JPH08279170A - Device and method for tracking servo - Google Patents

Abstract

PURPOSE: To optimally set a bias value to compensate for the offset value against a tracking error signal while conducting a tracking control of an opotical disk. CONSTITUTION: While rotating an optical disk, an optical pickup is moved by a driving signal generating circuit 61 in the radial direction of the disk. Under this condition, the center value of a track error signal TE from an RF amplifier 9 is extracted by a low pass filter 53. The frequency components to move the pickup are added against the frequency components of a traverse signal so that the components close to a direct current are eliminated from the traverse signal. Thus, the center value of the traverse signal is exactly extracted without lowering the cutoff frequency of the filter 53 while forming a bias value from the lower frequency components of the signal TE.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking servo circuit and method used for optical disks such as compact disks.

[0002]

2. Description of the Related Art In an optical disk reproducing apparatus such as a compact disk, a focus servo circuit for controlling a beam spot to focus on a disk surface and a tracking servo circuit for controlling a beam to trace along a track. A servo circuit is provided.

That is, in the optical pickup of the optical disk reproducing apparatus, a reproducing RF signal is detected and a four-division photodetector for detecting a focus servo error signal by detecting whether the beam is circular or not, and a side beam are detected for tracking. A photodetector for determining the error signal is provided. The outputs of these photo detectors are calculated to obtain a focus servo error signal and a tracking servo error signal. These servo error signals are, for example, 0 when there is no error, and have a positive or negative error voltage depending on the error direction and the error amount. When an unbalanced power supply is used, if there is no error, for example, V _CC / 2 (V _CC
Is the power supply voltage), and depending on the error direction and error amount,
The error voltage becomes higher or lower around V _CC / 2.

The objective lens of the optical pickup is movable in the focusing direction and the tracking direction by a biaxial device. The focus drive coil and tracking drive coil of the biaxial device are controlled based on the focus servo error signal and tracking error signal obtained by calculating the output of the photodetector, and the objective lens is in the tangential direction of the disc and the direction in which it is separated from the disc. Driven to. As a result, tracking servo and focus servo control is performed.

As described above, the servo error signal becomes 0 when there is no error, and becomes a positive or negative error voltage depending on the error direction and the error amount.
However, an offset component may occur in the servo error signal due to variations in the sensitivity of the photodetector, offset voltage of an operational amplifier that calculates the output of the photodetector, and the like. Therefore, it is necessary to perform offset adjustment such that the servo error signal when there is no error is set to 0, for example. Conventionally, this offset adjustment has been manually performed at the time of factory shipment, but the manual offset adjustment has poor productivity and cannot cope with a change over time.
Therefore, a method has been proposed in which the offset adjustment is automated.

For example, in the case of tracking servo,
When the disc is rotated, a traverse signal is obtained due to the eccentricity of the disc. A bias value for compensating the offset is obtained by extracting the traverse signal through a low-pass filter and obtaining the center value of the traverse signal. The bias value thus obtained is given to the tracking error signal. As a result, offset adjustment in tracking servo can be automatically performed.

That is, FIG. 10 shows an example of a circuit for automatically adjusting the offset in the conventional tracking servo circuit. In FIG. 10, the input terminal 101 is
A tracking error signal is provided. Input terminal 101
The tracking error signal from the low pass filter 1
02 and the subtractor 103.
The output of the low pass filter 102 is supplied to the register 104. The output of the register 104 is supplied to the terminal 105A of the switch circuit 105. The terminal 105B of the switch circuit 105 is grounded. The output of the switch circuit 105 is supplied to the subtractor 103. The output of the subtractor 103 is output from the output terminal 106.

When adjusting the offset of the tracking servo, the switch circuit 105 is set on the terminal 105B side. When the disc is rotated, a so-called traverse signal appears in the tracking error signal due to the eccentricity of the disc, as shown in FIG. This traverse signal is passed through the low-pass filter 102 and the low-pass filter 1
The center value of the traverse signal is extracted from the output of 02. The center value of this traverse signal is the register 10
Stored in 4. The center value of this traverse signal is used as the bias value for removing the offset.

