JP2005085355A - Optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical disk device capable of preventing the saturation of a main tracking error signal MTE and a sub tracking error signal STE used for a differential push-pull system caused by deflection, or the like of an optical disk and generating a stable tracking error signal TE. <P>SOLUTION: The optical disk device has a differential push-pull system tracking error signal generation circuit. There are provided: AGCs 35, 36 capable of controlling a gain in MTE and STE; and a variable gain attenuator ATT 37 for amplifying and attenuating the differential signal of output from the AGCs 35, 36. The amount of deflection in the optical disk is obtained, based on TE that is the output of the ATT 37. When the amount of deflection is large, the gains in the AGCs 35, 36 are decreased. Additionally, the gain in the ATT 37 is controlled so that the product with the gains in the AGCs 35, 36 is fixed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical disc apparatus that records and reproduces information on a recordable optical disc, and more particularly, to an optical disc tracking control technique.

  As optical disc devices, audio CDs, CD-ROMs, CD-R / RWs, DVDs, and the like have already been put into practical use, and application to various fields and development for high performance are being actively conducted. . Particularly recently, with the rapid market expansion of personal computers, the penetration rate of built-in optical disk devices in personal computers has increased.

  In an optical disc apparatus, in order to accurately record or reproduce information, it is necessary that the laser light applied to the optical disc accurately follows the track of the optical disc. This control is performed by generating a tracking error signal and controlling the tracking actuator based on the tracking error signal.

  In addition, it is necessary for the laser light applied to the optical disc to be properly focused on the reflecting surface of the optical disc, and for that purpose, means for controlling the distance between the objective lens that collects the laser light and the optical disc is important. It becomes. This control is performed by generating a focus error signal and controlling the focus actuator based on the focus error signal.

  Here, the tracking error signal is a signal indicating a deviation in the optical disc radial direction between the light spot and the information track of the optical disc. The focus error signal is a signal indicating a shift in the focal direction between the light beam spot emitted from the objective lens provided in the optical pickup and the recording surface of the optical disc.

  The configuration of the optical disc apparatus will be described with reference to FIG.

  In FIG. 7, the optical disc apparatus irradiates the optical disc 1 with laser light and receives the light reflected by the optical disc 1, and converts the reflected light from the optical disc 1 obtained by the optical pickup 2 into a current. , A detector 3 for outputting a signal for reading data from the optical disc 1, an output signal for detecting a focus error, and an output signal for detecting a tracking error, and an output signal output from the detector 3, from which a tracking error signal (hereinafter referred to as an error signal) And a signal calculation unit 4 that generates a focus error signal (hereinafter sometimes abbreviated as “FE signal”).

  The TE signal and the FE signal are input from the signal calculation unit 4 to a digital servo processor (hereinafter abbreviated as “DSP”) 7. The DSP 7 controls the tracking actuator 6 that drives the optical pickup 2 so that the optical pickup 2 follows the track of the optical disc 1 on the basis of the TE signal, and between the objective lens provided in the optical pickup 2 and the optical disc 1. The focus actuator 5 is controlled so that the distance between them is appropriate.

  The above processing of the DSP 7 is executed under the control of the CPU 8.

  Here, means for generating a tracking error signal from the output signal output from the detector 3 will be described with reference to FIGS.

  FIG. 8 is an explanatory diagram for generating a tracking error signal by a differential push-pull (Differential Push Pull, hereinafter referred to as DPP) system, and FIG. 9 shows an example of a conventional tracking error signal generating circuit.

  In FIG. 8, (a) irradiates the main beam 14 to the recorded track 11 in order to record information on the optical disc, and applies the sub beams 15 and 16 to the mirror regions 12 and 13 on both sides of the track 11, respectively. A three-beam system for irradiation is shown. The reflected light of the main beam 14 and the sub beams 15 and 16 is incident on a detector (not shown) via the objective lens of the optical pickup 2 and converted into an electric signal.

  At this time, as shown in FIG. 8A, the reflected light of the main beam 14 is received by a detector in which one spot is divided into four regions, and the output signals at each detector are A, B, C, and D, respectively. . The reflected lights of the sub beams 15 and 16 are received by one detector, and the output signals are E and F, respectively.

  With these signals, the following equation is obtained when the optical pickup 2 is stationary in the tracking direction.

