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Claims  |
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What is claimed is:
1. A prepit detecting apparatus which uses an optical recording medium
having information recording tracks on which information is recorded and
guide tracks each of which guides a light beam to the information
recording track and on which prepits for saving pre-information are formed
such that the light beam is applied to said information track of said
optical recording medium, said prepit detecting apparatus comprising:
light receiving means divided into a first divided light receiving portion
and a second divided light receiving portion divided along an optically
parallel division line in a direction of tangent of the information tracks
and arranged to receive reflected light of the light beam with which the
optical recording medium has been irradiated; and
a difference calculator for calculating the difference between a first read
signal output from said first divided light receiving portion and a second
read signal output from said second divided light receiving portion so
that the prepit is detected in response to a differential signal output
from said difference calculator;
wherein said difference calculator incorporates amplitude correction means
for causing the amplitudes of the first read signal and the second read
signal to coincide with a reference level, and the difference between the
first read signal and the second read signal corrected by said amplitude
correction means is calculated.
2. A prepit detecting apparatus according to claim 1, wherein said
amplitude correction means incorporates first amplitude extracting means
for extracting an average amplitude level of the first read signal, second
amplitude extracting means for extracting an average amplitude level of
the second read signal, first comparison means for comparing an output of
said first amplitude extracting means and an output of said second
amplitude extracting means with each other, second comparison means for
comparing an output of said second amplitude extracting a means and the
reference level with each other, first amplitude adjustment means for
adjusting the amplitude of the first read signal in accordance with an
output of said first comparison means and said second adjustment means for
adjusting the amplitude of the second read signal in accordance with an
output of said second comparison means.
3. A prepit detecting apparatus according to claim 1, wherein said
amplitude correction means incorporates first amplitude extracting means
for extracting an average amplitude level of the first read signal, second
amplitude extracting means for extracting an average amplitude level of
the second read signal, first comparison means for comparing an output of
said first amplitude extracting means with a reference level, second
comparison means for comparing an output of said second amplitude
extracting means with a reference level, first amplitude adjustment means
for adjusting the amplitude of the first read signal in accordance with an
output of said first comparison means and second adjustment means for
adjusting the amplitude of the second read signal in accordance with an
output of said second comparison means.
4. A prepit detecting apparatus arranged to use an optical recording medium
which has information recording tracks on which information is recorded,
which is wobbled in response to a wobble signal having a predetermined
frequency and which has guide tracks for guiding a light beam to said
information recording track having prepits for saving pre-information and
incorporating light receiving means divided into a first divided light
receiving portion and a second divided light receiving portion divided
along an optically parallel division line in a direction of tangent of the
information tracks and arranged to receive reflected light of the light
beam with which the optical recording medium has been irradiated and a
difference calculator for calculating the difference between a first read
signal output from said first divided light receiving portion and a second
read signal output from said second divided light receiving portion so
that the prepit is detected in response to a differential signal output
from said difference calculator, said prepit detecting apparatus
comprising:
DC clamping means for clamping the differential signal output from said
difference calculator with a DC level; and
prepit detecting means which compares an output from said clamping means
and a reference slice level with each other to detect the prepit signal;
wherein said DC clamping means clamps the wobble signal component
corresponding to detection timing of the prepit signal detected by said
prepit detecting means to the-predetermined DC level.
5. A prepit detecting apparatus according to claim 4, wherein said DC
clamping means incorporates relay means for relaying the differential
signal at detection timing of the prepit signal, an integrating circuit
for integrating the differential signal supplied through said relay means
and superimposing means for superimposing an output of said integrating
circuit on the differential signal, and said integrating circuit
incorporates a calculation amplifier having a non-inverted input terminal
to which a predetermined DC level is input and an integrating capacitor
connected between an output terminal of said calculation amplifier and an
inverted input terminal.