When tracking servo control is performed, the switch circuit 105 is set to the terminal 105A side. Therefore, the center value of the traverse signal is supplied from the register 104 to the subtractor 103 as a bias value. The subtractor 103 subtracts this bias value from the tracking error signal from the input terminal 101. As a result, the offset component in the servo error signal is removed.

[0010]

As described above, conventionally, for the tracking error signal, the center value of the traverse signal is obtained by passing the traverse signal through a low-pass filter, and this is used as the bias value. However, since the frequency of the traverse signal obtained due to the eccentricity of the disk fluctuates greatly, in such a configuration, how to set the cutoff frequency of the low-pass filter 102 becomes a problem.

That is, as shown in FIG. 11, the waveform of the traverse signal contains a component close to DC. That is, in FIG. 11, a portion indicated by P is a point where the radial movement of the disk is reversed due to the eccentricity, and at the point where the radial movement of the disk is reversed due to the eccentricity, the eccentric velocity is almost 0. Therefore, the traverse signal becomes almost direct current. As described above, since the traverse signal has almost DC component, it is necessary to lower the cutoff frequency of the low-pass filter 102 in order to accurately extract the center value of the traverse signal. However, low pass filter 1
If the cutoff frequency of 02 is lowered, the low-pass filter 10
There is a problem that the time required to extract the center value of the traverse signal from the output of 2 becomes long.

Therefore, an object of the present invention is to provide a tracking servo device and method in which a bias value for compensating an offset value for a tracking error signal can be optimally set.

[0013]

The present invention is directed to an optical disc having an information signal recorded thereon, an optical information reading unit for irradiating the optical disc with a beam and reading information from the optical disc, and a tracking error signal from the output of the optical information reading unit. , A bias value forming means for forming a bias value by detecting a low frequency component of the tracking error signal, a bias applying means for giving a bias value to the tracking error signal, and an optical information reading A pickup moving means for moving the means in the radial direction of the optical disc, and a beam varying means for controlling the beam applied to the optical disc from the optical information reading means in the tracking direction based on the tracking error signal are provided, and the optical disc is rotated. The optical information reading means is Is moved in a direction, a bias value is formed from the low-frequency component of the tracking error signal, a tracking servo apparatus being characterized in that so as to give a bias value to the tracking error signal.

[0014]

The center value of the traverse signal is extracted from the output of the optical information reading means in a state where the optical information reading means is moved in the radial direction of the optical disk while the optical disk is rotated. Therefore, the frequency component when the optical information reading means is sent is added to the frequency component of the traverse signal, and the traverse signal has no component close to direct current. Therefore, the center value of the traverse signal can be accurately extracted without lowering the cutoff frequency of the low-pass filter when forming the bias value from the low-frequency component of the tracking error signal.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of an optical disk reproducing apparatus to which the present invention can be applied. In FIG. 1, reference numeral 1 is an optical disc. The optical disc 1 is, for example, a compact disc in which digital audio signals are recorded as a pit train. The optical disc 1 is a spindle motor 2
Is rotated by.

An optical pickup 3 is provided for the optical disc 1. The optical pickup 3 includes a laser diode that emits a laser beam, an optical system such as a polarization beam splitter and an objective lens 4, and a photodetector that receives reflected light. The objective lens 4 of the optical pickup 3 is controllable by a biaxial device 5 in a tangential direction (tracking direction) of the disc and a direction (focusing direction) away from the disc. The biaxial device 5 is provided with a TRK coil 6 for moving the objective lens in the tracking direction and an FC coil 7 for moving the objective lens in the focus direction. The entire optical pickup 3 is movable in the radial direction of the optical disc 1 by a sled motor 8.

The output of the optical pickup 3 is the RF amplifier 9
Is supplied to. The RF amplifier 9 forms an RF signal RF, a tracking error signal TE, and a focus error signal FE from the output of the optical pickup 3.

