JP2013168196A - Optical disk drive - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical disk drive for generating an accurate tracking error signal even in the case that a movement direction of a light spot changes in a multilayer disk.SOLUTION: Three beams (a main spot 20 and two sub spots 21, 22) are irradiated to an optical disk recording surface, and a tracking error signal is generated by a differential push-pull system. In the case that light spots for recording are moved from an outer circumference in an inner circumferential direction, detection signals from the two sub spots 21, 22 are respectively divided into detection signals (E, G) of an inner circumferential side light reception surface and detection signals (F, H) of an outer circumferential side light reception surface, and gain correctors 47, 48 perform gain correction of a detection signal from each sub spot such that respective gains of an addition signal (E+G) of an inner circumferential side detection signal and an addition signal (F+H) of an outer circumferential side detection signal become constant.

Description

  The present invention relates to an optical disc apparatus that records and reproduces information while performing tracking control using a DPP tracking error signal.

  In tracking control of an optical disk device, a tracking error (TE) signal by a differential push pull (DPP) method using three beams (three spots) is generally used. The TE signal is a temperature around the device. The initial state may be changed due to the influence of the change, the emission heat of the semiconductor laser, the recording state of the disk, and the like. Particularly when recording is in progress, the upper / lower amplitude balance (TE balance) with respect to the reference potential of the TE signal is caused by the change in the reflectance of the recording surface due to the change in the recording state (recorded / unrecorded) of the disk The offset will change. At that time, since recording is being performed (the tracking servo is in an ON state), there is no opportunity to correct TE balance or the like, and in the worst case, recording failure occurs. In a multi-layer disc, when the recording layer changes, the moving direction of the light spot on the recording surface (from the inner periphery to the outer periphery, or from the outer periphery to the inner periphery) is reversed, so the arrangement of the recording state with respect to each light spot changes. As a result, there is a problem that the balance of signals from the sub-spots changes and the TE balance tends to deteriorate.

  Patent Document 1 discloses a TE signal normalization technique obtained by the differential push-pull method for the factor that the reflectance at each beam position is different. In this document, the push-pull signal of the main beam is normalized based on the received light amount of the main beam, the push-pull signal of one sub beam is normalized based on the received light amount of the sub beam, and the push-pull signal of the other sub beam is normalized. Normalize based on the received light quantity of the sub beam, take the difference between the normalized push-pull signal of the main beam and the sum signal of the normalized push-pull signals of both sub beams, and output the output signal as a tracking error signal The servo operation is performed.

  Further, in Patent Document 2, regarding a factor that reverses the moving direction of the beam (the spiral direction of the guide groove) in the multilayer disk, the first and second sub-beams condensed on the recording surface are compared with the condensed main beam. And at least 3/2 tracks or more.

JP 2002-230803 A JP 2006-147120 A

  According to the above-mentioned patent document 1, even if the reflectance of the portion irradiated by each beam is different, the effect of removing the DC offset due to the optical axis deviation can be made effective and a stable tracking servo operation can be performed. It is stated. However, when the spot moving direction is reversed during multi-layer disk recording, that is, when moving from the inner periphery to the outer periphery, and when moving from the outer periphery to the inner periphery, the reflectance of the portion irradiated with the sub-beam is different, and thus cannot be applied as it is. .

  In the above-mentioned Patent Document 2, it is stated that a good sub-beam signal can be obtained on any recording surface in a multilayer disk in which the spiral direction of the guide groove is different on each recording surface. However, since the interval (beam interval) d between the position of the sub beam and the position of the main beam is arranged at least 3/2 tracks apart from the normal 1/2 track, disks having different track pitches Tp are used. In this case, there is a problem that the beam position deviation is enlarged. For example, let us compare the beam position deviation amount ΔP when using a DVD-RAM with Tp = 0.61 μm and a DVD-ROM with Tp = 0.74 μm. When the beam interval is d = 1/2 Tp, ΔP = 0.06 μm. However, when d = 3/2 Tp, the distance is enlarged to ΔP = 0.19 μm, making it difficult to correct the tracking error signal.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical disc apparatus that generates an accurate tracking error signal even in a case where a moving direction of a light spot changes in a multi-layer disc and has little beam position deviation even on discs having different track pitches.

