JP2002150575A - Optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical disk device, in which the stray light of an optical pickup and the offset of an analog amplifier circuit are exactly removed and also the setting of the offset removal can be easily carried out. SOLUTION: This device is provided with a photodetector of the optical pickup for converting the return light from the optical disk into a signal, the analog amplifier circuit for amplifying an output of the above photodetector, an A/D converter for converting the output of the above analog amplifier circuit into a digital signal, a processor for computing the stray light of the photodetector or the offset of the analog amplifier circuit, an offset removing circuit for removing the stray light of the photodetector and the offset of the analog amplifier circuit from the output of the above A/D converter, and a servo signal processing circuit for making out a tracking signal and a focus signal from the output of the offset removing circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an offset remover for removing stray light in an optical pickup unnecessary for a tracking signal and a focus signal obtained from an optical disc and a circuit offset generated in an analog amplifier circuit (hereinafter abbreviated as an RF amplifier). The present invention relates to an optical disk device having a circuit.

[0002]

2. Description of the Related Art Conventionally, in an optical disk device such as a CD or MD, a laser beam from an optical pickup is reflected on an optical disk, a return light thereof is detected by a photodetector in an optical pick, and a focus error (hereinafter referred to as FE) is obtained from these signals. Abbreviation) signal, tracking error (hereinafter abbreviated as TE)
Signals are generated to perform focus servo control and tracking servo control.

With the miniaturization of the pick, unnecessary signals (stray light) other than the return light from the optical disk entering the photodetector in the optical pickup are becoming a problem, and the stray light and the circuit offset as shown in FIG. 4 are removed. There is a circuit.
Figure 4 shows how to create a tracking signal with two sub-beams.
This is a circuit that generates a beam type tracking error signal. The sub-beam signal is converted from a photodetector of the optical pickup into two electric signals of E and F, and input signals E and
Enter F. The E signal passes through an analog adder 40 that can add a DC offset and enters the (+) input of an analog subtractor 42 that generates a TE signal. The F signal passes through an analog adder 41 capable of adding a DC offset and enters the (-) input of an analog subtractor 42 that generates a TE signal. This is a circuit that generates a TE signal (E−F) by an analog subtractor 42 and supplies the TE signal to a final servo circuit through an analog adder 43 that can add an offset.

In such a configuration, the analog subtractor 4
2, the (+) and (-) input terminals and the TE signal are input to an AD converter (hereinafter abbreviated as AD) 44 for converting analog to digital. TE input to AD44
For the signal, a deviation from the set reference voltage is used as a circuit offset until the signal reaches the stray light of the photodetectors E and F and the TE signal, and the deviation value is converted into a digital value and stored. The value is converted by a microcomputer or the like so as to cancel the stored value. A signal is sent from a microcomputer to a DA converter (hereinafter, abbreviated as DA) 45 that converts digital to analog in order to convert the value to analog, and is applied to an analog adder 43. Therefore, an analog subtractor 4 is added to the TE signal from the stray light of E and F and the input.
The circuit offset up to the output 2 can be removed. The input terminal of the subtractor 42 for producing (E−F) is AD4
The monitor at 4 is for correcting the amplitude of the TE signal so that it is vertically symmetrical with respect to the reference level (balance correction). The balance is applied to the analog adders 40 and 41 from the DA 45 by a command from the microcomputer. Make corrections.

However, since this circuit removes stray light and circuit offset in analog by an adder, it is necessary to convert the stray light and circuit offset to be removed to digital by AD and store the digital value in a storage circuit. Further, when removing stray light and circuit offset, a DA for returning the stored digital signal to analog is required, and the circuit scale increases.

Next, in order to simplify the structure of the optical pickup, a push-pull method for obtaining a tracking signal by a one-beam method from a three-beam method in which a tracking signal is generated by two sub-beams has been used. Since the tracking signal detected by the push-pull method basically has a DC offset due to the movement of the objective lens of the optical pickup as shown in FIG. 5, a photodetector for detecting the movement amount of the objective lens is provided. A method of correcting an offset generated in a tracking signal by a detection signal is used.