That is, the photodetector of the optical pickup 5 is the photodetector 21 as shown in FIG.
A, 21B, 21C, 21D, and 21E and 21F. The beam spots focused on the photodetectors 21A to 21D via the cylindrical lens are circular in just focus, and are elliptical if they are separated from the front and the back. Therefore, the photo detector 21
The outputs of A, 21B, 21C, and 21D are S _A , S _B , and
If S _C and S _D , the focus error signal is (S _A
+ S _C )-(S _B + S _D ). Further, the reproduction RF signal can be obtained by (S _A + S _C + S _B + S _D ). Further, when the beam spot is tracing the track center, if the output of the photodetector 21E is S _E and the output of the photodetector 21F is S _F , the tracking error signal becomes (S _F −S
_E ) required.

As shown in FIG. 2, the RF amplifier 9 includes an adder 25 for obtaining a reproduction RF signal, adders 22 and 23 for obtaining a focus error signal, and a subtractor 2.
4. Subtractor 26 for obtaining the tracking error signal
Is provided.

Photodetectors 21A, 21B, 21
C, the output of the 21D are added by the adder 25, thereby comprising _{(S A + S C + S} B + S D) operation is performed, RF
The signal RF is obtained. The RF signal RF is output from the output terminal 27.

The output of the photo detector 21A and the output of the photo detector 21C are added by the adder 22, the output of the photo detector 21B and the output of the photo detector 21D are added by the adder 23, and the output of the adder 22 and the output of the adder 23 are added. And are subtracted by the subtractor 24. _{Thus, (S A + S C)} - (S B + S D) consisting operation is made, the focus error signal FE is obtained. The focus error signal FE is output from the output terminal 28.

The outputs of the photo detectors 21E and 21F are subtracted by the subtractor 26, which results in (S _F -S _E ).
Then, a tracking error signal TE is obtained. The tracking error signal TE is output from the output terminal 29.

In FIG. 1, the RF signal RF, the focus error signal FE, and the tracking error signal TE from the RF amplifier 9 are supplied to the digital servo circuit 10. In addition, the digital servo circuit 10 includes a signal processing circuit 15
Is supplied with the bit clock PCK reproduced by the PLL. Further, the digital servo circuit 10 is given a command such as a track jump or an access command from the system controller 14.

The digital servo circuit 10 forms a tracking servo signal S _TRK and a focus servo signal S _FCK in the form of a PWM signal based on these error signals and commands. This tracking servo signal S
_{The TRK} and focus servo signal S _FCK are sent to the driver 11
And 12 to the TRK coil 6 and the FCK coil 7 of the biaxial device 5 of the optical pickup 3, respectively. Also, based on this tracking error signal TE,
Sled servo signal S _SM is formed, the Suredosabo signal S _SM is supplied to the thread motor 8 through the driver 13. Further, the spindle servo signal S _SPDL is formed based on the reproduction bit clock PCK from the signal processing circuit 15. The spindle servo signal S _SPDL is supplied to the spindle motor 2 via the driver 14. As a result, the rotation of the optical disc 1 is controlled at a constant linear velocity.

The signal processing circuit 15 extracts the bit clock of the reproduction RF signal, EFM demodulates the reproduction signal, and further,
Performs error correction processing and the like by CIRC. If the error cannot be corrected, interpolation processing or the like is performed. Furthermore, the signal processing circuit 11 performs a subcode data extraction process. The extracted subcode data is stored in the system controller 1
Sent to 4.

The output of the signal processing circuit 15 is supplied to the D / A converter 16. The D / A converter 16 converts the digital audio signal into an analog audio signal. Then, this analog audio signal is output from the output terminal 17.

The system controller 14 is provided with inputs such as reproduction, stop and pause from the key input unit 18. In addition, the output of the system controller 14 is displayed on the display unit 1.
9. The display unit 19 displays operation states such as reproduction, stop, and pause, reproduction time, reproduction music number, and the like.

FIG. 3 shows the details of the tracking servo circuit in the above-mentioned optical disk reproducing apparatus. This invention provides an optical disc reproducing apparatus as described above,
It can be used to automatically adjust the tracking servo offset.

