JP2001067689A - Offset correcting method for tracking error signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the offset correcting method of a tracking error signal, in which the offset of a tracking error signal (push-pull signal) which is generated due to the shift of an objective lens or the like is corrected over a wide range. SOLUTION: When the push-pull signal obtained from outputs of the photodetecting sensor 20 having bisected segments C, D of an optical pickup by irradiating the groove part of a disk with a laser beam is used as a tracking error signal TE, the offset of the tracking error signal is corrected by calculating a middle-point level from the + peak level and the - peak level being in roughly the same amplitude states of a land prepit signal which are generated due to left and right prepits 24 on a land part which is formed alternately with the groove part of an optical disk 2 and by operating the middle-point level and the push-pull signal so that the reference level of the push-pull signal is overlapped on the middle-point level. Thus, the offset of the tracking error signal (push-pull signal) which is generated due to the shift of the objective lens or the like is corrected over the wide range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method for correcting an offset of a tracking error signal used when performing tracking control of an optical pickup.

[0002]

2. Description of the Related Art Generally, a compact disc (CD) is used.
c) and DVD (Digital Versatile)
An optical pickup is used to record / reproduce information on / from an optical disc such as a disc, and as the track pitch becomes smaller, higher tracking control is required. Here, focusing on tracking control, FIG. 6 shows a schematic configuration diagram of an optical disk structure of a DVD-RW and an optical system of an optical pickup for recording / reproducing a signal to / from the optical disk. On the substrate of the optical disc 2, land portions 4 and groove-shaped groove portions 6 are formed alternately spirally or concentrically. The read or write laser light L output from the laser element 10 of the optical pickup 8 passes through a grating (not shown), is converted into parallel light by a collimator lens 12, passes through a half mirror 14, and then passes through a half mirror 14. After passing through a quarter-wave plate or the like, which is not focused on the optical disc 2 by the objective lens 16, for example, the reading spot 1
8 is formed.

[0003] Here, the reading spot 1 is
8 shows a state in which irradiation is performed. The reflected light from the optical disc 2 returns along the reverse path to that described above, and the optical path is bent by 90 degrees by the half mirror 14 and passes through a not-shown cylindrical lens or the like.
0 is detected. Here, the principle of detecting the tracking error signal is shown, and the focus error signal and R
The F signal detection system is not shown. The light detection sensor 20 is divided into two segments C and D along the track direction. The difference between the outputs of the two segments C and D is obtained by a differential amplifier 22 to generate a push-pull signal, That is, the tracking error signal TE is obtained.

FIG. 7 shows a dimensional relationship between a groove, a land, and a reading spot, which are tracks of the optical disk. FIG. 7 also shows the wobble shape.
This optical disc 2 has a track pitch L1 of 0.74 μm.
The width L2 of the groove portion 6 is about 0.3 μm,
Data is recorded in the groove 6. The groove 6 is used to rotate the optical disc 2 under CVL control.
In order to obtain a wobble signal of 40 KHz, the wobble signal has a meandering width L3 of about 10 nm. Also, in the land part 4,
26 syncs for physical sector address signals
As data composed of frames, 16 syncs and 64
And a land pre-pit 24, which is a land break of 2T in length recorded with a relative address of 128 and data of 128. Here, T indicates a channel clock cycle.
In the push-pull tracking control system, the diffracted light of the reading spot 18 (see FIG. 6) radiated onto the groove 6 is divided into two light-detecting sensors 2 when off-track.
Tracking control is performed using the fact that the light amount becomes unbalanced on 0.

[0005]

As shown in FIG. 8, in this type of push-pull system, the objective lens 1
No. 6, there is a problem that an offset is generated in the tracking error signal TE due to a lens shift and a disc tilt in the radial direction. When the objective lens 16 shifts as shown in FIG.
Since the middle point of 6 and the dividing line of the two-divided light detection sensor 20 match each other, the reading spot 18 moves in the tracking direction (radial direction) by the movement amount ΔX of the lens shift. In connection with this, the center of the light spot 18A (see FIG. 6) on the light detection sensor 20 has a center point of the Gaussian distribution when considering the Gaussian intensity distribution of the parallel light.
Moves to a position symmetrical to the optical axis by the light detection sensor 2
The midpoint of the light beam on 0 moves by 2 × ΔX. For this reason, even if the reading spot 18 is at the middle point of the track, the tracking error signal TE of the push-pull system does not become 0 level, and an offset occurs. In addition, when the optical pickup is moved in the radial direction of the disk at the time of accessing to read the data of the eccentricity limit disk or a predetermined track, if the objective lens is offset by the inertia force, the AC amplitude of the tracking error signal may be exceeded. Unless this offset is corrected, reproduction of a large eccentric disc and tracking pull-in during an access operation cannot be performed.