FIG. 3 is a diagram showing a conventional one-beam type optical pickup, RF amplifier, and signal processing. Reference numeral 2 in FIG. 3 denotes an optical pickup. Inside the optical pickup 2, a photo detector 21r for detecting return light from the optical disk
f1, 21rf2, 21f1, 21f2, 21a, 21
b, 21c and 21d. RF signals for detecting data on the optical disk are output from the photodetectors 21rf1 and 21rf2 as current output RF1 and RF2 of the optical pickup through current-voltage conversion amplifiers 22a and 22b. The RF1 and RF2 are input to the RF amplifier 4 and pass through the buffer amplifiers 23rf1 and 23rf2 and enter the RF operation circuit 24. In the RF operation circuit 24, the type of the optical disk (the same pit structure as the CD, or the MO
The signal is switched to an adder and a subtractor according to the groove structure of the disk) and is output as an RF signal. The buffer amplifiers 23rf1 and 23rf2 detect the level difference between the outputs RF1 and RF2 of the optical pickup 2 due to the difference in the reflectivity of the optical disk (for example, the difference in the reflectivity of a read-only pit disk or a recordable / reproducible MO disk in an MD disk device). Change the gain to compensate. The output RF of the RF amplifier 4 is input to the signal processing LSI 6 and is demodulated by the EFM demodulation circuit 7 into EFM. Although not described here, the EFM signal is output as a final music signal or the like after error correction. The EFM demodulation circuit 7 performs CLV control in which EFM period information or frequency information enters the servo circuit 9 and rotates the spindle motor at a constant linear speed.

Next, the photodetectors 21f1 and 21f2 of the optical pickup 2 output focusing signals to output F signals.
Give out 1, F2. Outputs F1 and F2 of the optical pickup 2 are input to the RF amplifier 4 and output from the current / voltage conversion amplifier 23f.
AD3 of the signal processing LSI 6 as a total light amount (hereinafter abbreviated as AS)
2 and is converted into a digital signal and sent to the servo circuit 9. AS is used for normalizing the level of the FE signal in order to correct the difference in the reflectivity of the disc on the MO disc, for example, ON tracking when the beam spot of the laser beam from the optical pickup moves in the tracking direction,
Used as OFF tracking information.

On the other hand, the outputs of the current-voltage conversion amplifiers 23f1 and 23f2 are connected to the focusing balance circuits 25a,
The signal is input to the FE subtractor 26 through 5b. The FE subtractor 26 generates an FE signal and outputs the FE signal to the signal processing circuit LSI6.
It enters D32 and is converted into a digital signal. The digitized FE signal is sent to the servo circuit 9. Servo circuit 9
Then, the FE level is corrected by AS and a focus drive signal for the optical pickup is generated, and the objective lens of the optical pickup is driven in the focus direction. Thereby, focus servo control is performed. Also, the defocus characteristic is optimal (R
The focus balance correction signal is also sent to the focus balance circuits 25a and 25b of the RF amplifier 4 so that the jitter of the F signal is optimized.

Next, the photodetectors 21c and 21d of the optical pickup use a tracking-system signal generated by the one-beam push-pull method and a current-voltage conversion amplifier 22c.
The light is output from the optical pickup as outputs C and D through 22d. Output signals C and D of the optical pickup 2 are input to the RF amplifier 4 and enter the buffer amplifiers 23c and 23d, pass through the tracking balance circuits 30a and 30b, and generate a tracking signal by the tracking subtractor 31. A direct current (DC) offset is generated in the tracking signal obtained by the tracking subtractor 31 when the objective lens moves in the tracking direction as shown in FIG. Since this DC offset adversely affects the tracking servo control, it cannot be used as a tracking servo control signal as it is.

Therefore, the optical pickup 2 is provided with photodetectors 21a and 21b for detecting the position in the tracking direction. The photodetectors 21 a and 21 b are configured to generate a DC component when the objective lens moves in the tracking direction, and are output as outputs A and B and input to the RF amplifier 4. The input signals A and B pass through the current-voltage conversion amplifiers 23a and 23b and are input to the subtracter 27, and output (AB) as shown in FIG. 6A.
(AB) passes through the gain adjuster 28 and outputs K (AB). The gain K added by the gain adjuster 28
Is used to adjust the tracking signal to have the same slope as the DC offset of the tracking signal obtained by the tracking subtractor 31. Then, the output K (AB) of the gain adjuster 28 and the tracking subtractor 31 are provided to the TE subtractor 29.
(C-D) is input and (C-D) -K (A
Issue -B).