6. A prepit detecting apparatus which uses an optical recording medium
having information recording tracks on which information is recorded and
guide tracks each of which guides a light beam to the information
recording track and on which prepits for saving pre-information are formed
such that the light beam is applied to said information track of said
optical recording medium, said prepit detecting apparatus comprising:
a light receptor divided into a first divided light receiving portion and a
second divided light receiving portion divided along an optically parallel
division line in a direction of tangent of the information tracks and
arranged to receive reflected light of the light beam with which the
optical recording medium has been irradiated; and
a difference calculator for calculating the difference between a first read
signal output from said first divided light receiving portion and a second
read signal output from said second divided light receiving portion so
that the prepit is detected in response to a differential signal output
from said difference calculator;
wherein said difference calculator incorporates an amplitude correction
unit for causing the amplitudes of the first read signal and the second
read signal to coincide with a reference level, and the difference between
the first read signal and the second read signal corrected by said
amplitude correction unit is calculated.
7. A prepit detecting apparatus according to claim 1, wherein said
amplitude correction unit incorporates a first amplitude extracting unit
for extracting an average amplitude level of the first read signal, a
second amplitude extracting unit for extracting an average amplitude level
of the second read signal, a first comparison unit for comparing an output
of said first amplitude extracting unit and an output of said second
amplitude extracting unit with each other, a second comparison unit for
comparing an output of said second amplitude extracting unit and the
reference level with each other, a first amplitude adjustment unit for
adjusting the amplitude of the first read signal in accordance with an
output of said first comparison unit, and a second adjustment unit for
adjusting the amplitude of the second read signal in accordance with an
output of said second comparison unit.
8. A prepit detecting apparatus according to claim 1, wherein said
amplitude correction unit incorporates a first amplitude extracting unit
for extracting an average amplitude level of the first read signal, a
second amplitude extracting unit for extracting an average amplitude level
of the second read signal, a first comparison unit for comparing an output
of said first amplitude extracting unit with a reference level, a second
comparison unit for comparing an output of said second amplitude
extracting unit with a reference level, a first amplitude adjustment unit
for adjusting the amplitude of the first read signal in accordance with an
output of said first comparison unit, and a second adjustment unit for
adjusting the amplitude of the second read signal in accordance with an
output of said second comparison means.
9. A prepit detecting apparatus arranged to use an optical recording medium
which has information recording tracks on which information is recorded,
which is wobbled in response to a wobble signal having a predetermined
frequency and which has guide tracks for guiding a light beam to said
information recording track having prepits for saving pre-information and
incorporating a light receptor divided into a first divided light
receiving portion and a second divided light receiving portion divided
along an optically parallel division line in a direction of tangent of the
information tracks and arranged to receive reflected light of the light
beam with which the optical recording medium has been irradiated and a
difference calculator for calculating the difference between a first read
signal output from said first divided light receiving portion and a second
read signal output from said second divided light receiving portion so
that the prepit is detected in response to a differential signal output
from said difference calculator, said prepit detecting apparatus
comprising:
a DC clamping unit for clamping the differential signal output from said
difference calculator with a DC level; and
a prepit detector which compares an output from said DC clamping unit and a
reference slice level with each other to detect the prepit signal;
wherein said DC clamping unit clamps the wobble signal component
corresponding to detection timing of the prepit signal detected by said
prepit detector to the predetermined DC level.
10. A prepit detecting apparatus according to claim 4, wherein said DC
clamping unit incorporates a relay for relaying the differential signal at
detection timing of the prepit signal, an integrating circuit for
integrating the differential signal supplied through said relay and a
superimposing unit for superimposing an output of said integrating circuit
on the differential signal, and said integrating circuit incorporates a
calculation amplified having a non-inverted input terminal to which a
predetermined DC level is input and an integrating capacitor connected
between an output terminal of said calculation amplifier and an inverted
input terminal. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a prepit detecting apparatus arranged to
detect prepit information in an optical recording/reproducing apparatus
for recording/reproducing information with respect to a recording-enabled
optical recording medium having pre-information, such as address
information, in the form of prepits.