[0008] Further, as shown in FIG. 8 (B), the center of the reflected light also moves due to the tilt of the optical disk 2, causing an offset in the tracking error signal TE. At this time, if f is the focal length, the center movement is f × 2Δθ. Here, Δθ indicates a tilt angle. Several methods are known as methods for correcting such an offset of the push-pull signal. As one of them, a DPP (Differential Push-Pul) as shown in FIG.
l) The scheme is known. Three beams are formed as reflected light, and three split photodiodes 26 are formed as sensors.
A, 26B and 26C are provided. Then, the sub-beams on both sides of the disk surface trace on the land,
The arithmetic circuit 28 performs a differential operation on an offset signal generated by a lens shift or a disc tilt having the same phase as the push-pull signal having the opposite phase to the tracking error signal detected at the main spot for reading, thereby generating the offset. There is a method to get rid of. However, in this method, the amount of light for reading is reduced due to three beams, resulting in light loss. Also, a grating for forming three beams is required, and the number of components increases. In addition, it is necessary to adjust the grating with respect to the track, and the number of adjustment steps increases. In addition, the preceding spot traces a recorded track, and the following spot is on an unrecorded track, causing a problem that the balance may be lost.

As another solution, there is a method using an objective lens integrated drive light beam splitting element. In this method, an objective lens and a light beam splitting element are adhered to a lens bobbin part of an actuator, and this element is shifted in the radial direction in the same manner as the objective lens. This is a method for correcting the offset of the push-pull signal generated by moving from the optical axis. In this case, there is a problem that components are mounted on the light beam splitting element and the movable portion, and a problem that a light detection sensor having a special pattern shape is required. Further, in the case of forming a hologram element in which the light emitting section and the light receiving section are integrated, there is another problem that the light beam splitting element is so far away that it cannot be adjusted by a single component.

On the other hand, Japanese Patent Application Laid-Open No. 11-16174 discloses a push-pull signal using a land pre-pit signal by a land pre-pit formed on a land portion of a DVD-R or DVD-RW disc in order to compensate for the above drawback. The method of correcting the offset of the above is shown. As described above, the cause of the offset is the radial tilt of the optical disk and the shift of the objective lens. However, in this publication, only the lens shift causes the offset, and the offset is corrected (the radial tilt of the disk is adjusted by the drive using a tilt servo system). In the case of absorption, the lens shift is the cause of the offset of the push-pull signal).

However, the change of the land prepit signal due to the lens shift is caused by the fact that the center (effective diameter) of the objective lens does not coincide with the center of the parallel light beam, and the influence on the land prepit signal will be described later. There is little imbalance. On the other hand, since the track offset is such that the spot is displaced from neutral with respect to the land prepits on both sides of the groove, the change to the land prepit signal is different from the lens shift and the land prepit signal on both sides is different from the lens shift. Unbalance occurs, the amount of which is large compared to the lens shift. The above-mentioned known example is intended for correction of an offset generated in a push-pull signal in a range where a tilt is small and a lens shift is relatively small because correction is performed by imbalance of a land pre-pit signal.

Therefore, the eccentricity limit disk (large eccentricity of about ± 0.5 mm) used for testing the characteristics of the drive is reproduced, and the eccentricity is large even when it is desired to reproduce the disk without any problem or in actual use. In such a case, or when a relatively large lens displacement is assumed as in the case of high-speed access control, a large offset occurs in the push-pull signal. In this case, as shown in FIG. 8C, the push-pull tracking signal is greatly offset, exceeds the level of the tracking signal (which is assumed to be an AC component) that changes while crossing the track, and is synchronized with the lens shift. The swell of the push-pull signal due to the offset may occur. In other words, as shown in FIG. 8C, the track pull-in points of the push-pull tracking signal, for example, P1 and P2 greatly deviate from 0 V, and the polarity of the tracking signal (AC component) becomes only + or only-
The servo loop cannot perform the pulling operation at 0 V. Assuming this time, the correction method of the known example is a correction method using the level difference of the land prepit signal as AC coupling by removing the DC component of the differential signal. When the offset fluctuation of the push-pull signal becomes equal to or more than the AC component of the push-pull signal, the level cannot be corrected by the level difference of the land pre-pit signal from which the AC component is removed, and a wide range of tracking control can be performed only with the proposed correction circuit configuration. There was a problem that could not be done. In the present invention, even if there is a lens shift of the objective lens,
In principle, when the spot is located on the groove portion, the level difference of the land pre-pit signal is basically small, and the midpoint signal of the land pre-pit including the DC component includes an offset due to lens shift. It is due to being.