In FIG. 3, the RF amplifier 9 outputs the RF signal RF, the focus error signal FE, and the tracking error signal TE, as described above. The tracking error signal TE from the RF amplifier 9 is supplied to the A / D converter 51 of the digital servo circuit 10. The tracking error signal TE is digitized by the A / D converter 51. The output of the A / D converter 51 is the subtractor 52.
And to the low pass filter 53.

The low-pass filter 53 is for obtaining the center value of the traverse signal due to the eccentricity of the disc. By thus obtaining the center value of the traverse signal, the bias value for compensating the offset is set. The output of the low-pass filter 53 is the register 54
Is supplied to.

The output of the register 54 is supplied to the terminal 55A of the switch circuit 55. Terminal 55 of switch circuit 55
B is grounded. The output of the switch circuit 55 is the subtractor 52.
Is supplied to.

The output of the subtractor 52 is the registers 56 and 57.
Is supplied to. The output of the register 56 is the phase compensation circuit 5
8 and is supplied to the TRK coil 6 via the driver 11.

The output of the register 57 is supplied to the phase compensation circuit 59. The output of the phase compensation circuit 59 is the switch circuit 6
0 terminal 60A. The output of the drive signal generation circuit 61 is supplied to the terminal 60B of the switch circuit 60. The output of the switch circuit 60 is supplied to the sled motor 8 via the driver 13.

When performing automatic offset adjustment, first,
The optical disc 1 is rotated with the focus servo turned on. Then, the switch circuit 55 is set to the terminal 55B side, and the switch circuit 60 is set to the terminal 60B side. When the switch circuit 60 is set to the terminal 60B side, the sled motor 8 is driven by the drive signal from the drive signal generation circuit 61, and the optical pickup 3 is sent. In this way, the center value of the traverse signal is taken out from the low-pass filter 53 while the optical disc 1 is being rotated and the optical pickup 3 is being sent. The output of the low pass filter 53 is supplied to the register 54.

When performing tracking servo, the switch circuit 55 is set to the terminal 55A side and the switch circuit 60 is set to the terminal 60A side. Therefore, the output of the register 54 is subtracted from the tracking error signal TE. In the register 54, the center value of the traverse signal obtained through the low-pass filter 53 while the optical disc 1 is being sent is stored as a bias value. The subtractor 52 subtracts the bias value thus obtained from the tracking error signal TE, thereby removing the offset component in the servo error signal.

As described above, in the embodiment of the present invention, the center value of the traverse signal is detected as the bias value while the optical pickup 3 is being sent. Therefore, the center value of the traverse signal can be accurately extracted without lowering the cutoff frequency of the low-pass filter 53 for extracting the center value of the traverse signal.

That is, conventionally, the optical pickup 3 is stopped when the center value of the traverse signal is detected. When the optical disc 1 is rotated, a traverse signal is obtained due to the eccentricity of the disc.
4A is stopped, as shown in FIG. 4A, the eccentricity causes the radial movement of the disk to be reversed in the vicinity P, P,
In P, ..., the traverse signal has a component close to DC.
Therefore, in order to accurately obtain the center value of the traverse signal, the cutoff frequency of the low-pass filter 53 must be set low.

On the other hand, in one embodiment of the present invention,
The center value of the traverse signal is detected while sending the optical pickup 3. When the traverse signal is detected while sending the optical pickup 3, the optical pickup 3
Is added to the frequency component of the traverse signal, and as shown in FIG. 4B, the traverse signal has no component close to DC. Therefore, the center value of the traverse signal can be accurately obtained without setting the cutoff frequency of the low-pass filter 53 to a low value.

This will be further considered. now,
As shown in FIG. 5, it is assumed that the center L2 of the disk is eccentric with respect to the center L1 of the rotating shaft. Thus, when the center L1 of the rotating shaft and the center L2 of the disk are eccentric,
When the disc is rotated, the position of the disc moves in the radial direction X _D perpendicular to the tangent of the track on which the beam spot LB hits, as shown in FIGS. 6A to 6E.