2. Description of the Related Art
In general, a recording-enabled optical recording medium is enabled to
record information on an optical recording medium on which no information
has been recorded by recording address information and reference signals
for generating a clock signal for use in a recording/reproducing operation
in the form of prepits and pregrooves. A DVD-R (Digital Versatile
Disc-Recordable) has energetically been researched and developed, the
DVD-R being a recording medium which is capable of recording information
in a quantity which is about seven times information which can be recorded
on the CD. The DVD-R has pregrooves which are regions on which
information, such as video data and audio data, which must be recorded is
recorded as recording pits. Moreover, the DVD-R has prepits (hereinafter
called land prepits (LPP)) in land portions which are regions formed among
the foregoing pregrooves. The LPP is formed on a straight line which
perpendicularly intersect the direction of a tangent of the pregrooves
such that the land prepits are not formed adjacent to each other across a
pregrooves. The pregrooves are slightly swung (wobbled) in the radial
direction of the disc at a frequency based on a reference clock which is
used to control the rotation of the DVD-R. When the rotation of the DVD-R
is controlled, the wobbling frequency is detected. Thus, feedback control
is performed in such a manner that the detected wobbling frequency
coincides with the frequency of the reference clock.
The LPP is detected such that reflected light of a light beam with which
the pregrooves has been irradiated is received by a light receiving device
divided into two sections at least along a division line optically
parallel with the direction of a tangent of the pregrooves. Moreover, the
difference of output signals from the regions (the divided regions) of the
light receiving device in a direction perpendicular to the pregrooves is
calculated. The obtained differential signal is compared with a
predetermined threshold so that the LPP is detected as a binary signal
(hereinafter called an "LPP signal"). When the recording medium is an
optical disc, the divided regions of the light receiving device are formed
in the radial direction of the disc owing to the foregoing division line.
Therefore, the foregoing differential signal is called a "radial push-pull
signal".
The reason why the LPP can be detected by using the radial push-pull signal
is that the LPP is formed as described above such that the level does not
exist in adjacent land portions on a straight line which perpendicularly
intersects the direction of tangent of the pregroove. That is, when one
pregroove is irradiated with a light beam, reflection components of the
LPP do not simultaneously exist in light reflected from the two side land
portions (the LPP component exists in only reflected light from either of
the land portions). Thus, the foregoing calculation for obtaining the
difference enables only the component of light reflected from the LPP to
be extracted. Usually, only either (for example, the positive pole
component) of the two polar components obtained by the calculation for
obtaining the difference is compared with the predetermined threshold. An
obtained binary signal is employed as the LPP signal.
A record-enabled optical recording medium, such as the DVD-R, encounters
reduction in the reflectance owing to thermal energy of a recording beam
when a high output light beam is applied to form the recording pit for
storing information. That is, the quantity of light reflected from the
position of a pregroove in which the recording pit has been formed (which
has been irradiated with the recording beam) is smaller than the quantity
of light reflected from the position of a pregroove having no recording
pit. If the recording beam is also used as the reproducing beam (when the
recording beam is used as the reproducing beam, a low output is employed)
or if a tracking error signal can be generated by one beam, the structure
of a recording/reproducing apparatus can conveniently be simplified.
Therefore, the diameter of the recording beam is usually adjusted to be
slightly larger than the width of the pregroove. Thus, when information is
recorded, also a portion of the land adjacent to the position of the
pregroove on which the recording pit for storing information is formed is
irradiated with the recording beam. Therefore, if an LPP exists in the
land portion adjacent to the position of the pregroove on which the
recording pit is formed, the following problem arises when the LPP is read
after the recording pit has been formed.
That is, irradiation with the high-output recording beam for the purpose of
forming the recording pit causes the reflectance of the land portion
having the LPP to be reduced. Therefore, when the LPP is read, the
quantity of light reflected from the LPP is reduced. Also the amplitude
level of the differential signal of the LPP which is extracted as the
radial push-pull signal is lowered. As a result, there arises a problem in
that the S/N ratio (signal-to-noise ratio) of the LPP signal with respect
to unnecessary noise deteriorates.
On the other hand, the pregrooves are wobbled in the radial direction of
the disc as described above. Therefore, the radial push-pull signal is
formed into a composite signal in which the prepit is superimposed on the
component of the wobbling frequency. A disc, such as the DVD-R, on which
information has densely been recorded, sometimes encounters a fact that
the component of the wobble signal of a pregroove adjacent to a pregroove
which is irradiated with a light beam is leaked and introduced owing to
crosstalk. If the foregoing leakage and introduction occur, the component
of the wobble signal in the foregoing composite signal is undesirably
caused to interfere. Thus, the amplitude is undesirably changed.