A + D = αsin (ωt)
B + C = αcos (ωt)
E = −α cos (ωt)
F = −αsin (ωt)
Where α is a constant, ω is the angular velocity of the optical disk, and t is time.

  A + D and B + C obtained from the reflected light of the main beam 14 have waveforms shown in FIG. E and F obtained from the reflected light of the sub beams 15 and 16 have waveforms shown in FIG.

  Using these signals, the tracking error signal TE is generated by the circuit of FIG.

  In FIG. 9, 20 denotes an analog processing unit, and 30 denotes a digital processing unit. The analog processing unit 20 includes amplifiers 21 to 24 that amplify signals from the respective light receiving elements and low-pass filters 25 to 28 that remove noise components.

  The digital processing unit 30 includes D / A converters 31a to 31a for converting the signals F and E and (A + D) and (B + C) after being amplified and filtered in the analog unit from analog signals to digital signals. 34a, amplifiers 31 to 34 for balancing the signals F and E and (A + D) and (B + C) after the D / A converters 31a to 34a, automatic gain controllers (AGC) 35 and 36, It comprises a gain attenuator (ATT) 37 and a low-pass filter 38 for noise removal.

  In the digital processing unit 30, the output of the amplifier 31 and the output of the amplifier 32 are subtracted to generate a sub-tracking error signal STE and output to the AGC 35. The output of the amplifier 33 and the output of the amplifier 34 are subtracted to generate a main tracking error signal MTE, which is output to the AGC 36. The variable gains of the two AGCs 35 and 36 are controlled by the AS signal (A + B + C + D) which is the sum signal of the common main beam so that the signal amplitude of the MTE signal and the STE signal is always constant. Since the characteristics of the chips constituting each of the amplifiers 31 to 34 are different, OFSET is given to align the characteristics.

FIG. 10 is an explanatory diagram showing a control operation using a tracking error signal by the DPP method.

  If the track of the optical disk 1 is eccentric with respect to the spindle hole, the distance between the lens of the optical pickup 2 and the track fluctuates, and offset due to lens shift occurs. FIG. 10 shows the relationship between the lens shift amount and the offset of the F or (A + D) signal and the E or (B + C) signal.

  The offset amount with respect to the lens shift amount of each signal of (A + D), (B + C), F, and E can be expressed as the following equation.

ΔE = β × x + r
ΔF = −β × x + r
Δ (A + D) = − β × x + r
Δ (B + C) = β × x + r
Here, β is a constant, x is a lens shift amount, and r is an offset amount.

  Therefore, the main tracking error signal MTE and the sub tracking error signal STE are as follows.

MTE = {(A + D) + Δ (A + D)} − {(B + C) + Δ (B + C)}
= {Αsin (ωt) −β × x + r} − {αcos (ωt) + β × x + r}
= Αsin (ωt) −αcos (ωt) −2β × x
STE = (F + ΔF) − (E + ΔE)
= {-Αsin (ωt) −β × x + r} − {− αcos (ωt) + β × x + r}
= -Αsin (ωt) + αcos (ωt) -2β × x
The MTE or STE waveform at this time is shown in FIG. MTE and STE have the same offset amount and opposite phases.

  Therefore, in the DPP method, the offset component β × x is canceled by calculating the tracking error signal TE as follows.

TE = MTE−STE = 2α {sin (ωt) −cos (ωt)}
Thus, the TE signal by the DPP method is strong against the lens shift characteristic, that is, the DC offset generated by the optical position shift between the objective lens and the light receiving element, and the generated TE signal is caused by eccentricity or the like. It has the feature that offset is hard to occur.

For example, Patent Documents 1 and 2 propose techniques for generating such tracking error signals.
JP 2000-293868 A JP 2000-163765 A

  As described above, at the stage of the main TE (MTE) and sub TE (STE) signals used in the TE signal generation process by the DPP method, an offset component due to the eccentricity of the optical disk is included.

  On the other hand, in order to use the tracking error signal TE as a servo signal on the circuit, since it is desired to output it as a signal as large as possible, the gains of the AGC 35 and AGC 36 before subtraction are increased as much as possible in the circuit of FIG.

  The amount of eccentricity of the CD-ROM disc itself is regulated to be within 140 μm in the standard book Yellow Book.