FIGS. 6A, 6B and 6C show (A-
B), K (AB), the tracking signal (CD) of the output of the tracking subtractor 31, and the TE signal (CD) -K (AB) of the output of the TE subtractor 29. It shows the relationship. This output TE signal (CD) -K (A
6B, the DC offset does not occur even if the objective lens moves in the tracking direction as shown in FIG. 6C, so that it enters the AD32 of the signal processing LSI 6 as a TE signal, is converted into a digital signal, and is converted into a servo signal. Sent to In the servo circuit 9, the tracking balance circuit 30 of the RF amplifier 4 is adjusted so as to correct the balance of the input TE signal.
Returning the correction signals to a and 30b and creating a tracking drive signal for servo-controlling the objective lens of the optical pickup in the tracking direction, and performing tracking servo control. The matrix circuit 34 includes an AS signal necessary for servo, T
A block including an arithmetic circuit for generating the E signal and the FE signal includes a focus AS adder 33, focus balance circuits 25a and 25b, a FE subtractor 26 and a position detection subtractor 27, a gain adjuster 28, Tracking balance circuits 30a and 30b, tracking subtractor 3
1. A block including the TE subtractor 29.
This block is composed of analog arithmetic circuits.

As described above, in the one-beam optical pickup, an objective lens position detecting circuit is required because the DC offset is removed particularly in the tracking servo system, and stray light entering the photodetector of the optical pickup and signal processing LSI
The circuit offset before entering the AD has a great adverse effect on the tracking servo and the like. Therefore, it is necessary to design an optical pickup and a circuit in which attention is paid to reducing stray light of the optical pickup and reducing circuit offset of the RF amplifier.

In FIG. 3, a circuit for removing the stray light and the offset of the analog circuit is made unnecessary by the method described below. For stray light, a pair of photo detectors,
The optical pickups are selectively used so that the stray light amounts leaking to the photodetectors of the pairs F1 and F2, A and B, and C and D are substantially equal. Therefore, the FE subtractor 26,
When the light passes through the position detecting subtractor 27 and the tracking subtractor 31, the stray light is canceled by subtraction, and the level becomes practically no problem. Therefore, no stray light removing circuit is provided. Further, since the circuit offset is designed to be minimized in each block, there is no practical problem. However, since the AS adds and outputs the photodetectors F1 and F2, stray light poses a problem. Although not shown here, in order to cancel the stray light, the AS adder 33 includes a cancel circuit for canceling the sum of the stray light amounts leaking into F1 and F2 in advance.

However, in recent portable MDs and the like, in order to reduce the size of the set and extend the battery life, adoption of a one-beam method suitable for downsizing the optical pickup and reduction of circuit power consumption have been advanced. Unnecessary signals (stray light) other than the return light from the optical disk are increasing in the photodetector and integration is progressing due to the miniaturization of the optical pickup, and the yield is deteriorated in sorting and the like. Therefore, it is necessary to remove the stray light by an electric circuit. There is. In order to reduce power consumption, it is particularly important to reduce the scale of the analog circuit, and there is a need for a method of removing stray light and circuit offset without increasing the scale of the analog circuit.

In the case of an MD or the like, the stray light and the removal value of the circuit offset removal circuit are set to a pit disk, MO disk reproduction,
It is necessary to set three types of MO disk recording. In other words, since the number of times of storage in the storage means is large, it is necessary to be able to set a removal value so that offset removal can be performed easily, quickly and reliably.

[0017]

In such an offset removing circuit of an optical disk device, it is possible to remove stray light, which has been increasing due to miniaturization of an optical pickup such as a one-beam system, and to reduce an analog circuit current. There is a need for a circuit offset removal circuit. The conventional example using the TE signal shown in FIG. 4 is effective for removing stray light and a circuit offset of an optical pickup. However, since this circuit removes stray light and a circuit offset in an analog manner, the stray light to be removed and the circuit offset are AD. It is necessary to convert the digital value and store the digital value in a storage circuit. Further, when removing stray light and circuit offset, a DA and an analog adder for returning the stored digital signal to analog are required, and the circuit scale increases. In particular, an increase in the scale of an analog circuit such as an RF amplifier increases a chip area and a circuit current, which is disadvantageous in extending the life of a battery of a portable MD or the like.