That is, the component of the LPP signal is superimposed on the wobble
signal having the amplitude which is changed. Since the amplitude of the
wobble signal serving as the base-line voltage is undesirably changed,
comparison with a fixed slice level for detecting the LPP signal in the
form of a binary signal cannot easily be performed.
To overcome the interference of the wobble signal component, for example, a
method exists with which the amplitude of the wobble is AM-detected to
obtain the amplitude change component. Moreover, the obtained change
component is reduced to a slice level for binary-coding the LPP, followed
by performing binary coding while following to change in the wobble is
being performed.
However, when the foregoing method is employed, an AM wave detecting
circuit, a variety of filters and suppression of the LPP component when
the wobble amplitude is detected are required. Therefore, the size of the
circuit is enlarged excessively. Moreover, setting and adjustment of a
quantity of injection of the change in the wobble must be performed.
Therefore, there arises a problem in that the operation for adjusting the
circuit becomes complicated.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide
a prepit detecting apparatus having a simple structure and capable of
accurately extracting an LPP signal.
To solve the problem, according to one aspect of the present invention,
there is provided a prepit detecting apparatus which uses an optical
recording medium having information recording tracks on which information
is recorded and guide tracks each of which guides a light beam to the
information recording track and on which prepits for saving
pre-information are formed such that the light beam is applied to the
information track of the optical recording medium, the prepit detecting
apparatus comprising: light receiving means divided into a first divided
light receiving portion and a second divided light receiving portion
divided along an optically parallel division line in a direction of
tangent of the information tracks and arranged to receive reflected light
of the light beam with which the optical recording medium has been
irradiated; and a difference calculator for calculating the difference
between a first read signal output from the first divided light receiving
portion and a second read signal output from the second divided light
receiving portion so that the prepit is detected in response to a
differential signal output from the difference calculator, wherein the
difference calculator incorporates amplitude correction means for causing
the amplitudes of the first read signal and the second read signal to
coincide with a reference level, and the difference between the first read
signal and the second read signal corrected by the amplitude correction
means is calculated.
Another aspect of the present invention has a structure that the amplitude
correction means incorporates first amplitude extracting means for
extracting an average amplitude level of the first read signal, second
amplitude extracting means for extracting an average amplitude level of
the second read signal, first comparison means for comparing an output of
the first amplitude extracting means and an output of the second amplitude
extracting means with each other, second comparison means for comparing an
output of the second amplitude extracting means and the reference level
with each other, first amplitude adjustment means for adjusting the
amplitude of the first read signal in accordance with an output of the
first comparison means and second adjustment means for adjusting the
amplitude of the second read signal in accordance with an output of the
second comparison means.
Another aspect of the present invention has a structure that the amplitude
correction means incorporates first amplitude extracting means for
extracting an average amplitude level of the first read signal, second
amplitude extracting means for extracting an average amplitude level of
the second read signal, first comparison means for comparing an output of
the first amplitude extracting means with a reference level, second
comparison means for comparing an output of the second amplitude
extracting means with a reference level, first amplitude adjustment means
for adjusting the amplitude of the first read signal in accordance with an
output of the first comparison means and second adjustment means for
adjusting the amplitude of the second read signal in accordance with an
output of the second comparison means.
Since the recording pits for storing data are formed, the amplitude
correction means corrects reduction in the amplitude of each of the first
and second read signals if the quantity of light reflected from the LPP
which must be extracted is reduced. Therefore, the LPP signal can
accurately be extracted.
According to another aspect of the present invention, there is provided a
prepit detecting apparatus arranged to use an optical recording medium
which has information recording tracks on which information is recorded,
which is wobbled in response to a wobble signal having a predetermined
frequency and which has guide tracks for guiding a light beam to the
information recording track having prepits for saving pre-information and
incorporating light receiving means divided into a first divided light
receiving portion and a second divided light receiving portion divided
along an optically parallel division line in a direction of tangent of the
information tracks and arranged to receive reflected light of the light
beam with which the optical recording medium has been irradiated and a
difference calculator for calculating the difference between a first read
signal output from the first divided light receiving portion and a second
read signal output from the second divided light receiving portion so that
the prepit is detected in response to a differential signal output from
the difference calculator, the prepit detecting apparatus comprising: DC
clamping means for clamping the differential signal output from the
difference calculator with a DC level; and prepit detecting means which
compares an output from the clamping means and a reference slice level
with each other to detect the prepit signal, wherein the DC clamping means
clamps the wobble signal component corresponding to detection timing of
the prepit signal detected by the prepit detecting means to the
predetermined DC level.