  However, some optical discs on the market have an eccentricity that greatly exceeds the standard. In such a disk with a large eccentricity, the MTE and STE signals output from the AGC 35 and 36 in FIG. 9 have a larger amplitude as shown in FIG. It will be saturated. For this reason, even if the saturated MTE and STE are subtracted, the TE signal that is the original transverse component disappears partially or entirely, and tracking control may become impossible.

  Users are required to be able to use optical discs whose eccentricity greatly exceeds the standard.

  The present invention provides an optical disc apparatus capable of preventing a main tracking error signal and a sub tracking error signal used in a differential push-pull method from being saturated due to eccentricity of an optical disc and generating a stable tracking error signal. The purpose is to provide.

  The optical disc apparatus of the present invention amplifies the main tracking error signal and the sub-tracking error signal so that the gain becomes small when the eccentric amount of the optical disc is large, and becomes a predetermined gain when the eccentric amount is small. The gains of the first and second gain controllers are interlocked to increase / decrease so that the main tracking error signal and the sub-tracking error signal are not saturated, and the gain of the output amplifier at the subsequent stage is increased / decreased by the increased / decreased gain. Thus, the main feature is to make the overall gain constant.

  As described above, the optical disc apparatus of the present invention adjusts the gain of the gain controller in accordance with the eccentricity of the optical disc, so that the main tracking error signal and the sub tracking error signal used in the differential push-pull method can be obtained. Therefore, it is possible to obtain an optical disc apparatus that can prevent saturation due to eccentricity of the optical disc and generate a stable tracking error signal.

In order to solve the above-mentioned problem, the invention of claim 1 is characterized in that the main tracking and the sub-beam reflected by the main beam irradiated to the optical disc are respectively received and the main tracking obtained by dividing the light receiving spot of the main beam into four parts. In an optical disc apparatus that generates a tracking error signal based on a difference signal between an error signal and a sub-tracking error signal obtained based on a signal obtained by receiving reflected light of the two sub beams, and performs tracking control of the optical disc. A first automatic gain controller and a second automatic gain controller capable of controlling gains of the main tracking error signal and the sub tracking error signal, an output of the first automatic gain controller, and the second automatic gain controller While amplifying the difference signal of the output of An output amplifier capable of controlling the gain of the optical disk, means for obtaining an eccentricity amount of the optical disk based on the tracking error signal that is an output of the output amplifier, and a bias so that the gain is reduced when the eccentricity amount of the optical disk is large. Means for controlling the gains of the first and second gain controllers in conjunction with each other so as to obtain a predetermined gain when the core amount is small; and the gains of the first and second gain controllers and the output amplifier An optical disc apparatus comprising: gain control means comprising means for controlling the gain of the output amplifier so that the product of gain is constant. As a result, when the eccentricity is large, the gains of the first and second gain controllers are lowered so that the main tracking error signal and the sub-tracking error signal are not saturated, and the gain of the output amplifier at the subsequent stage is increased by the lowered gain. As a result, the amplitude of the tracking error signal can be kept constant, and a stable tracking error signal can be generated.

  In order to solve the above-mentioned problem, the invention of claim 2 is the invention of claim 1, wherein the means for determining the eccentricity of the optical disc includes a counting unit for counting the number of crossings of the tracking error signal, and the counting result. Is an optical disc apparatus characterized by comprising an eccentricity measuring unit for obtaining an eccentricity amount from the above, and using the fact that the eccentricity amount correlates with the number of crossings of the tracking error signal, The amount can be determined.

  In order to solve the above-mentioned problem, the invention according to claim 3 is the invention according to claim 1 or 2, wherein the gain control means is a threshold having an eccentricity amount obtained by the means for obtaining the eccentricity amount. When the gain of the first and second gain controllers and the gain of the output amplifier are changed by a preset multiple, and when one or more thresholds are exceeded The gain can be adjusted by changing the gain stepwise. One threshold may be used, but a plurality of thresholds may be set.

  According to a fourth aspect of the present invention for solving the above-mentioned problem, in the first or second aspect of the present invention, the gain control means has a threshold value having an eccentricity amount obtained by the means for obtaining the eccentricity amount. The gain magnification of the first and second gain controllers is calculated when the value exceeds the value, and the gain of the first and second gain controllers and the gain of the output amplifier are changed by the multiples thereof. By calculating the gains of the first and second gain controllers corresponding to the amount of eccentricity and using the gains, a gain without saturation can be determined at a time.

