking error
signal is the one obtained by integrating phase difference pulses
with a predetermined time constant. For this reason, by employing
the foregoing method, the contribution ratio of long mark signals
at edges to generate a tracking error signal can be relatively
increased.
Two possible methods for achieving the present invention are as
follows.
(1) A First Method: a Method in which the Height of a Phase
Difference Pulse is Weighted According to the Gradient of a Signal
at a Concerned Edge
In this method, the height of a phase difference pulse is made
approximately proportional to the gradient of a signal at a
concerned edge. As has been described, the longer the length of a
mark/space adjacent to a concerned edge, the larger the gradient of
a signal at an edge. For this reason, the height of a phase
difference pulse is changed so that a phase difference pulse at an
edge of a signal with a longer mark, which is a more stable signal,
would have the higher height.
(2) A Second Method: a Method in which the Width of a Phase
Difference Pulse is Weighted According to the Length of a
Mark/Space Following a Concerned Edge
In this method, a phase difference pulse having the width equal to
the length of a mark/space is outputted. In other words, a phase
difference pulse is outputted during a period from an edge to the
immediately subsequent edge. In this way, a phase difference pulse
starting from an edge of a long mark signal has the larger width
than a phase difference pulse starting from an edge of a short mark
signal. In this case, however, a phase difference pulse does not
include information on the phase difference if the height of a
phase difference pulse is set constant. In order to avoid this, the
height of a pulse is also made approximately proportional to the
gradient at an edge as is the case with the first method.
FIG. 9 shows a summary of the tracking error signal generating
methods of the present invention, which have been described above,
in terms of information contained in the height and the width of a
phase difference pulse.
In the case of the conventional DPD method, the height of a pulse
does not contain any information, since the height is constant. The
width of a pulse indicates a phase difference, as it is, between
signals A1 and A2.
In the case of the first method, that is, the method in which the
height of a phase difference pulse is weighted according to the
gradient of a signal at a concerned edge, the height of a pulse
contains information on a phase difference between signals A1 and
A2, and information on the mark length. The width of a pulse
indicates a phase difference, as it is.
In the case of the second method, that is, the method in which the
width of a phase difference pulse is weighted according to the
length of a mark/space following a concerned edge, the width of a
pulse indicates the length of the mark/space, as it is, and the
height of a pulse contains the same information as that in the
first method.
As has been described, in both of the first and second methods, a
phase difference pulse is made to contain information on a phase
difference, and the area of a pulse is weighted according to the
length of a mark/space adjacent to a concerned edge. Thus, the
contribution ratio of long mark signals at edges to generate a
tracking error signal is increased, and thereby the foregoing
problem of the accuracy of detecting a phase difference in the
conventional DPD tracking method is solved.
According to the present invention, a tracking error signal becomes
stable, and thus a stable tracking servo can be performed even
during reproduction of data on a high density disc, or during
high-speed reproduction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram for explaining the principle of a conventional
DPD method.
FIG. 2 is a diagram showing a tracking error signal generating
block by using a conventional DPD method.
FIG. 3 is a diagram showing signal waveforms in a process where a
tracking error signal is generated in the conventional DPD
method.
FIG. 4 is a diagram showing a readout signal and the gradients at
edges thereof.
FIG. 5 is a schematic diagram of an optical disc reproduction
apparatus to which the present invention is applied.
FIGS. 6A and 6B show a tracking error signal generating method
according to a first embodiment of the present invention.
FIGS. 7A and 7B show a tracking error signal generating method
according to a second embodiment of the present invention.
FIGS. 8A and 8B show a tracking error signal generating method
according to a third embodiment of the present invention.
FIG. 9 is a table showing relationships between tracking error
signal generating methods and information included in the heights
and the widths of phase difference pulses.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, descriptions will be given of embodiments of the
present invention.
First Embodiment
FIG. 5 is a diagram showing a configuration example of an optical
disc reproduction apparatus to which the present invention is
applied.
Laser light 102 that is linearly polarizing light is generated from
a laser diode 101. It becomes a parallel pencil of light through a
collimating lens 103, and then enters a polarizing beam splitter
104. The polarizing beam splitter 104 is an optical element having
properties allowing linearly polarizing light in a certain
direction to pass therethrough almost without any loss, and causing
the linearly polarizing light in a direction perpendicular to the
certain direction to be reflected almost without any loss. The
polarizing beam splitter 104 is disposed so as to allow the laser
light 102 generated from the laser diode 101 to pass therethrough
almost without any loss. The laser light 102 having passed through
the polarizing beam splitter 104 is reflected by a half mirror 105,
and thus the travel direction of laser light 102 is changed
vertically. At the same time, the half mirror 105 also allows very
small part of the entered laser light to pass therethrough.
Accordingly, the laser light 102 having passed through the half
mirror 105 is concentrated by a front monitor lens 106 which is
located ahead of the half mirror 105, and then enters a front
monitor 107 which is located ahead of the front monitor lens 106.