Another aspect of the present invention has a structure that the DC
clamping means incorporates relay means for relaying the differential
signal at detection timing of the prepit signal, an integrating circuit
for integrating the differential signal supplied through the relay means
and superimposing means for superimposing an output of the integrating
circuit on the differential signal, and the integrating circuit
incorporates a calculation amplifier having a non-inverted input terminal
to which a predetermined DC level is input and an integrating capacitor
connected between an output terminal of the calculation amplifier and an
inverted input terminal.
Therefore, if the base voltage of the first transition of the LPP signal is
changed, the LPP can accurately be extracted.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram schematically showing the structure of a prepit
detecting apparatus according to an embodiment of the present invention;
FIG. 2 is a block diagram showing the internal structure of an RF pit
correction circuit 2 according to the embodiment of the present invention;
FIG. 3 is a graph showing an input/output characteristic of a nonlinear
circuit realized by a circuit realized by approximation using broken lines
according to the embodiment of the present invention;
FIG. 4 is a block diagram showing the structure of an RF level correction
circuit 3 according to the embodiment of the present invention;
FIG. 5 is a block diagram showing the structure of an RF level correction
circuit according to another embodiment of the present invention;
FIG. 6 is a block diagram showing a binary-coding circuit according to the
embodiment of the present invention;
FIG. 7 is a graph showing timing of LPP and that of a sample pulse;
FIGS. 8A and 8B are graphs showing an example of an LPP signal superimposed
on a wobble signal;
FIG. 9 is a diagram showing an example of a DVD-R having prepits formed on
land tracks thereof; and
FIG. 10 is a diagram showing a recording format of the DVD-R according to
the embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will now be described with
reference to the drawings.
Referring to FIG. 9, the structure of a DVD-R will now be described.
Referring to FIG. 9, the DVD-R disc is a pigment-type DVD-R incorporating a
pigment film 105 and permitted to write information only one time. The
DVD-R has pregrooves 102 on which information is recorded and lands 103
for guiding light beam B, such as a laser beam, which is reproducing light
or recording light to the pregroove 102. Moreover, the DVD-R has a
protective film 107 for protecting the foregoing elements and a reflecting
surface 106 for reflecting the light beam B when recorded information is
reproduced. Moreover, LPP 104 are provided for the lands 103.
The pregrooves 102 of the foregoing DVD-R are wobbled at a frequency
corresponding to the rotational speed of the DVD-R disc. When information
(pre-information and information, such as image information which must be
recorded except for the synchronizing signal), is recorded, an information
recording apparatus detects the frequency of wobbling of the pregrooves
102. Thus, a synchronizing signal is obtained to control the rotation of
the DVD-R disc to a predetermined rotational speed. Moreover, the LPP 104
is detected to obtain pre-information. In accordance with the
pre-information, an optimum output of the light beam B which is the
recording light is determined. Moreover, address information which
indicates the position on the DVD-R on which information must be recorded
is obtained. In accordance with the address information, information which
must be recorded is recorded at the corresponding recording position.
When information is recorded, the light beam B is applied in such a manner
that the center of the light beam B coincides with the center of the
pregroove 102. Thus, recorded information pit corresponding to the
information which must be recorded is formed on the pregroove 102. Thus,
recorded information is formed. The size of a light spot is determined
such that a portion of the light spot is also applied to the land 103 as
well as the pregroove 102, as shown in FIG. 9. Reflected light of a
portion of the light spot applied to the land 103 is used to detect
pre-information from the LPP 104 so as to obtain the pre-information.
Moreover, reflected light of the light spot applied to the pregroove 102
is used to detect a wobbling signal from the pregroove 102. Thus, a clock
signal for controlling rotation can be obtained.