  In the optical disc apparatus of the present invention, the tracking error signal is generated by the differential push-pull method by irradiating the optical disc recording surface with three beams. Sub-spots preceding the main spot are arranged on the outer peripheral side, and subsequent sub-spots are arranged on the inner peripheral side with a shift of ½ track. When the beam spot moves from the outer periphery to the inner periphery for recording, the detection signals from the two sub-spots are divided into the detection signal for the inner peripheral light receiving surface and the detection signal for the outer peripheral light receiving surface, respectively. Gain correction is performed on the detection signals from the sub-spots so that the gains of the detection signal addition signal and the outer periphery side detection signal addition signal are constant.

  According to the present invention, it is possible to provide an optical disc apparatus that generates an accurate tracking error signal even when the moving direction of a light spot changes during recording in a multi-layer disc, and has little beam position deviation even with discs having different track pitches.

1 is a configuration block diagram showing an embodiment of an optical disk device of the present invention. The figure which shows the example of arrangement | positioning of the beam spot on a disc recording surface. The figure which shows the DPP signal arithmetic circuit (arithmetic circuit 1) used with respect to forward movement (inner periphery-> outer periphery). The figure which shows the DPP signal arithmetic circuit (arithmetic circuit 2) used with respect to a reverse direction movement (outer periphery-> inner periphery). The figure which shows the 1st prior art example (arithmetic circuit 3) of a DPP signal arithmetic circuit. The figure which shows the 2nd prior art example (arithmetic circuit 4) of a DPP signal arithmetic circuit. The figure explaining operation | movement of the low-pass filter (LPF) used for the arithmetic circuits 1 and 2. FIG.

FIG. 1 is a block diagram showing the configuration of an embodiment of an optical disk apparatus according to the present invention.
The configuration of the optical disc apparatus includes a spindle motor 2 that rotates the optical disc 1 as a mechanism system, and an optical pickup 3 that records or reproduces information on the optical disc 1. Further, as a signal / servo system, an A / D converter 4 that digitally converts a detection signal from the optical pickup 3, a detected digital signal is calculated and a tracking error signal (TE signal), a focus error signal (FE signal), etc. A servo error calculation unit 5 that generates a servo error signal, a servo filter 6 that compensates for frequency characteristics of the servo control system, and an actuator driving unit 7 that drives an objective lens actuator in the optical pickup 3 are provided. Other than this, as shown in the figure, a sled motor that moves the optical pickup 3 in the radial direction of the optical disc 1 and a recording signal that supplies a recording signal to the optical pickup 3 and generates a reproduction signal from the detection signal of the optical pickup 3 A reproduction signal processing unit, a laser driving unit that drives a semiconductor laser in the optical pickup 3, and a control unit that controls the operation of the entire apparatus are provided.

  The optical disc 1 can be used not only for a single layer disc but also for a multilayer disc having a plurality of recording layers. The optical pickup 3 has a semiconductor laser that emits a light beam, and a diffraction grating that diffracts the light beam and separates it into a main beam (0th order diffracted light) and two sub beams (± 1st order diffracted light). One recording surface is irradiated with three light beams (light spots). The light beam is reflected by the recording surface of the optical disc 1 and enters the photodetector in the optical pickup 3 through the objective lens. The photodetector receives three light beams, each of which is divided into a plurality of regions (light receiving surfaces), and a detection signal is output from each light receiving surface according to the intensity of the light beam.

  The servo error calculation unit 5 calculates a plurality of detection signals output from the photodetector, and generates a tracking error signal (TE signal) and a focus error signal (FE signal) that are servo error signals. The TE signal is generated by a three-beam differential push-pull (DPP) method, but in order to compensate for changes in the recording state (recorded / unrecorded) at the irradiation position (spot position) of each beam, An automatic gain control (AGC) function is provided. Further, as will be described later, in order for the AGC function to operate effectively according to the moving direction of the spot (inner periphery → outer periphery or outer periphery → inner periphery), the arithmetic expressions of the two DPP signals are switched.

  FIG. 2 is a diagram showing an arrangement example of beam spots on the disk recording surface. (A) is a case where the spot moves from the inner periphery to the outer periphery direction (hereinafter referred to as a forward direction), and (b) is a case where the spot moves from the outer periphery to the inner periphery direction (hereinafter referred to as a reverse direction). Reference numeral 25 denotes a track (groove), and reference numeral 26 denotes a recording mark formed on the track by being irradiated with the main beam.