That is, first, as shown in FIG. 6A, it is assumed that the center L2 of the disc is at a position deviated from the center L1 of the rotating shaft by d. From this position, take one disc
When rotated / 4, in the radial direction X _D , the disc moves in the direction indicated by arrow A1 as shown in FIG. 6B. When the disk is rotated 1/2 turn, the position becomes such that the center L2 of the disk deviates from the center L1 of the rotating shaft by -d as shown in FIG. 6C (the position indicated by disk 1'in FIG. 5). When the disc is further rotated, the moving direction of the disc in the radial direction X _D is reversed to the direction shown by the arrow B1, and when the disc is rotated 3/4, the position becomes as shown in FIG. 6D. When the disc is rotated once, As shown in 6E, the center L2 of the disc returns to the state where it is displaced from the center L1 of the rotation axis by d, and the moving direction of the disc in the radial direction X _D is reversed.

Therefore, the amount of movement x of the disc along the radial direction X _D is represented by x = dcos ωt (1) where ω is the rotational angular velocity of the disc and d is the eccentricity. Then, the velocity v of the movement of the disk along the radial direction X _D is obtained from the equation (1) as follows: v = −dωsin ωt = 2πdfsin ωt (2)

The position where the disk is rotated 1/2 turn (see FIG. 6C).
Position) and a position where the disc is rotated once (see FIGS. 6E and 6E).
In position) of A, since the direction of movement of the disk in the radial direction X _D reversed, the speed v of movement along a radial direction X _D of the disk is minimized. Then, at an intermediate position at which the direction is reversed to move the disk in the radial direction X _D, velocity v of movement along a radial direction X _D of the disk is maximized. That is, at the position where the disc is rotated 1/4 (the position in FIG. 6B) and the position where the disc is rotated 3/4 (the position in FIG. 6D), the moving direction of the disc in the radial direction X _D is maximum. Becomes

The velocity v of movement along a radial direction X _D of the disk is maximized when the velocity of motion along a radial direction X _D of the disk at a position where the disc one quarter turn and v _a, If the rotation period of the disk is T, because it is sin 2πfT = sin ((2π / T) · T / 4) = sin (π / 2) = 1, speed v _a is, v _a = -2πfd ... ( 3) Similarly, if the speed of movement of the disk along the radial direction X _D at the position where the disk is rotated 3/4 is v _b, and the rotation cycle of the disk is T, then sin 2πfT = sin ((2π / T ) · 3T / 4) = sin (3π / 2) = because it is -1, v _b is a speed, a _{v b = -2πfd ... (4)} .

From the equation (3), when the track pitch of the disk is p, the frequency f _a of the traverse signal due to the eccentricity at the position where the disk is rotated ¼ is f _a = 2πfd / p (5) Becomes Similarly, the frequency f _b of the traverse signal due to the eccentricity at the position where the disk is rotated 3/4 becomes f _b = 2πfd / p (6) (5) From the equation (6), the maximum frequency of the traverse signal can be obtained by the eccentricity of the disc when the optical pickup 3 is not sent.

In one embodiment of the present invention, the traverse signal is detected while the optical pickup 3 is being sent.
Therefore, if the feed speed V of the optical pickup 3 is added to the speed obtained by the equation (3), the disc becomes 1/4.
It is the relative speed between the movement of the optical pickup 3 along the radial direction X _D at the rotated position and the movement of the optical pickup 3.
'When, v _a' the relative speed of the movement of the movement and the optical pickup 3 in a radial direction X _D of the disk of the disk at a position one quarter turn v _a is v _a '= V-v _{It is} calculated as _a = V + 2πfd (7). Similarly, if the feed speed V of the optical pickup 3 is added to the speed obtained by the equation (4),
Is relative velocity between the motion and the motion of the optical pickup 3 in a radial direction X _D of the disk at a position by 3/4 turn the disk, the relative velocity v _b 'When, v _b'
Is calculated as v _b ′ = V−v _b = V−2πfd (8)

(7) From the equation (8), when the traverse signal is detected while sending the optical pickup 3 at the speed V, the frequency of the traverse signal is due to the eccentricity of the disc.
It can be seen that the value varies from (V-2πfd) / p to (V + 2πfd) / p.