Recording formats for pre-information and rotation control information
previously recorded on the DVD-R will now be described with reference to
FIG. 10. The upper portion of FIG. 10 shows the recording format for
information which must be recorded, while waves in the lower portion of
FIG. 10 show a wobbling state (a plan view of the pregroove 102) on which
the information is recorded. Upward arrows between information which must
be recorded and the pregrooves 102 schematically indicate positions at
which the LPP 104 is formed. The wobbling state of the pregrooves 102
shown in FIG. 10 is illustrated with a greater amplitude as compared with
an actual amplitude to cause the illustration to easily be understood.
Information which must be recorded is recorded on the center line of the
pregroove 102.
As shown in FIG. 10, information which is recorded on the DVD-R is
previously divided for each sync-frame. Twenty six sync-frames form one
recording sector which is an information unit. Moreover, sixteen recording
sectors form one ECC block which is an information block. One sync-frame
has a length which is 1488 times (1488T) a unit length (hereinafter called
"T") corresponding to a bit interval which is determined in accordance
with the recording format when information, which must be recorded, is
recorded. In a leading end portion of one sync-frame having a length of
14T, synchronization information SY for synchronization for each
sync-frame is recorded.
Pre-information which is recorded on the DVD-R is recorded for each
sync-frame. When pre-information is recorded by using the LPP 104, one LPP
104 indicating the synchronizing signal of pre-information must be formed
in a land 103 adjacent to a region of each sync-frame for information
which must be recorded and on which the synchronization information SY is
recorded. Moreover, one or two LPP 104 are formed which indicate the
contents (address information) of pre-information which must be recorded
in the land 103 adjacent to the front-half portion in the sync-frame
except for the synchronization information SY (note that the LPP 104 is
not sometimes formed in the front-half portion in the sync-frame except
for the synchronization information SY depending on the contents of
pre-information which must be recorded. Moreover, three LPP 104 are
continuously formed in the front-half portion of the leading sync-frame in
one recording sector without exception). At this time, in one recording
sector, the LPP 104 is formed in only even-number sync-frames (hereinafter
called "EVEN frames") or only odd-number sync-frames (hereinafter called
"ODD frames"). Thus, pre-information is recorded. That is, when the LPP
104 is formed in the EVEN frame in a case shown in FIG. 10 (as indicated
with upward arrows on a solid line shown in FIG. 10), the LPP 104 is not
formed in the adjacent ODD frame.
As for the relationship between wobbling of the pregrooves 102 and the LPP
104, an LPP 104 is formed at the position of a maximum amplitude in the
wobbling.
The pregrooves 102 in all of the sync-frames are wobbled at predetermined
wobbling frequency of which is 145 kHz (a frequency at which one
sync-frame corresponds to eight change waves of the pregrooves 102). The
information recording apparatus detects the predetermined wobbling
frequency of so that a synchronizing signal for controlling the rotation
of a spindle motor 8 for rotating the DVD-R is extracted.
Referring to FIGS. 1 to 8, an embodiment of the prepit detecting apparatus
according to the present invention will now be described.
FIG. 1 is a block diagram showing the overall shape of the prepit detecting
apparatus according to the embodiment of the present invention.
In this embodiment, an assumption is made that recording pits having
recorded information have been formed in a portion of pregrooves 102 of
the DVD-R.
As shown in FIG. 1, the prepit detecting apparatus according to this
embodiment incorporates a DVD-R; the spindle motor 8 for rotating the
DVD-R; a pickup 1 which irradiates an information recording surface of the
DVD-R which is being rotated with a reproducing beam, which causes a light
receiving device (the divided light receiving portions of the light
receiving device are called a first divided light receiving portion and a
second divided light receiving portion for convenience) divided into two
sections in a division line at least optically parallel with a direction
of tangent of the pregroove on the DVD-R to receive reflected light of the
applied reproducing beam from the information recording surface, which
outputs electric signals corresponding to the quantities of light received
by the first and second divided light receiving portions as first divided
RF signal SDRF1 and second divided RF signal SDRF2 and which outputs the
sum of the first divided RF signal SDRF1 and the second divided RF signal
SDRF2 as the sum RF signal SRF; an RF pit correction circuit 2 serving as
an amplitude correction means for correcting the amplitude level of each
of the first and second divided RF signals SDRF1 in accordance with a
recording pit component contained in the sum RF signal SRF output from the
pickup 1; an RF level correction circuit 3 for uniforming the levels of
the divided RF signals corrected by the RF pit correction circuit 2; a
radial push-pull signal generating circuit 4 which is a dividing unit for
generating radial push-pull signal Srpp which is a differential from the
uniformed first and second divided RF signals SDRF2; a prepit detecting
circuit 5 which binary-code the generated radial push-pull signal Srpp to
detect the same as the LPP signal; and a decoder 6 which decodes the
detected LPP signal to extract address information represented by the LPP
signal. Address information output from the decoder 6 is supplied to a
system control portion (not shown) for totally controlling the prepit
detecting apparatus.