According to the present invention, a matrix circuit which has conventionally been constituted by an analog circuit is moved to a digital signal processing LSI and constituted by a digital circuit, and the analog circuit current is reduced by greatly reducing the scale of the analog circuit. To provide an offset removing circuit of an optical disc apparatus that removes both stray light and a circuit offset of an optical pickup by a circuit configuration including only an adder that removes a circuit offset of each digital photodetector signal and obtains an accurate FE signal and a TE signal. It is intended to set the offset removal value so that the offset removal value can be easily and reliably removed in a short time.

[0019]

To solve this problem, an optical disk apparatus according to the present invention comprises a photodetector of an optical pickup for converting return light from an optical disk into a signal, and an RF amplifier for amplifying an output of the photodetector. An AD for converting the output of the RF amplifier into a digital signal, a processor for calculating stray light of the photodetector and an offset of the RF amplifier from the output of the AD, and an output of the AD and an output of the processor. An offset removing circuit that removes the stray light of the photodetector and the offset of the RF amplifier from the output of the AD, and a servo signal processing circuit that creates a tracking signal and a focus signal from the output of the offset removing circuit. It was done.

As a result, the number of analog circuits is greatly reduced, stray light from the optical pickup and circuit offset are accurately removed, and accurate FE and TE signals can be obtained.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a digital signal processing LSI for converting a matrix circuit which has conventionally been constituted by an analog circuit.
To reduce the analog circuit current by greatly reducing the scale of the analog circuit, and eliminate the circuit offset of each digital photodetector signal. Since the FE signal and the TE signal can be accurately removed, an accurate FE signal and an accurate TE signal can be obtained.

An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows an overall configuration of an offset removing circuit of an optical disk apparatus according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an optical disk, 2 denotes an optical pickup, 50 denotes an RF amplifier that amplifies a photodetector signal in an analog manner, and 3 denotes a servo. Amplified photodetector signal line required for, 51 is a signal processing LSI, 7 is EF
M demodulation circuit, 5 is AD. 8 is an offset removing circuit, 10
Is a matrix circuit, 9 is a servo circuit, 11 is a driver for driving the objective lens and the spindle motor of the optical pickup 2, and 12 is a spindle motor.

The operation of the offset removing circuit of the optical disk device configured as described above will be described below.

First, the optical pickup 2 has a photodetector inside for detecting return light from the optical disk 1, and includes an RF component of signal data recorded on the optical disk and servo signals (focus, tracking, position detection) necessary for servo control. Signal). Each photodetector signal is sent to the RF amplifier 50 and amplified. Although not described in this figure, the RF amplifier 50 can switch the gain so as to correct the difference in reflectivity depending on the type of the disc and the difference in output of the photodetector due to the difference in recording and reproducing laser power.

From the pickup 2, the RF shown in FIG.
RF1 and RF2 are sent as component outputs, and an RF signal is generated in the RF amplifier. The RF signal is EF by the signal processing LSI
M is demodulated, and spindle control is performed using the EFM period information and frequency information. This operation is the same as the operation described with reference to FIG.

The servo signal output from the RF amplifier 50 enters AD5 of the signal processing LSI 51 and is converted into a digital signal. Each photodetector digital signal converted into a digital signal by the AD 5 enters the offset removing circuit 8, where the stray light and the offset of the analog circuit are removed and enters the matrix circuit 10. The matrix circuit 10 generates an AS signal, an FE signal, and a TE signal required for servo control by a digital circuit and sends the signal to the servo circuit 9. A spindle control signal required for controlling the rotation speed of the optical disc 1 by CLV is sent from the EFM circuit 7 to the servo circuit 9. Therefore, the servo circuit 9 performs focus control, tracking control, and spindle control, and sends a drive signal to the drive driver 11 so that the driver 11 drives the objective lens of the optical pickup 2 and the spindle motor.