The front monitor 107 outputs an electric current proportional to
the power of the entered laser light. The electric current
outputted by the front monitor 107 is generally used for
controlling the power by monitoring it, the power being outputted
by the laser diode 101. The laser light 102 reflected by the half
mirror 105 is changed to be circularly polarizing light by passing
through a quarter-wave plate 108. The laser light 102 which has
been changed to the circularly polarizing light is concentrated by
an objective lens 109, and then enters an optical disc 110.
The laser light 102 reflected by an information recording surface
111 of the optical disc 110 again passes through the objective lens
109, and then passes through the quarter-wave plate 108, thereby
becoming the linearly polarizing light again. In this linearly
polarizing light, the polarization direction is perpendicular to
the polarization direction for approaching the optical disc 110.
For this reason, the laser light 102 is reflected by the half
mirror 105, and then reflected by the polarizing beam splitter 104
almost without any loss. Thereafter, the laser light 102 is
concentrated by a detection lens 112, and enters a photo detector
113.
The photo detector 113 is a quadrant photo detector as shown in
FIG. 1. The output from the photo detector 113 is processed by a
readout signal generating circuit 114, a tracking error signal
generating circuit 120 and a focus error signal generating circuit
121. Thus, these circuits 114, 120 and 121 generate a readout
signal, a tracking error signal and a focus error signal,
respectively. The tracking error signal and the focus error signal
are supplied to a lens actuator 122, and the lens actuator 122
performs tracking control and focus control by driving the
objective lens 109 in a track width direction and in a focus
direction. In general, the foregoing configuration except for the
optical disc 110 is built up by using an optical pickup as an
optical system. The readout signal which is a sum signal of outputs
from four segment elements of the quadrant photo detector, is
processed to be user data by a signal processing circuit 115 and a
decoding circuit 116 of a controlling unit. The user data thus
processed is transferred to an upper apparatus via a micro
processor 117. The micro processor 117 also monitors the output
from the front monitor 107, and controls a laser driver circuit 118
so that the output would become a predetermined value.
Hereinafter, descriptions will be given of a configuration example
of the tracking error signal generating circuit 120 by using FIGS.
6A and 6B. FIG. 6A is a waveform diagram showing two kinds of
diagonal sum signals A1=(I.sub.A+I.sub.C) and A2=(I.sub.B+I.sub.D)
of the quadrant photo detector, and pulse signals generated from
the diagonal sum signals. FIG. 6B is a diagram showing a
configuration example of a circuit which generates the pulses shown
in FIG. 6A.
A phase difference pulse generating circuit of this embodiment is
configured of binarizing circuits, sample/hold circuits, summing
circuits, switch circuits and a subtracting circuit. The binarizing
circuits 63 and 66 output the binary signals of input signals. The
binarizing circuits 64 and 65 output the level inverted signals of
the binary signals of the input signals. The sample/hold circuits
(S/H) 67, 68, 69 and 610 hold the levels of the input signals D at
an edge of a sample/hold signal A until the time when the next edge
of the sample/hold signal A comes. The switch circuits 613 and 614
output the input signals D themselves in a case where both signals
A and B are at high levels (H), and output zero 0 in the other
cases. The subtracting circuit 615 outputs a difference signal
indicating the difference between the outputs from the switch
circuit 613 and the switch circuit 614.
As shown in FIG. 6A, in this configuration, a phase difference
pulse is outputted during a period from an edge time of a signal A1
to the immediately subsequent edge time of a signal A2. In other
words, the width of the phase difference pulse is equal to the
absolute value of the phase difference between the signals A1 and
A2. The height of the pulse is equal to the sum of the signal level
of the signal A2 at the edge time (a time at which a signal crosses
over the zero level) of the signal A1, and the signal level of the
signal A1 at the edge time of the signal A2.
As shown in FIG. 6A, here, assume that p denotes the signal level
of the diagonal sum signal A2 at the edge time of the diagonal sum
signal A1, and that q denotes the signal level of the signal A1 at
the edge time of the signal A2, when the diagonal sum signals A1
and A2 cross over the zero level in the minus-to-plus direction. In
addition, assume that r denotes the signal level of the signal A2
at the edge time of the signal A1, and that s denotes the signal
level of the signal A1 at the edge time of the signal A2, when the
diagonal sum signals A1 and A2 cross over the zero level in the
plus-to-minus direction.
In this situation, when the diagonal sum signals cross over the
zero level in the minus-to-plus direction, a pulse 61 is outputted
during a period from an edge time of the signal A1 to the
immediately subsequent edge time of the signal A2 whose phase
delays. The signal level of this pulse 61 is equal to the sum (s+p)
of the signal level p of the signal A2 at the edge time of the
signal A1, which is a pulse starting time, and the signal level s
of the signal A1 at the immediately preceding edge time of the
signal A2. On the other hand, when the diagonal sum signals cross
over the zero level in the plus-to-minus direction, a pulse 62 is
outputted during a period from an edge time of the signal A1 to the
immediately subsequent edge time of the signal A2. The signal level
of this pulse 62 is equal to the sum (q+r) of the signal level r of
the signal A2 at the edge time of the signal A1, which is a pulse
starting time, and the signal level q of the signal A1 at the
immediately preceding edge time of the signal A2. The signs of
these two pulses are different from each other. Hence, a tracking
error signal is generated by amplifying the difference between
these two pulses, that is, the pulse 61 with the signal level (s+p)
and the pulse 62 with the signal level (q+r), and then by adding up
the differences thus amplified for a certain period of time.