  In order to solve the above-mentioned problem, the invention of claim 5 is the invention of claim 4, wherein means for detecting the maximum value of the input signal of the first and second gain controllers is provided, and there is the maximum value. When a set saturation threshold is exceeded, the gains of the first and second gain controllers and the gain of the output amplifier are changed at a magnification according to the eccentricity amount, which exceeds a specified value. It is possible to determine whether or not the optical disk has an eccentric amount, and to perform an operation of adjusting the gain only for an optical disk that exceeds a specified value.

(Embodiment 1)
Hereinafter, Embodiment 1 of the present invention will be described with reference to FIGS.

  1 is a configuration diagram of a tracking error signal generation circuit according to the first embodiment of the present invention, FIG. 2 is a detailed block diagram of an eccentricity detecting unit according to the first embodiment of the present invention, and FIG. 3 is a first embodiment of the present invention. 2 is a flowchart for generating a tracking error signal.

In FIG. 1, 20 denotes an analog processing unit, and 30 denotes a digital processing unit. The analog processing unit 20 includes amplifiers 21 to 24 that amplify signals from the detectors and low-pass filters 25 to 28 that remove noise components. The digital processing unit 30 includes D / A converters 31a to 31a for converting the signals F and E and (A + D) and (B + C) after being amplified and filtered in the analog unit from analog signals to digital signals. 34a, amplifiers 31 to 34 for balancing the signals F and E and (A + D) and (B + C) after the D / A converters 31a to 34a, automatic gain controllers (AGC) 35 and 36, A variable gain attenuator (ATT) 37 as an example of an output amplifier and a low-pass filter 38 for noise removal are included. Up to this point, the configuration is the same as the conventional configuration of FIG.
In the first embodiment, the eccentricity detection unit 39 that detects the eccentricity amount of the optical disk from the tracking error signal TE that is the output of the low-pass filter 38, and the eccentricity obtained by the eccentricity detection unit 39. The gain of AGC 35 and 36 and the attenuation rate of ATT 37 are adjusted.

  Details of the eccentricity detector 39 are shown in FIG. In FIG. 2, the eccentricity detection unit 39 includes a track cross counter unit (TKC) 41 for counting the number of track crossings of the tracking error signal TE, a comparator 42 that compares a threshold value with the output of the TKC 41, and an optical disc deviation. A threshold value setting unit 43 for setting the degree of the core amount, an eccentricity determination unit 44 for determining whether or not the optical disk is an eccentric disk having an eccentricity greater than a specified value from the output of the comparator 42, and AGCs 35 and 36 An AGC gain 1 / x unit 45 that multiplies the gain by 1 / x and an ATT gain x multiplication unit 46 that multiplies the attenuation rate of the ATT 37 by x times the reference attenuation rate.

  The operation of the first embodiment will be described with reference to the flowchart of FIG.

  However, threshold 1> threshold 2> threshold 3 and x1> x2> x3.

  First, when an optical disk is set in the optical disk device, the optical disk rotates. The optical pickup is driven with focusing on and tracking off, and the number of track crossings, that is, the amount of eccentricity of the optical disk is measured by the output of TKC 41 (S1). The comparator 42 compares the eccentricity amount with the threshold value 1 (S2). If the amount of eccentricity is greater than or equal to the threshold value 1, the gains of AGC 35 and 36 are set to 1 / x1, and the attenuation rate of ATT 47 is set to x1 (S3, S4).

  If the eccentricity amount does not exceed the threshold value 1 in S2, it is compared with the second threshold value 2 (S5), and if it is equal to or greater than the threshold value 2, the gains of the AGCs 35 and 36 are set to 1 / x2 times the current gain (S6). , The attenuation factor of ATT 47 is set to x2 (S7).

  If the amount of eccentricity does not exceed the threshold value 2 in S5, it is compared with the third threshold value 3 (S8), and if it is greater than or equal to the threshold value 3, the gains of the AGCs 35 and 36 are set to 1 / x3 times the current gain (S9). , The attenuation factor of ATT 47 is set to x3 (S10).

  If the amount of eccentricity does not exceed the threshold value 3 in S8, the gains of AGC 35 and 36 and the attenuation rate of ATT 47 are left as they are.