In this way, the optical pickup 4 is moved to the speed V
, The frequency of the traverse signal is (V-2πf
It will vary from d) / p to (V + 2πfd) / p, and there will be no direct current component. Then, as the low-pass filter 53 for extracting the center value of the traverse signal from the traverse signal, it is sufficient that the low-pass filter 53 has a characteristic capable of sufficiently attenuating the signal components in these frequency ranges.

Therefore, the minimum frequency required for measurement is f
_min , and when a component with a minimum frequency f _min or higher is to be attenuated by the low-pass filter 53, f _min = (V _min -2πfd) / p, so the minimum feed speed V of the optical pickup 3 is V.
_min is V _min = f _min · p + 2πfd (9) At this time, the maximum frequency f _{max of the} signal included in the traverse signal is f _max = (V _min + 2πfd) / p.

The feeding speed of the optical pickup 3 is set to V _min.
Then, the traverse frequency of the tracking error input to the low-pass filter 53 varies from f _min to f _max due to the eccentricity of the disc. If this is attenuated by the low-pass filter 53 having a cutoff frequency f _C ,
Minimum frequency f attenuation G _min of the low-pass filter 53 when the _{min is, G min = 20n · log (} f c / f min) = 20n · log
(F _c · p / (V _min −2πfd)) dB. Further, the attenuation amount G _max of the low-pass filter 53 at the maximum frequency f _max is G _max = 20n · log (f
_c / f _max ) = 20 n · log (f _c · p / (V _max +2
πfd)) dB. Therefore, the cutoff frequency f of the low-pass filter 53
_The lower the value of _c or the higher the feeding speed of the optical pickup 3, the greater the amount of attenuation by the low-pass filter 53,
It can be seen that the measurement accuracy of the traverse center value increases. Also, if the feed speed V _min is large enough to ignore the term (2πfd) due to the eccentricity of the disk, the fluctuation of the measured value is small. That is, if the cutoff frequency of the low-pass filter 53 is fixed, increasing the feeding speed of the optical pickup 3 does not influence the eccentricity of the disk, and the accuracy of extracting the traverse center value can be improved.

As described above, by detecting the center value of the traverse signal while the optical pickup 3 is sending, the center value of the traverse signal can be accurately obtained without lowering the cutoff frequency of the low-pass filter 53. Figure 7
~ FIG. 9 verifies this.

FIG. 7 shows that when the eccentricity of the disk is small,
The low-pass filter 5 is used depending on whether the center value of the traverse signal is detected without sending the optical pickup 3 or when the center value is detected while sending the optical pickup 3.
3 is a comparison of the outputs. 7A shows the output of the low-pass filter 53 when the optical pickup 3 is not sent, and FIG. 7B shows the output of the low-pass filter 53 when the optical pickup 3 is sent. When the eccentricity of the disk is small, the frequency component due to the eccentricity of the traverse signal contains a lot of components close to direct current, and the output of the low-pass filter 53 fluctuates due to the influence as shown in FIG. 7A. On the other hand, when the center value is detected while sending the optical pickup 3, a stable center value of the traverse signal is obtained as shown in FIG. 7B.

FIG. 8 shows a case where the center value of the traverse signal is detected without sending the optical pickup 3 and a case where the center value is detected while sending the optical pickup 3 when the eccentricity of the disk is medium. , The output of the low-pass filter 53 is compared. 8A shows the output of the low-pass filter 53 when the optical pickup 3 is not sent, and FIG. 8B shows the output of the low-pass filter 53 when the optical pickup 3 is sent. When the eccentricity of the disk becomes slightly large, the frequency component due to the eccentricity of the traverse signal rises, so the output of the low-pass filter 53 becomes stable, but as shown in FIG. 8A, it still fluctuates due to the influence of the component close to DC. . On the other hand, when the center value is detected while sending the optical pickup 3, a stable center value of the traverse signal is obtained as shown in FIG. 8B.