FIG. 2 shows an optical pickup 2 in the overall configuration diagram.
FIG. 3 is a diagram for explaining in detail servo control of the RF amplifier 50 and the signal processing LSI 51 (spindle control is omitted).
Since the optical pickup of FIG. 2 has the same configuration as the pickup of FIG. 3, the optical pickup 2 of FIG. 2 omits the photodetectors 21rf1 and 21rf2 for detecting the RF components of FIG. 3 and the current-voltage conversion amplifiers 22a and 22b. Further, a circuit that generates an RF signal from the RF component of the RF amplifier 50 is also a buffer amplifier 23rf1, 23rf2, R in FIG.
It is omitted because it is the same as the F operation circuit 24. Similarly, the signal processing LSI 51 includes an EFM demodulation circuit 7, which is also the same as FIG.

The operation of the detailed configuration of FIG. 2 will be described.

First, signals related to the focus system enter the RF amplifier 50 at the outputs F1 and F2 from the optical pickup 2 and are converted and amplified into voltages by the current / voltage converters 23f1 and 23f2, and the output is sent to the AD5 in the signal processing LSI 51. send.
Outputs FF1 and FF2 of AD5 are digitized signals. The FF1 and FF2 are stored in a signal processor (hereinafter abbreviated as DSP) 61 inside the signal processing LSI 51 and stored. FF1 and FF2 are added to the adders 53f1,
It enters the matrix circuit 10 via 53f2.

An AS adder 62 for producing an AS signal inside the matrix circuit 10, a focusing balance circuit 54a, 54b for producing an FE signal and a FE subtractor 55
Of the AS adder 3 of the matrix circuit 34 in FIG.
3. The operation is the same as that of the focus balance circuits 25a and 25b and the FE subtractor 26, and thus the description thereof is omitted.

The adder 53f1 provided between the AD5 and the matrix circuit 10 in the servo signal processing circuit supplies the input signal FF
1 and the output from the DSP 61 are added. In order to remove the circuit offset, first stop the laser emission (the photodetector has no return light from the optical disk and no stray light generated by the optical pickup) AD5
Is stored as FF1k, the stored value is R
The circuit offset of the F amplifier 50 is measured and stored.
Therefore, the stored value (circuit offset value) is inverted and the DSP
If the signal is output from 61, only the circuit offset is canceled by the adder 53f1 and the circuit enters the matrix circuit 10. If the digital output FF1 of the AD5 in a state where the laser is emitted and the disk is lost (there is no return light from the optical disk) is stored as FF1mk, the sum of the stray light of the optical pickup and the circuit offset of the RF amplifier appears in FF1mk. . Therefore, the stray light of the optical pickup is (F
F1mk−FF1k) can be obtained by using the DSP 61. Similarly, the adder 53f2 stores the circuit offset and stray light in the DSP 61. There are three types: stray light of the optical pick, circuit offset, and stray light of the optical pick and the total of circuit offset from the time when the laser of the optical pickup is illuminated without returning light from the optical disk and the time when the laser is not illuminated. Offset removal pattern can be made.

Next, the signals of the tracking system are output from the outputs of A, B, C and D of the optical pickup 2 from the current / voltage conversion amplifiers 23a and 23b of the RF amplifier 50 and the buffer amplifier 23.
AD5 in the signal processing LSI 51 amplified by c and 23d
Digital photodetector signals AA and B
It is output from AD5 as B, CC, DD. Between the output of AD5 and the matrix circuit 10, adders 53a, 5a, 5b,
3b, 53c and 53d are inserted. This adder 5
The signals passing through 3a, 53b, 53c and 53d enter the matrix circuit 10. The position detection subtractor 56, the gain adjuster 57, the tracking balance circuits 58a and 58b, the tracking subtractor 59, and the TE of the matrix circuit 10.
The operation of the subtractor 60 for the position is the same as that of the subtractor 27 for position detection, the gain adjuster 28, the balance circuits 30a and 30b for tracking, and the subtractor 31 for tracking and the subtractor 29 for TE of the matrix circuit 34 in FIG. Is omitted.

The four adders 53a, 53b, 53c and 53d are the adders 53f1 and 53f5 described in the focus signal.
Works the same as 3f2. Therefore, the adder 53f1,
The stray light generated in each photodetector and the analog circuit offset up to AD5 can be removed by the six of 53f2, 53a, 53b, 53c and 53d. 5
Although 3f1, 53f2, 53a, 53b, 53c and 53d are shown as adders in the present embodiment, they may be formed as subtractors. The offset removing circuit 8 includes an adder 53f1,
This is a block showing 53f2, 53a, 53b, 53c, 53d.