  Note that x1, x2, and x3 are values such as 18 dB, 12 dB, and 6 dB, for example.

  In the flowchart of FIG. 3 described above, the threshold value 1, the threshold value 2, and the threshold value 3 are compared with the eccentricity amount in descending order. However, as a comparison method, there are a descending order, a range comparison, and the like. Also, the number of thresholds is not limited to three, but may be other numbers.

  In this way, the gains of the AGCs 35 and 36 are controlled in accordance with the amount of eccentricity, and the gain is reduced to such an extent that saturation does not occur when the amount of eccentricity of the optical disk is large. In order to make the total gain of the circuit constant, the attenuation rate of the ATT 47 is automatically adjusted as much as the gain is reduced.

  In this way, during amplification of MTE and STE, saturation due to excessive eccentricity of the optical disk is prevented, and no error occurs in the output of the tracking error signal.

(Embodiment 2)
Next, a second embodiment of the present invention will be described with reference to FIGS. 4 is a configuration diagram of a tracking error signal generation circuit according to the second embodiment of the present invention, FIG. 5 is a detailed block diagram of an eccentricity detecting unit according to the second embodiment of the present invention, and FIG. 6 is a second embodiment of the present invention. 2 is a flowchart for generating a tracking error signal in FIG.

  In this Embodiment 2 shown in FIG. 4, since the components 21-38 are the structures similar to Embodiment 1, description is abbreviate | omitted. In the second embodiment, the eccentricity detecting unit 50 that detects the eccentricity amount from the tracking error signal, the gain calculating unit 51, and the maximum value detection that detects the maximum values of the main tracking error signal MTE and the sub-tracking error signal STE. It is characterized by comprising a unit 52, a CPU 53 that performs arithmetic processing, and a balance control unit 54 that controls the gain of AGC 35 and 36 and the attenuation rate of ATT 47.

  The details of the eccentricity detection unit 50 are shown in FIG. In FIG. 5, the eccentricity detection unit 50 includes a track cross counter unit (TKC) 61 for counting the number of track crossings of the tracking error signal TE, a comparator 62 that compares the threshold value with the output of the TKC 61, and an optical disc. A threshold value setting unit 63 that sets a value for determining whether or not the disk is a core disk, an eccentricity determination unit 64 that determines whether or not the optical disk is an eccentric disk, and the gains of the AGCs 35 and 36 are 1 / AGC gain 1 / x unit 65 for multiplying x, ATT gain x multiplying unit 66 for multiplying the attenuation rate of ATT 37 by x times the reference attenuation rate, and a saturation threshold setting for distinguishing whether MTE and STE are at the saturation level Unit 67, a maximum value detection unit 68 for detecting the maximum values of MTE and STE, a gain magnification of AGC 35 and 36, and an offset. And a calculation block 69 Metropolitan for detecting an abnormality.

  The operation of the second embodiment will be described with reference to the flowchart of FIG.

  First, when an optical disk is set in the optical disk device, the optical disk rotates. The optical pickup is driven with focusing on and tracking off, the eccentricity of the optical disk is measured from the number of track crossings and the track pitch length by the output of TKC61, and the maximum value of MTE and STE is detected by the maximum value detector 68 (S21). The comparator 62 compares the maximum value of MTE and STE with the saturation threshold (S22). If the maximum value is less than or equal to the saturation threshold, the gains of AGC 35 and 36 and the attenuation rate of ATT 47 remain as they are.

  If the maximum value of MTE and STE exceeds the saturation threshold value in S22, the eccentricity is compared with the threshold value (S23), and if it is less than the threshold value, an appropriate AGC 35, 36 that does not cause saturation is calculated by the calculation block 69. The gain magnification 1 / x is calculated (S24), 1 / x times the current gain of the AGC 35, 36 (S25), and the attenuation factor of ATT 47 is x times (S26). Here, the appropriate gain magnification of the AGC 35, 36 can be easily obtained from the maximum level that can be amplified determined by the electrical characteristics of the AGC 35, 36 and the maximum value of the MTE and STE.

  If the amount of eccentricity exceeds the threshold value in S23, an alarm is displayed on the display as an offset abnormality, or a sound is notified.