The present invention focuses on the above problems,
The purpose of the present invention is to solve this problem effectively, and its purpose is to provide a tracking error signal capable of correcting an offset of a tracking error signal (push-pull signal) generated due to a shift or the like of an objective lens to a wide range. An object of the present invention is to provide a signal offset correction method.

[0012]

According to a first aspect of the present invention, a push-pull signal obtained from an output of a light detection sensor of a two-segment segment of an optical pickup by irradiating a groove portion of an optical disk with laser light is provided. When used as a tracking error signal, the positive and negative peak levels of the land pre-pit signal having substantially the same amplitude due to the left and right land pre-pits on the lands formed alternately with the groove portions of the optical disc. The midpoint level is obtained from the above, and the offset of the tracking error signal is corrected by calculating the midpoint level and the push-pull signal so that the reference level of the push-pull signal overlaps the midpoint level. It is like that.

This makes it possible to correct the offset of the tracking error signal (push-pull signal) generated due to the shift of the objective lens or the like to a wide range.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a method for correcting an offset of a tracking error signal according to the present invention will be described in detail with reference to the accompanying drawings. Generally, a push-pull signal, which is a detection signal for applying tracking servo to follow the read spot to the eccentricity of the disc, for example, causes an offset due to a radial shift of the objective lens or disc tilt, and in this state, the tracking servo is performed. Is operated to pull in the off-track state when the tracking range is narrow. When the lens shift is large and the polarity of the push-pull tracking signal cannot be maintained, the servo cannot perform the following operation. In the present invention, the offset of the tracking error signal generated by such a lens shift is corrected so that the tracking servo operation can be performed in a wide range.

Specifically, a land pre-pit signal is formed on a land portion of the disk for address detection. In the present invention, a land formed on the land portions on both sides of a groove portion to which servo is applied is provided. The pre-pit signal is detected by a push-pull signal detection system, and the track offset amount is measured using the level difference, thereby correcting the offset of the push-pull signal (tracking error signal). However, since the lens shift is not basically off-track, the correction of the lens shift is not accurate only with the land pre-pit differential signal generated on the off-track. In addition, correction cannot be performed for a large lens shift. Therefore, this is solved by a method of detecting the midpoint of the land pre-pit signal which enables correction even when the objective lens shifts greatly.

That is, the offset of the push-pull signal in a state where the level difference of the land pre-pit signal does not relatively occur is detected as a DC fluctuation from the land pre-pit signal, and the offset due to a large lens shift is corrected by this. FIG. 1 is a block diagram of an offset correction circuit for implementing the method of the present invention, FIG. 2 is a diagram schematically showing a relative positional relationship between an objective lens, a groove portion, and a light detection sensor, and FIG. FIG. 4 is a diagram showing characteristics, and FIG. 4 is a diagram showing a position of a light spot on an optical disc and a relationship between a push-pull signal and a land pre-pit signal at that time.
FIG. 6 is a diagram showing a relationship between a push-pull signal and a land pre-pit signal at the time of lens shift generated in an on-track state.

First, a block diagram of the offset correction circuit shown in FIG. 1 will be described. Outputs from the segments C and D of the two-divided light detection sensor 20 are input to a differential amplifier 22, where a difference signal is obtained. The difference signal is input to the + peak detecting unit 30, the -peak detecting unit 32, the first low-pass filter 34, and the second low-pass filter 36, respectively. The first low-pass filter 34 cuts a high-frequency signal of the difference signal, and has a cutoff frequency of, for example, about 1 KHz. The second low-pass filter 36 is for obtaining a push-pull signal, and has a set-off frequency of, for example, about 30 KHz. The peak detectors 30 and 32 are composed of a diode, a capacitor, a resistor, and an operational amplifier, and detect the peak level of the signal based on the nonlinear characteristics of the diode.

The differential amplifiers 38, 40, and 42 form required signals from outputs selected from the above-described elements, and the differential amplifier 44 performs differential operation on the outputs of the two differential amplifiers 38, 40. I do. The adder / subtractor 46 outputs a signal obtained by multiplying the output of the differential amplifier 44 by the gain G1 and a signal obtained by multiplying the output of the differential amplifier 42 by the gain G2 from the push-pull signal output from the second low-pass filter 26. Is connected via an analog switch 52, and this switch 52 is turned ON / OFF according to the level state of the amplitude differential signal S2,
When it is ON, the tracking error signal TE is created by subtraction. First, in FIG. 2, the reflected light from the groove portion 6 of the optical disk 2 is received by a light detection sensor 20, and the outputs of the two segments C and D are compared by a differential amplifier 22, and a push-pull signal (differential signal) is obtained. ), The tracking error signal TE is obtained. FIG. 3 shows an S-shaped characteristic of a tracking error signal TE for tracking control detected by a deviation between a read spot and a track center. Reference numerals in FIG. 3 correspond to the respective aspects in FIG.