As described above, according to the present embodiment, the circuit which conventionally constitutes a matrix circuit by analog is shifted to the digital side, so that the circuit of the RF amplifier can be largely reduced. In addition, since each photodetector digital signal is provided with each offset removal circuit, the stray light of each photodetector and the circuit offset of the RF amplifier are removed by each offset removal circuit. Thereby, each photodetector digital signal is completely a signal necessary for servo control. Also, since the inside of the matrix circuit after the removal of stray light and circuit offsets is calculated digitally, ideal FE and TE signals can be created without any circuit offset occurring at each stage as in analog circuits. There is no need to worry about circuit offset as in analog circuits. In order to form a matrix circuit with a digital circuit in this way, high-speed sampling of AD (to cope with an increase in the number of input ports) and high resolution are essential items.
A recent LSI fine process has realized high-speed sampling and high resolution of this AD.

[0035]

As described above, according to the present invention, the number of analog circuits can be greatly reduced, and the current consumption of the analog circuits can be reduced. Also, the stray light of the optical pickup and the offset of the analog circuit are removed before entering the servo signal processing circuit.Therefore, the digital calculation in the servo signal processing circuit thereafter does not include a circuit offset like an analog circuit.
Servo control can be performed with very accurate FE and TE signals. The stray light and the circuit offset may be summed and removed as an offset, and an advantageous effect that less stored information is required can be obtained.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an offset removing circuit of an optical disc device according to an embodiment of the present invention;

FIG. 2 is an optical pick, an RF amplifier, and a signal processing LSI of FIG. 1;
Detailed view of

FIG. 3 is an explanatory diagram of a conventional one-beam type optical disk device.

FIG. 4 is an explanatory diagram of a conventional offset removal circuit.

FIG. 5 is a characteristic diagram of a TE signal detected by a one-beam method.

FIG. 6 is an explanatory diagram of a position detection signal and a one-beam type TE signal.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 optical disk 2 optical pickup 3 photodetector signal line 5, 32, 45 AD converter 6, 51 signal processing LSI 7 EFM demodulation circuit 8 offset removal circuit 9 servo circuit 10, 34 matrix circuit 11 driver circuit 12 spindle motor 21 rf1, 21rf2 21f1, 21f2, 21
a, 21b, 21c, 21d Photodetectors 22a, 22b, 22c, 22d Current-voltage conversion amplifiers 23f1, 23f2, 23a, 23b, Current-voltage conversion amplifiers 23c, 23d Buffer amplifiers 24 RF operation circuits 25a, 25b, 54a, 54b For focusing Balance circuit 26,55 FE subtractor 27,56 Position detection subtractor 28,57 Gain adjuster 29,60 TE subtractor 30a, 30b, 58a, 58b Tracking balance circuit 31,59 Tracking subtractor 33, 62 AS adders 40, 41, 43 Analog adder 42 Analog subtractor 44 AD converter 50 RF amplifier 53f1, 53f2, 53a, 53b, 53c, 53d
Adder 61 DSP

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuhiro Teshirogi 1006 Kazuma Kadoma, Osaka Pref. Matsushita Electric Industrial Co., Ltd. 72) Inventor Kazutoshi Yamaguchi 1006 Kazuma Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. F-term (reference) 5D118 AA13 AA18 BA01 CA11 CA13 CB03 CC12 CD02 CD03 FB04

Claims (1)

[Claims] 1. A photodetector of an optical pickup for converting return light from an optical disk into a signal, an analog amplifier circuit for amplifying an output of the photodetector, and an AD for converting an output of the analog amplifier circuit to a digital signal.
A converter for calculating stray light of the photodetector and an offset of the analog amplifier circuit from an output of the AD converter; and an output of the AD converter and an output of the processor, using the AD converter. An optical disc comprising: an offset removing circuit that removes stray light of the photodetector and an offset of the analog amplifier circuit from an output of the optical detector; and a servo signal processing circuit that creates a tracking signal and a focus signal from the output of the offset removing circuit. apparatus.