Second Embodiment
By using FIGS. 7A and 7B, descriptions will be given of another
embodiment of a configuration which is based on the basic
configuration of the first embodiment, and which is different
therefrom only in a tracking error signal generation unit. FIG. 7A
is a waveform diagram showing two kinds of diagonal sum signals
A1=(I.sub.A+I.sub.C) and A2=(I.sub.B+I.sub.D) of a quadrant photo
detector, and pulse signals generated from the diagonal sum
signals. FIG. 7B is a diagram showing a configuration example of a
circuit which generates the pulses shown in FIG. 7A.
A phase difference pulse generating circuit of this embodiment is
configured of binarizing circuits, sample/hold circuits, switch
circuits and subtracting circuits. The operations of the binarizing
circuits, the sample/hold circuits and the switch circuits are the
same as those in a case of the first embodiment.
With this configuration, a phase difference pulse is outputted
during a period from an edge time of a signal A2 to the immediately
subsequent edge time of a signal A1. In other words, the width of
the phase difference pulse is approximately equal to the length of
the mark/space following the edge of the signal A2. On the other
hand, the height of the phase different pulse is the difference
between the signal level of the signal A1 at the edge time of the
signal A2, which is a starting time of the phase difference pulse,
and the signal level of the signal A2 at the immediately preceding
edge time of the signal A1.
As shown in FIG. 7A, here, assume that the signal levels p, q, r
and s are defined in the same manner as those shown in FIG. 6A. In
this embodiment, when the diagonal sum signals cross over the zero
level in the minus-to-plus direction, a pulse 71 is outputted
during a period from an edge time of the signal A2 to the
immediately subsequent edge time of the signal A1. The signal level
of this pulse 71 is equal to the value of (p-q). On the other hand,
when the diagonal sum signals cross over the zero level in the
plus-to-minus direction, a pulse 72 is outputted during a period
from an edge time of the signal A2 to the immediately subsequent
edge time of the signal A1. The signal level of this pulse 72 is
equal to the value of (r-s). The signs of these two pulses are
different from each other. Hence, a tracking error signal is
generated by amplifying the difference between these two pulses,
that is, the pulse 71 with the signal level (p-q) and the pulse 72
with the signal level (r-s), and then by adding up the differences
thus amplified for a certain period of time.
Third Embodiment
By using FIGS. 8A and 8B, descriptions will be given of another
embodiment of a configuration which is based on the basic
configuration of the first embodiment, and which is different
therefrom only in a tracking error signal generation unit. FIG. 8A
is a waveform diagram showing two kinds of diagonal sum signals
A1=(I.sub.A+I.sub.C) and A2=(I.sub.B+I.sub.D) of a quadrant photo
detector, and pulse signals generated from the diagonal sum
signals. FIG. 8B is a diagram showing a configuration example of a
circuit which generates the pulses shown in FIG. 8A.
A phase difference pulse generating circuit of this embodiment is
configured of binarizing circuits, sample/hold circuits, a summing
circuit, a switch circuit and subtracting circuits. The operations
of the binarizing circuits, the sample/hold circuits, the
subtracting circuits and the summing circuit are the same as those
in a case of the first embodiment. The switch circuits 813 and 814
output input signals D themselves in a case where a signal A is at
a high level (H), and outputs zero 0 in a case where the signal A
is at a low level (L).
With this configuration, an average level signal A3 indicating the
average level of the signals A1 and A2 is used, and a phase
difference pulse is outputted during a period from a first edge
time of the signal A3 to the immediately subsequent edge time of
the signal A3. The height of the phase difference pulse is equal to
the difference between the signal levels of the signals A1 and A2
at the first edge of the signal A3.
As shown in FIG. 8A, here, when the diagonal sum signals cross over
the zero level in the minus-to-plus direction, assume that, at an
edge time of the signal A3: the signal level of the signal A1 is q;
and the signal level of the signal A2 is p. Moreover, when the
diagonal sum signals cross over the zero level in the plus-to-minus
direction, assume that, at an edge time of the signal A3: the
signal level of the signal A1 is s; and the signal level of the
signal A2 is r. At this time, a pulse 81 with the signal level
(p-q) is outputted during a period when the signal A3 is plus. On
the other hand, a pulse 82 with the signal level (r-s) is outputted
during a period when the signal A3 is minus. The signs of these two
pulses are different from each other. Hence, a tracking error
signal is generated by amplifying the difference between these two
pulses, that is, the pulse 81 with the signal level (p-q) and the
pulse 82 with the signal level (r-s), and then by adding up the
differences thus amplified for a certain period of time.
* * * * *