  The beam spot has a main spot 20 and two sub-spots 21 and 22, and the main spot 20 follows the recording target track to form a recording mark 26. The two sub-spots 21 and 22 are shifted from the main spot 20 in the front-rear direction of the disk travel, and the side preceding the main spot 20 is referred to as a front spot 21 and the subsequent side is referred to as a rear spot 22. Further, the two sub-spots are arranged so as to be shifted by 1/2 track in the radial direction with respect to the main spot, the front spot 21 is arranged on the outer peripheral side, and the rear spot 22 is arranged on the inner peripheral side. As described above, since the sub-spot shift amount is set to the ½ track which is the minimum value, the beam position shift with respect to the track position can be reduced even in a disc having a different track pitch.

  The three beams are reflected by the disk recording surface and enter the divided light receiving surface of the photodetector. The reflected light from the main spot 20 is incident on the four-divided light receiving surface and generates main detection signals A, B, C, and D. The reflected light from the front spot 21 is incident on the two-divided light receiving surface and the sub detection signals E and F are reflected. The reflected light from the rear spot 22 is incident on the two-divided light receiving surface and the sub detection signals G and H are received. generate. The servo error calculation unit 5 uses these detection signals A to D, E to F, and G to H to perform calculation by a differential push-pull (DPP) method to generate a tracking error signal (TE signal).

Here, attention is focused on the recording state of the recording surface at the two sub-spots 21 and 22 positions.
(A) is a case where the spot moving direction is a forward direction (inner circumference → outer circumference direction), and a rewritable medium such as a DVD-R or a rewritable like a DVD-RW or BD-RE. This corresponds to the first recording of the medium. The front spot 21 is arranged at approximately the center of two adjacent tracks, and the recording state of these two tracks is unrecorded, while the recording state of the two adjacent tracks is already recorded in the rear spot 22. Since the reflectance is different between the unrecorded surface and the recorded surface, the amplitudes of the detection signals (E, F) obtained from the front spot 21 and the detection signals (G, H) obtained from the rear spot 22 in the photodetector. A difference will occur (the balance will be lost).

  (B) corresponds to a case where the spot moving direction is the reverse direction (outer periphery → inner periphery direction), and recording is performed on the second layer of a multilayer disc such as DVD +/− RDL or BD-RDL. In this case, the recording states of two adjacent tracks are different in each of the sub-spots 21 and 22, and recording is not performed on the inner peripheral side (E, G), and recording is performed on the outer peripheral side (F, H). Since the reflectance is different between the unrecorded surface and the recorded surface, the detection signal (E, G) obtained from the inner peripheral side of each of the sub-spots 21 and 22 and the detection signal (F, A difference occurs in the amplitude of H).

  In the servo error calculation unit 5 of this embodiment, the tracking error signal (TE signal) is calculated while correcting the difference in signal amplitude (balance loss) caused by the difference in the recording state. The correction method in the conventional calculation will be described.

  FIG. 5 is a diagram showing a first conventional example (arithmetic circuit 3) of the DPP signal arithmetic circuit. The basic configuration of the DPP method is to generate a main push-pull (MPP) signal from detection signals A, B, C, and D from the main spot 20 and detect detection signals E, F, G, and D from the two sub-spots 21 and 22. A sub push-pull (SPP) signal is generated from H. A DPP signal that is a TE signal is calculated by multiplying the SPP signal by a coefficient K and subtracting it from the MPP signal. Here, the coefficient K is a coefficient that determines the amplitude ratio of the SPP signal to the MPP signal, and by adjusting this, the offset component of the TE signal generated when the objective lens is shifted in the radial direction from the neutral position is removed. I am doing so.

  In the correction method of the arithmetic circuit 3, the detection signals A to D from the main spot 20 are added by the adder 50, and the detection signals E to H from the sub spots 21 and 22 are added by the adder 51. Further, the addition signals of both are added to form a sum signal of the detection signals A to H, and sent to the gain corrector 53. The gain corrector 53 performs gain correction (coefficient Gall) on the DPP signal using the sum signal (A + B +... + H) to generate a corrected DPP signal (DPP3). A high-frequency component is removed by a low-pass filter (LPF) 52 with respect to the sum signal (A + B +... + H). By this correction (AGC function), a DPP signal (DPP3) having a constant amplitude corresponding to the 3-spot total signal (A + B +... + H) is obtained.