Normally, the servo system is servoed by an error signal in a band of 30 KHz or less. However, the radial push-pull is provided with a wideband differential signal system for reading a wobble signal for applying a CLV to a land pre-pit signal which is an address signal. The land pre-pit signal is a signal having a width of 2T when the channel clock cycle is 1T. When this signal is approximated by a sine wave, it is 13 MHz. If there is a band of 20 MHz, a signal having no waveform distortion can be read. The land pre-pit signal detection system is composed of such a wide-band circuit system. When the tracking servo is applied and the light spot is scanning the center of the groove, the land prepit signals on both sides of the track are reproduced at the same level. However, the land pre-pit signal on the inner peripheral side on the two-divided sensor 20 is received such that the light quantity of the inner peripheral side segment is reduced by diffraction. Further, the land pre-pit signal on the outer peripheral side is detected as a change in which light reception decreases in the inner peripheral side segment.

When the spot is off-track toward the inner circumference side (FIG. 4A), the intensity of the diffracted light by the groove portion becomes unbalanced on the outer circumference side, and the outer circumference side segment is negatively polarized with the negative polarity. A tracking signal that performs a differential operation on a segment with a positive polarity has a positive voltage output. However, the land pre-pits 24 are on the lands adjacent to the grooves. In the on-track state shown in FIG. 4B, the periphery of the read spot hits the land pre-pit 24. When the read spot is offset to the inner side, the level of the land prepit signal in the segment C on the inner side increases and the land prepit signal in the segment D on the outer side decreases. When this signal is subjected to differential calculation, a signal is detected in which the level of the negative land prepit signal increases and the positive land prepit signal decreases.

Conversely, when the reading spot moves to the outer peripheral side as shown in FIG. 4C, an opposite signal is output. Also,
At the time of on-track, the land pre-pit signals on both sides are at the same level, so that an offset correction signal due to lens shift cannot be formed only by the amplitude difference between the land pre-pit signals. Therefore, an appropriate tracking error signal can be obtained by calculating the difference between the peak values of the land pre-pit signals on the plus side and the minus side and correcting the push-pull signal based on the midpoint. Here, the characteristics of the land pre-pit signal with respect to the lens shift of the objective lens will be described with reference to FIG.

At the time of lens shift, the objective lens follows the track and moves from the neutral position, so that the center of the light beam does not coincide with the optical axis of the objective lens. Since the division line of the two-division sensor 20 is aligned with the optical axis of the reflected light when the objective lens 16 is at the center position, when the objective lens shifts, the center of the image of the reflected light coincides with the division line. Is gone. If the radial rim intensity of the objective lens is set to 50% of the intensity on the optical axis, the movement of the objective lens means that the Gaussian intensity distribution is not at the lens center. Further, when the lens is further shifted, the reading spot is formed at the focal position of the objective lens. For this reason, the center of the spot image on the sensor 20 is shifted from the dividing line (the midpoint of the Gaussian distribution is 2 × ΔX
), The light amount of the spot on the sensor 20 becomes unbalanced, and becomes an offset of the push-pull signal.

That is, when the lens shifts inward as shown in FIG. 5A, the S-shaped characteristic and the push-pull signal also shift to the + side, and a land pre-pit signal having substantially the same level is output on both sides. Conversely, when the lens is shifted outward as shown in FIG. 5C, the S-shaped characteristic and the push-pull signal also shift to the negative side, and land prepit signals having substantially the same level are output on both sides. Now, based on such a phenomenon, the control operation will be described with reference to FIG. From the outputs from the segments C and D of the above-mentioned two-divided sensor 20, a broadband land pre-pit signal is detected by passing it through a differential amplifier 22. This signal is passed through a + peak detection unit 30, a first low-pass filter 34, and a -peak detection unit 32, respectively, so that a peak signal Epa of the land prepit signal on the + side and a high-frequency signal including a wobble signal of 140 KHz are converted. The cut low frequency signal Ec,
A peak signal Epb of the negative land prepit signal is formed. Here, when each signal is differentially calculated by the differential amplifiers 38 and 40 with reference to the low-frequency signal Ec, a positive signal amplitude and a negative signal amplitude are detected. Further, in order to detect the midpoint between the + peak and the −peak, the operation of ((+ peak) + (− peak)) / 2 is performed by the differential amplifier 42 so that the midpoint signal (voltage ) S1 is formed.