FIG. 9 shows that when the disc eccentricity is large,
The low-pass filter 5 is used depending on whether the center value of the traverse signal is detected without sending the optical pickup 3 or when the center value is detected while sending the optical pickup 3.
3 is a comparison of the outputs. 9A shows the output of the low-pass filter 53 when the optical pickup 3 is not sent, and FIG. 9B shows the output of the low-pass filter 53 when the optical pickup 3 is sent. When the eccentricity of the disk becomes large, the frequency component due to the eccentricity of the traverse signal further increases, so the output of the low-pass filter 53 becomes stable, but as shown in FIG. 9A, it still fluctuates due to the influence of the component close to DC. . On the other hand, if the center value is detected while the optical pickup 3 is being sent, FIG.
As shown in, a stable center value of the traverse signal is obtained.

In the above-mentioned one embodiment, the example applied to the optical disk reproducing apparatus such as the compact disk reproducing apparatus has been described, but the present invention is also applicable to the case where another disk such as a magneto-optical disk is used. Can be applied to.

[0055]

According to the present invention, the center value of the traverse signal is extracted from the output of the optical information reading means while the optical disk is rotated and the optical information reading means is moved in the radial direction of the disk. Therefore, the frequency component when the optical information reading means is sent is added to the traverse signal, and the component close to direct current is eliminated in the traverse signal. Therefore, the center value of the traverse signal can be accurately extracted without lowering the cutoff frequency of the low-pass filter when forming the bias value from the low-frequency component of the tracking error signal.

[Brief description of drawings]

FIG. 1 is a block diagram of an example of an optical disc reproducing apparatus to which the present invention can be applied.

FIG. 2 is a block diagram used for explaining an optical disc reproducing apparatus to which the present invention can be applied.

FIG. 3 is a block diagram of an embodiment of the present invention.

FIG. 4 is a waveform chart used for explaining one embodiment of the present invention.

FIG. 5 is a schematic diagram used to describe an embodiment of the present invention.

FIG. 6 is a schematic diagram used to describe an embodiment of the present invention.

FIG. 7 is a waveform chart used for explaining one embodiment of the present invention.

FIG. 8 is a waveform chart used for explaining one embodiment of the present invention.

FIG. 9 is a waveform chart used for explaining one embodiment of the present invention.

FIG. 10 is a block diagram of an example of a conventional tracking servo circuit.

FIG. 11 is a waveform diagram used to describe a conventional tracking servo circuit.

[Explanation of symbols]

 1 Optical Disc 2 Spindle Motor 6 TRK Coil 7 FCK Coil 8 Thread Motor 10 Digital Servo Circuit 53 Low Pass Filter 61 Drive Signal Generation Circuit

Claims (4)

[Claims] 1. An optical disc on which an information signal is recorded, an optical information reading unit for irradiating the optical disc with a beam to read information from the optical disc, and a tracking error for extracting a tracking error signal from the output of the optical information reading unit. Signal extracting means, bias value forming means for forming a bias value by detecting a low frequency component of the tracking error signal, bias applying means for giving the bias value to the tracking error signal, and the optical information reading means. And a beam changing means for controlling, in the tracking direction, the beam emitted from the optical information reading means to the optical disk based on the tracking error signal. Optical information A tracking servo device, wherein the reading means is moved in the radial direction of the optical disc, a bias value is formed from the low-frequency component of the tracking error signal, and the bias value is given to the tracking error signal. 2. The moving speed of the optical reading means is f _min which is the minimum frequency required to form the bias value, p is the track pitch, f is the frequency due to the rotation of the optical disc, and d is the eccentricity of the optical disc. Then, the tracking servo device according to claim 1, wherein f _min · p + 2πfd or more is set. 3. A tracking error signal is extracted from the output of the optical information reading means in a state where the optical information reading means is moved in the radial direction of the optical disk while the optical disk is being rotated, and the tracking error signal is extracted from the low frequency component of the tracking error signal. A tracking servo method comprising a step of forming a bias value and a step of performing tracking control by adding the bias value to a tracking error signal from the output of the optical information reading means. 4. The moving speed of the optical reading means is f _min , which is the minimum frequency required to form the bias value, p is the track pitch, f is the frequency due to the rotation of the optical disk, and d is the eccentricity of the optical disk. Then, the tracking servo method according to claim 3, wherein f _min · p + 2πfd or more is set.