The above arithmetic expression is shown in the following expression (3).
MPP = (A + D) − (B + C),
SPP = (E−F) + (G−H),
DPP3 = Gall * (MPP-K * SPP) (3)

FIG. 6 is a diagram showing a second conventional example (arithmetic circuit 4) of the DPP signal arithmetic circuit.
In the correction method of the arithmetic circuit 4, the detection signals A to D from the main spot 20 are added by the adder 60 and sent to the gain corrector 64 via the LPF 62. The gain corrector 64 performs gain correction (coefficient Gmain) on the MPP signal before the DPP calculation, using the main spot addition signal (A + B + C + D). On the other hand, the detection signals E to H from the sub-spots 21 and 22 are added by the adder 61 and sent to the gain corrector 65 through the LPF 63. The gain corrector 65 performs gain correction (coefficient Gsub) on the SPP signal before the DPP calculation, using the sub-spot addition signal (E + F + G + H). Thereafter, a corrected DPP signal (DPP4) is generated using the corrected MPP signal and the corrected SPP signal. By this correction (AGC function), a DPP signal (DPP4) having a constant amplitude corresponding to the main spot addition signal (A + B + C + D) and the sub spot addition signal (E + F + G + H) is obtained.

The above arithmetic expression is shown in the following expression (4).
MPP = (A + D) − (B + C),
SPP = (E−F) + (G−H),
DPP4 = Gmain * MPP-K * Gsub * SPP (4)

  The conventional arithmetic circuits 3 and 4 shown in FIGS. 5 and 6 can be expected to be effective against the influence of the temperature change around the apparatus and the light emitted from the semiconductor laser, but the recording state of the disk shown in FIG. It cannot respond to changes.

  When the spot moves in the forward direction as shown in FIG. 2A, the amount of reflected light differs between the front spot 21 and the rear spot 22, so that the sub-beam detection signals (E, F) and (G, H) obtained from them. Level difference occurs. For this reason, there is also a difference in the offsets generated in the difference signals (E−F) and (G−H), and the MPP signal and the SPP signal are calculated at the stage of calculating the DPP signal by calculating the difference of the SPP signal from the MPP signal. A part of the offset remains without being canceled. That is, even if the gain corrector 53 in FIG. 5 or the gain correctors 64 and 65 in FIG. 6 are used, the difference in offset generated between the difference signals (EF) and (GH) cannot be solved. is there. This residual offset causes the main spot to shift from the track center, and the tracking servo becomes unstable.

  In addition, when the spot moves in the opposite direction as shown in FIG. 2B, the amount of reflected light differs between the inner peripheral side and the outer peripheral side of each of the sub-spots 21, 22, and as a result, detection signals (E, G) obtained therefrom. And (F, H) cause a level difference. In this case, an unnecessary offset is added to the difference signals (E−F) and (G−H) itself, and the SPP signal offset is calculated at the stage of subtracting the SPP signal from the MPP signal and calculating the DPP signal. Will remain without being canceled. That is, even if the gain corrector 53 in FIG. 5 or the gain correctors 64 and 65 in FIG. 6 are used, the unnecessary offset added to the difference signals (EF) and (GH) itself cannot be eliminated. is there. In this case, the remaining offset is larger than that in the forward direction of (a), and the tracking servo is expected to become more unstable.

  Next, the arithmetic circuit of this embodiment that solves these problems will be described. In this embodiment, the two arithmetic circuits are switched and used in accordance with the moving direction of the spot. A circuit used for forward movement is referred to as an arithmetic circuit 1, and a circuit used for backward movement is referred to as an arithmetic circuit 2, and the configuration will be described.

  FIG. 3 is a diagram showing a DPP signal arithmetic circuit (arithmetic circuit 1) used for forward movement (inner circumference → outer circumference).