Further, an amplitude differential signal S2 is obtained by taking a difference between ± signal amplitudes in the differential amplifier 44. Then, the adder / subtracter 46 converts the above-mentioned middle-point signal S from the push-pull signal PP obtained from the second low-pass filter 36.
By subtracting 1 and the amplitude differential signal S2, a corrected tracking error signal TE can be obtained. Here, the reason for subtracting the amplitude differential signal S2 is that when the lens shift is small, the level difference between the land pre-pits accurately detects the off-track state. Can be used. However, if the lens shift is not large, even if the spot is in the on-track state, the PP tracking signal is greatly offset, and the amplitude differential signal S2 is in a balanced state, the signal level becomes 0, and does not become a correction signal. Therefore, the correction is performed using the midpoint signal. The offset of the push-pull signal is corrected by performing a differential operation on the amplitude differential signal S2 and the midpoint signal S1 from the PP (push-pull) signal. Since the midpoint signal can be used accurately in a state close to on-track, a configuration in which an ON / OFF control circuit is added so that correction is not performed in an off-track state of the amplitude differential signal S2 may be considered. In addition, the level of the land pre-pit signal by the left and right land pre-pits is monitored to detect when they are substantially at the same level.
2 is input to the window comparator 50. If the level of the land pre-pit signal is the same, the amplitude differential signal S
2 is close to 0 V, and a threshold voltage Vth is provided for comparison. As a result, when the difference is within ± Vth, the window comparator 50 outputs a logical level L (low), and when it exceeds ± Vth, the window comparator 50 outputs H (high). Thereby, the analog switch 52 is controlled to output the midpoint signal S1.
Is turned on when it is at the L level and turned off when it is at the H level.

As a control method, a comparator and an AND circuit are used to detect when the amplitude level of the single + and-land pre-pit signals satisfies a certain level, and control the correction operation. You may. Further, the lens shift becomes large when the tracking drive voltage is large. Therefore, the magnitude of the tracking drive current may be detected and linked with this to determine the correction operation period.

[0026]

As described above, according to the present invention,
An offset generated in the push-pull signal due to a shift of the objective lens or the like can be corrected to a wide range to form an appropriate tracking error signal. Further, the push-pull signal can be corrected by adding a simple electric system, the loss of light quantity can be eliminated, the cost of parts and the number of assembly steps can be reduced, and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of an offset correction circuit for implementing the method of the present invention.

FIG. 2 is a diagram schematically illustrating a relative positional relationship between an objective lens, a groove portion, and a light detection sensor.

FIG. 3 is a diagram illustrating an S-shaped characteristic of a push-pull tracking control signal detected based on a relationship between a track and a spot position.

FIG. 4 is a diagram showing the relationship between the position of a light spot on an optical disk and the push-pull signal and land pre-pit signal at that time.

FIG. 5 is a diagram showing a relationship between a push-pull signal and a land pre-pit signal at the time of a lens shift generated in an on-track state.

FIG. 6 is a schematic configuration diagram showing an optical disk structure of a DVD-RW and an optical system of an optical pickup for recording / reproducing a signal on / from the optical disk.

FIG. 7 is a diagram illustrating a dimensional relationship among a groove portion, a land portion, and a reading spot, which are tracks of an optical disc.

FIG. 8 is a diagram illustrating a state of a lens shift and a disc tilt in a radial direction of the objective lens.

FIG. 9: DPP (Differential Push)
FIG. 2 is a diagram illustrating a (Pull) type light detection sensor.

[Explanation of symbols]

2 ... optical disk, 4 ... land part, 6 ... groove part, 8 ...
Optical pickup, 16: Objective lens, 18: Reading spot, 20: Light detection sensor, 22: Differential amplifier, 24: Land prepit, 30 ... + Peak detector, 32 ...- Peak detector, 38, 40, 42 , 44 ... differential amplifier, 46
... Addition / subtraction unit, L: Laser beam, TE: Tracking error signal.

Claims (1)

[Claims] When a push-pull signal obtained from an output of a light detection sensor of a two-segment segment of an optical pickup by irradiating a laser beam onto a groove portion of an optical disc is used as a tracking error signal, the groove portion of the optical disc is used. The mid-point level is obtained from the + peak level and the -peak level of the land pre-pit signal having substantially the same amplitude due to the left and right land pre-pits on the land portion alternately formed. The offset of the tracking error signal is corrected by calculating the midpoint level and the push-pull signal so that the reference level of the push-pull signal overlaps with the offset of the tracking signal. Method.