The sum RF signal SRF and each of divided RF signals output from the pickup
1 have positive polarity with respect to light reflected from the portion
of the DVD-R on which the recording pit has been formed.
An operation of the RF pit correction circuit 2 for processing the signal
will now be described with reference to FIGS. 2 and 3.
FIG. 2 is a block diagram showing the internal structure of the RF pit
correction circuit 2 which incorporates an amplitude correction circuit
21, a nonlinear amplifier 22, a voltage controlled amplifier 23 and a
voltage controlled amplifier 24.
The amplitude correction circuit 21 has a function to correct change in the
amplitude which is caused from change in the pit length of the recording
pit which occurs owing to the spatial frequency characteristic of the
pickup 1 and represented by the sum RF signal SRF. In general, the spatial
frequency characteristic of the pickup is a low-pass characteristic. That
is, as the spatial frequency component of the recording pit which must be
read is raised (as the pit length of the recording pit is shortened), the
detection performance of the pickup deteriorates. Therefore, the amplitude
of the sum RF signal SRF is shortened. Since the length of the recording
pit varies according to information which must be recorded, long recording
pits and short recording pits (hereinafter called "long pits and short
pits" if necessary) are mixed in the sum RF signal SRF. Therefore, the
level of the amplitude of the sum RF signal SRF output from the pickup 1
varies according to the length of the recording pit. The amplitude
correction circuit 21 corrects the foregoing variation in the amplitude of
the sum RF signal SRF owing to the length of the recording pit. The
amplitude correction circuit 21, for example, comprises a so-called HBF
(High Boost Filter) having the amplification factor which can be enlarged
as the frequency of the signal is raised. Therefore, the reduction in the
amplitude of the sum RF signal SRF owing to the short pits is corrected by
the amplitude correction circuit 21 to have the amplitude which is
substantially the same as the amplitude caused from the long pits, and
then output to the nonlinear amplifier 22.
The nonlinear amplifier 22 has an input/output characteristic having
nonlinearity realized by approximation using broken lines shown in FIG. 3.
Specifically, amplification factor of an input signal with respect to a
positive input is larger than the amplification factor with respect to a
negative input. A signal input to the foregoing nonlinear amplifier is
formed into an output signal, in which the positive amplitude level is
emphasized. In this embodiment, the polarity of the sum RF signal SRF is
made to be positive with respect to the portion of the DVD-R disc on which
the recording pit has been formed. Therefore, the nonlinear amplifier 22
amplifies the signal level of the sum RF signal SRF corresponding to the
recording pit so as to have the amplitude which is larger than that of the
signal level which does not correspond to the recording pit, that is, the
negative polar portion of the sum RF signal SRF. The sum RF signal SRF
which has been nonlinear-amplified is output as amplification-factor
control signal Sac22 for the voltage controlled amplifiers 23 and 24.
On the other hand, the voltage controlled amplifier 23 amplifies the first
divided RF signal SDRF1 supplied from the pickup 1 with the amplification
factor represented by the amplification-factor control signal Sac22
supplied from the nonlinear amplifier 22. Specifically, the amplification
factor is an amplification factor with which the amplitude is enlarged as
the amplitude level of the amplification-factor control signal Sac22 is
enlarged. The amplified signal is, as a first correction signal Scs1,
output to the following RF level correction circuit 3.
Similarly, the voltage controlled amplifier 24 amplifies the second divided
RF signal supplied from the pickup 1 with the amplification factor
represented by the amplification-factor control signal Sac22 supplied from
the nonlinear amplifier 22. The amplified signal is, as a second
correction signal Scs2, output to the following RF level correction
circuit 3.