  In the correction method of the arithmetic circuit 1, gain correction is performed individually for each sub spot with respect to the detection signals from the two sub spots 21 and 22. That is, the detection signals E and F from the front spot 21 are added by the adder 31 and sent to the gain corrector 37 via the LPF 34. The gain corrector 37 performs gain correction (coefficient Gef) on the difference signal (E−F) using the front spot addition signal (E + F). On the other hand, the detection signals G and H from the rear spot 22 are added by the adder 32 and sent to the gain corrector 38 via the LPF 35. The gain corrector 38 performs gain correction (coefficient Ggh) on the difference signal (GH) using the rear spot addition signal (G + H). The detection signals A to D from the main spot 20 are added by the adder 30 and sent to the gain corrector 36 through the LPF 33, as in the conventional example (FIG. 6). The gain corrector 36 performs gain correction (coefficient Gmain) on the MPP signal using the main spot addition signal (A + B + C + D). Thereafter, the corrected differential signal (E-F) and the differential signal (GH) are added to obtain a corrected SPP signal, which is calculated with the corrected MPP signal, thereby correcting the DPP signal (DPP1). Is generated. By this correction (AGC function), a DPP signal (DPP1) is obtained when the gains of the main spot addition signal (A + B + C + D), the front spot addition signal (E + F), and the rear spot addition signal (G + H) are constant.

The above arithmetic expression is shown in the following expression (1).
MPP = Gmain * {(A + D)-(B + C)},
SPP = Gef * (EF) + Ggh * (GH),
DPP1 = MPP-K * SPP1 (1)
Comparing the calculation formula (1) with the calculation formula (4), the coefficient Gsub is divided into Gef and Ggh and corrected by a total of three correction coefficients.

  According to the arithmetic circuit 1, it is possible to correct when the spot moves in the forward direction as shown in FIG. In this case, a level difference occurs between the detection signal (E, F) of the front spot and the detection signal (G, H) of the rear spot, but the difference signals (E−F) and (G) are respectively obtained using coefficients Gef and Ggh. By performing the gain correction of −H), the SPP signal is normalized, and no offset remains in the DPP signal.

  FIG. 4 is a diagram showing a DPP signal arithmetic circuit (arithmetic circuit 2) used for backward movement (outer circumference → inner circumference).

  In the correction method of the arithmetic circuit 2, the detection signals from the two sub-spots 21 and 22 are divided into an inner peripheral detection signal and an outer peripheral detection signal, and gain correction is performed individually for each. That is, the inner circumference detection signal E of the front spot 21 and the inner circumference detection signal G of the rear spot 22 are added by the adder 41 and sent to the gain corrector 47 via the LPF 44. The gain corrector 47 performs gain correction (coefficient Geg) on the inner circumference side addition signal (E + G) using the inner circumference side addition signal (E + G). On the other hand, the outer periphery side detection signal F of the front spot 21 and the outer periphery side detection signal H of the rear spot 22 are added by the adder 42 and sent to the gain corrector 48 via the LPF 45. The gain corrector 48 performs gain correction (coefficient Gfh) on the outer peripheral side addition signal (F + H) using the outer peripheral side addition signal (F + H). The detection signals A to D from the main spot 20 are added by the adder 40 and sent to the gain corrector 46 through the LPF 43 as in the conventional example (FIG. 6). The gain corrector 46 performs gain correction (coefficient Gmain) on the MPP signal using the main spot addition signal (A + B + C + D). Thereafter, the corrected addition signal (E + G) is subtracted from the addition signal (F + H) to obtain a corrected SPP signal, which is calculated with the corrected MPP signal to generate a corrected DPP signal (DPP2). By this correction (AGC function), the DPP signal when the gains of the main spot addition signal (A + B + C + D), the sub spot inner circumference addition signal (E + G), and the sub spot outer circumference addition signal (F + H) are constant. (DPP2) is obtained.

The above equation is shown in the following equation (2).
MPP = Gmain * {(A + D)-(B + C)},
SPP = Geg * (E + G) −Gfh * (F + H),
DPP2 = MPP-K * SPP (2)
When the calculation formula (2) is compared with the calculation formula (1), the calculation order of F and G in the SPP calculation formula is switched.

  According to the arithmetic circuit 2, it is possible to correct when the spot moves in the reverse direction as shown in FIG. In this case, a level difference occurs between the inner peripheral detection signal (E, G) and the outer peripheral detection signal (F, H) of the sub spot, but the addition signals (E + G) and (F + H) using the coefficients Geg and Gfh, respectively. ), The SPP signal is normalized and no offset remains in the DPP signal.

  Switching between the arithmetic circuit 1 and the arithmetic circuit 2 is performed according to the moving direction of the spot. The servo error calculation unit 5 has the above two calculation circuits 1 and 2 and switches between the two calculation circuits depending on the recording direction (inner circumference → outer circumference or outer circumference → inner circumference) with respect to the optical disc.