  In this way, during amplification of MTE and STE, saturation due to excessive eccentricity of the optical disk is prevented, and no error occurs in the output of the tracking error signal.

  In the first and second embodiments described above, an example in which the variable gain ATT is used as the output amplifier has been described. However, not only the gain is attenuated, but also a variable gain amplifier that amplifies can be used.

  The present invention can be applied to an optical disc apparatus capable of performing stable tracking even for an optical disc having a large eccentricity.

Configuration diagram of a tracking error signal generation circuit in Embodiment 1 of the present invention Detailed block diagram of the eccentricity detection unit in Embodiment 1 of the present invention Tracking error signal generation flowchart in Embodiment 1 of the present invention Configuration diagram of tracking error signal generation circuit in Embodiment 2 of the present invention Detailed block diagram of eccentricity detection unit in Embodiment 2 of the present invention Tracking error signal generation flowchart in Embodiment 2 of the present invention Block diagram showing configuration of optical disc apparatus Explanatory diagram for generating tracking error signal by differential push-pull method Circuit diagram showing an example of a conventional tracking error signal generation circuit Explanatory drawing which shows the operation | movement of the control using the tracking error signal by a DPP system. Waveform diagram of MTE or STE

Explanation of symbols

20 Analog Processing Unit 21-24 Amplifier 25-28 Low Pass Filter 30 Digital Processing Unit 31-34 Amplifier 35, 36 Automatic Gain Controller (AGC)
38 Low-pass filter 39 Eccentricity detection unit 41 Track cross counter unit (TKC)
42 Comparator 43 Threshold Setting Unit 44 Eccentricity Determination Unit 45 AGC Gain 1 / x Unit 46 ATT Gain x Multiply Unit 50 Eccentricity Detection Unit 51 Gain Calculation Unit 52 Maximum Value Detection Unit 53 CPU
54 Balance control part 61 Track cross counter part (TKC)
62 Comparator 63 Threshold Setting Unit 64 Eccentricity Determination Unit 65 AGC Gain 1 / x Unit 66 ATT Gain x Multiplication Unit 67 Saturation Threshold Setting Unit 68 Maximum Value Detection Unit 69 Calculation Block

Claims (5)

Receiving the reflected light of the main beam and the two sub beams irradiated to the optical disc, respectively, and receiving the main tracking error signal obtained by dividing the light receiving spot of the main beam into four parts and the reflected light of the two sub beams. In an optical disc apparatus that generates a tracking error signal based on a difference signal from a sub-tracking error signal obtained based on the obtained signal and performs tracking control of the optical disc,
A first automatic gain controller and a second automatic gain controller capable of controlling gains of the main tracking error signal and the sub tracking error signal;
An output amplifier capable of amplifying a difference signal between the output of the first automatic gain controller and the output of the second automatic gain controller and controlling the gain;
Means for determining an eccentricity amount of the optical disk based on a tracking error signal which is an output of the output amplifier;
Means for controlling the gains of the first and second gain controllers in conjunction with each other so that the gain decreases when the eccentricity amount of the optical disc is large, and a predetermined gain when the eccentricity amount is small; And gain control means comprising means for controlling the gain of the output amplifier so that the product of the gain of the first and second gain controllers and the gain of the output amplifier is constant. Optical disk device. The means for determining the amount of eccentricity of the optical disk comprises a counting unit for counting the number of crossings of the tracking error signal, and an eccentricity measuring unit for determining the amount of eccentricity from the counting result. 1. The optical disc device according to 1. When the eccentricity obtained by the means for obtaining the eccentricity exceeds a certain threshold, the gain control means has a gain set by the first and second gain controllers and the output amplifier in a preset multiple The optical disk apparatus according to claim 1, wherein the gain of the optical disk apparatus is changed. The gain control means calculates the magnification of the gains of the first and second gain controllers when the eccentricity obtained by the means for obtaining the eccentricity exceeds a certain threshold, 3. The optical disc apparatus according to claim 1, wherein the gain of the first and second gain controllers and the gain of the output amplifier are changed. Means for detecting the maximum values of the input signals of the first and second gain controllers are provided, and the first and second gain controllers are scaled according to the eccentricity when the maximum value exceeds a preset saturation threshold. 5. The optical disc apparatus according to claim 4, wherein a gain of a second gain controller and a gain of the output amplifier are changed.

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