The first and second divided RF signals SDRF2 and the sum RF signal SRF
have the same phase. Therefore, the first and second correction signal
Scs2 are formed such that the signal level corresponding to the recording
pit is emphasized as compared with the signal level which does not
correspond to the recording pit, similarly to the amplification-factor
control signal Sac22.
The reason why the foregoing biased emphasis is performed will now be
described. When a prepit is formed on a land adjacent to a recording pit,
a signal component caused from the foregoing prepit is contained in the
positive pole portion of the first divided RF signal SDRF1. When the
positive pole component of each divided RF signal is emphasized, the S/N
of a signal of the prepit contained in a difference signal between the
first and second divided RF signal SDRF2 can be improved.
The RF level correction circuit 3 will now be described with reference to
FIG. 4.
FIG. 4 is a block diagram showing the RF level correction circuit.
A first voltage controlled amplifier 31 amplifies the first correction
signal Scs1 supplied from the RF pit correction circuit 2 with the
amplification factor represented by an amplification-factor control signal
Sac35 supplied from a first voltage generating portion 35 to be described
later. Thus, a first amplified signal Sap1 is generated so as to be output
to the first RF-signal-level detecting portion 33 and the following radial
push-pull signal generating circuit 4.
The RF-signal-level detecting portion 33 which is the first amplitude
extracting means comprises a time-constant circuit having a sufficiently
long time constant with respect to intervals of signals corresponding to
the long pits of the first divided RF signal SDRF1. The RF-signal-level
detecting portion 33 detects an average level of the amplitude level of
the supplied first amplifying signal Sap1 to output it as a first average
signal to the first voltage generating portion 35 which is a first
amplitude adjustment means.
The first voltage generating portion 35 comprises a so-called differential
circuit which calculates the difference between the average level supplied
from the RF-signal-level detecting portion 33 and a second average signal
supplied from the second RF-signal-level detecting portion 34. Thus, the
first voltage generating portion 35 supplies an obtained difference signal
to the first voltage controlled amplifier 31 as the amplification control
signal Sac35.
On the other hand, the second voltage controlled amplifier 32 amplifies the
second correction signal Scs2 supplied from the RF pit correction circuit
2 with the amplification factor represented by the amplification control
signal Sac36 supplied from the second voltage generating portion 36. Thus,
the second voltage controlled amplifier 32 generates the second amplified
signal Sap2 to output it to the second RF-signal-level detecting portion
34 and the radial push-pull circuit 4.
The RF-signal-level detecting portion 34 which is the second amplitude
extracting means comprises a time-constant circuit having a sufficiently
long time constant with respect to the intervals of signals corresponding
to the long pits of the second divided RF signal SDRF2. The
RF-signal-level detecting portion 34 detects an average level of the
amplitude level of the supplied second amplified signal Sap2 to output it,
as the second average signal, to the second voltage generating portion 36
which is the second amplitude adjustment means.
The second voltage generating portion 36 calculates the difference between
the average level supplied from the RF-signal-level detecting portion 34
and the reference level Vref set by the reference-level setting unit 37.
Thus, the second voltage generating portion 36 supplies the obtained
differential signal to the second voltage controlled amplifier 32 as the
amplification control signal Sac36. Therefore, the amplification factor of
the second voltage controlled amplifier 32 is controlled in such a manner
that the second average signal coincides with the reference level Vref set
by the second voltage generating portion 36.
As described above, the amplification factor of the voltage controlled
amplifier 31 is controlled in such a manner that the first average signal
coincides with the second average signal output from the second
RF-signal-level detecting portion 34. Therefore, the first voltage
controlled amplifier 31 and the second voltage controlled amplifier 32
amplify the first and second correction signals Scs1 and Scs2 with the
same amplification factor regulated with the reference level Vref. As a
result, the amplification factor of each of the first voltage controlled
amplifier 31 and the second voltage controlled amplifier 32 is determined
in such a manner that the average level of the amplitude of each of the
first amplified signal Sap1 and that of the second amplified signal Sap2
is the predetermined amplitude level regulated by the refe | | |