  The DPP signals (DPP1 and DPP2) generated by the arithmetic circuits 1 and 2 in FIGS. 3 and 4 are balanced in amplitude by a TE balance circuit (not shown) so that the DPP signal is vertically symmetrical with respect to the reference potential. It shall be corrected.

  FIG. 7 is a diagram for explaining the operation of the low-pass filter (LPF) used in the arithmetic circuits 1 and 2. (A) is a detection signal from a photodetector input to each LPF, and (b) schematically shows a detection signal after LPF processing. The input detection signal includes an AC amplitude component due to track crossing or lens shift, and a DC amplitude component proportional to the amount of reflected light. Among them, the DC component used for gain correction (AGC) is a DC component indicating a change in the amount of reflected light due to a disc recording state, a temperature change, etc., and does not respond to an AC component such as a lens shift caused by disc eccentricity (that is, DPP). Limit the response speed so that the basic functionality of the scheme is not compromised. Therefore, the LPF has a characteristic that sufficiently attenuates a frequency component equal to or higher than the disk rotation speed.

  According to the present embodiment, an accurate tracking error signal can be generated even when the moving direction of the light spot changes in the multilayer disk. Further, since the beam position deviation is small even with discs having different track pitches, it can be widely applied to optical discs of various standards.

1: Optical disc,
2: Spindle motor
3: Optical pickup
4: A / D converter,
5: Servo error calculation unit,
6: Servo filter,
7: Actuator drive unit,
20: Main spot
21: Sub spot (front spot),
22: Sub spot (back spot)
25: Track,
26: Record mark,
30, 31, 32, 40, 41, 42: adder,
33, 34, 35, 43, 44, 45: LPF,
36, 37, 38, 46, 47, 48: Gain corrector.

Claims (3)

In an optical disk apparatus that records and reproduces information by irradiating an optical disk with a light beam and performs tracking control using a tracking error signal based on a differential push-pull method,
A light beam generated by a semiconductor laser is separated into one main beam and two sub-beams and irradiated onto a recording surface of an optical disc through an objective lens, and a plurality of light receptions obtained by dividing the three light beams reflected from the optical disc An optical pickup for detection by a photodetector having a surface;
A servo error calculation unit having a calculation circuit that calculates a detection signal from each light receiving surface of the photodetector and generates a tracking error signal;
The spot of the two sub-beams (hereinafter referred to as “sub-spot”) irradiating the optical disc recording surface is shifted from the spot of the main beam (hereinafter referred to as “main spot”) in the front-rear direction of the disc travel. The front spot arranged on the outer side of the main spot is shifted by ½ track on the outer side, and the rear spot arranged on the rear side is shifted by ½ track on the inner side of the main spot. And
The arithmetic circuit calculates a differential push-pull signal by calculating a difference between a push-pull signal from the main spot and a push-pull signal from the sub spot, and in the difference calculation, Each detection signal is divided into a detection signal for the inner peripheral light receiving surface (hereinafter referred to as a sub-spot inner peripheral signal) and a detection signal for the outer peripheral light receiving surface (hereinafter referred to as a sub-spot outer peripheral signal). And a gain corrector that performs gain correction on the sub-spot inner-side signal and the sub-spot outer-side signal so that the gains of the sub-spot outer-side signal and the sub-spot outer-side signal are constant. An optical disk device. The optical disc apparatus according to claim 1,
The tracking error signal is generated by applying the arithmetic circuit when the light beam spot moves from the outer circumference to the inner circumference when recording information on the optical disc.
The servo error calculation unit further includes a calculation circuit applied when the spot of the light beam moves from the inner periphery to the outer periphery,
In the difference calculation, the arithmetic circuit divides a detection signal from the two sub-spots into a detection signal from the front spot and a detection signal from the rear spot, and an addition signal of the front spot detection signal and the rear spot An optical disc apparatus comprising a gain corrector that performs gain correction on the detection signals of the two sub-spots so that the gains of the addition signals of the detection signals are constant. The optical disc apparatus according to claim 1 or 2,
An optical disc apparatus comprising a low-pass filter for attenuating a frequency component equal to or higher than the rotational speed of the optical disc with respect to an addition signal of the detection signal used for the gain correction.

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