WO2007141768A1

WO2007141768A1 - Recording device and method

Info

Publication number: WO2007141768A1
Application number: PCT/IB2007/052182
Authority: WO
Inventors: Wooi Liang Chin; Wei Zhang
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-09
Filing date: 2007-06-11
Publication date: 2007-12-13
Also published as: JP2009540479A; TW200809823A

Abstract

A method of calibrating focus offset, tilt offset and radial offset for improving tracking performance during recording is described. A relationship between a focus offset and a radial error signal is found. A write focus offset value is found at which the radial error signal amplitude is substantially maximum. This is useful for all optical recording devices.

Description

Recording device and method

FIELD OF THE INVENTION

This subject matter relates to the field of recording devices, and more specifically to recording methods.

BACKGROUND OF THE INVENTION

Shirota, et.al. (US 2006/0002250) teach a method of recording data. Shirota's method comprises writing test data on to the disc, reading back the written test data and then finding a focus-offset value based on the quality of the signal read back. Shirota's method determines focus-offset value on the basis of jitter level of written test data using read laser power. The determined focus-offset value is used for writing data on the disc and also for reading data from the disc. Shirota's method is generally sufficient for writing data on to single layer discs but not suited for reliable writing to multi- layer discs. Shirota's method assumes that the focus-offset value that is optimum for reading data is optimum for writing data, which results in recording errors.

It would be advantageous to have a method that uses servo-offset values suitable for recording data before recording starts. It would also be advantageous to have a device that uses servo-offset values suitable for recording data before recording starts.

SUMMARY OF THE INVENTION

In a method of recording described herein, a relationship between a focus offset and a radial error signal is found. A write focus offset value is found at which the radial error signal amplitude is substantially maximum.

In a device for recording data described herein, an optical system scans recording tracks (T₁, T₂, .... T_n) of a record carrier. A light beam generating means generates a light beam; an objective lens focuses the light beam on the record carrier. An optical detector detects a reflected light beam. A controllable focus actuator axially displaces the objective lens with respect to a recording reference plane of the record carrier. A control circuit receives a signal from the optical detector and outputs a control signal to control the focus actuator. The control circuit further comprises a write focus offset unit arranged to find a write focus offset value at which the radial error signal amplitude is substantially maximum. The control circuit further comprises a focus offset correction unit arranged to find a focus-offset-correction value for writing.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages will be further explained by the following description, by way of example only, with reference to the accompanying drawings, in which same reference numerals indicate same or similar parts, and in which: Figs. IA and IB schematically illustrate one example of a recording device,

Fig. 2 shows an example flow chart illustrating steps of the method of recording,

Fig. 3 is an example graph, illustrating schematically the relationship between radial tracking error signal and focus-offset obtained for a DVD+R DL record carrier at Layer 1,

Fig. 4 shows an example block diagram, schematically illustrating the control circuit for controlling the focus actuator,

Fig. 5 is an example graph, illustrating schematically the relationship between jitter signal amplitude and tilt offset obtained for a DVD+R DL record carrier at Layer 1, Fig. 6 shows an example block diagram, schematically illustrating the control circuit for controlling the tilt actuator,

Fig. 7 is an example graph, illustrating schematically the relationship between radial offset and missing synchronization symbols obtained for a DVD+R DL record carrier at layer 1 , and Fig. 8 shows an example block diagram, schematically illustrating the control circuit for controlling the radial actuator.

Figs. IA and IB schematically illustrate one example of a recording device 1 (e.g. DVD recorder), suitable for writing information on to a record carrier 2 (typically a DVD). For rotating the record carrier 2, the recording device 1 comprises a motor 4. The motor 4 is typically fixed to a frame defining a rotation axis 5. For receiving and holding the record carrier 2, the recording device 1 may comprise a turntable or clamping hub 6, which in the case of a spindle motor 4 is mounted on the spindle axis 7 of the motor 4. The recording device 1 further comprises an optical system 30 for scanning tracks of the record carrier 2 by an optical beam. More specifically, the optical system 30 comprises a light generating means 31 (e.g. a laser diode), arranged to generate a light beam 32a. The light beam 32a passes through a beam splitter 33 and an objective lens 34. The objective lens 34 focuses the light beam 32b in a focal spot F on the record carrier 2. The light beam 32b reflects from the record carrier 2 (reflected light beam 32c) and passes through the objective lens 34 and the beam splitter 33 (beam 32d) to reach an optical detector 35.

A rectangular co-ordinate system XYZ is used. The rotation axis 5 is Z-axis. The radial direction is X-axis (perpendicular to the Z-axis, such that the focal spot F lies in the XZ plane). The tangential direction is Y-axis (perpendicular to the X-axis and the Z- axis). A polar co-ordinate system, r, Φ is also used to define the co-ordinates of the record carrier 2.

For achieving and maintaining correct focusing of the light beam 32b on a desired location (on the record carrier 2), the objective lens 34 is mounted axially displaceable (Z-direction). Further, the actuator system 40 of the recording device 1 comprises a focus actuator 42 arranged for axially displacing the objective lens 34 with respect to the recording reference plane of the record carrier 2. Due to spherical aberration, refractive index of the substrate etc, the focal spot F of the light beam on the record carrier 2 is shifted by an offset value. The focus-offset correction value is the amount of offset with respect to the focal distance to correct the influence of this aberration. To eliminate these influences, the focus-offset correction value is computed and is used to bring back the focus- offset to a suitable position. The focus-offset correction that gives good jitter performance in the read back signal is not necessarily the same as the correction that will result in good push- pull signal. Further, the focal spot F should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, the objective lens is mounted radially displaceable, and the recording device 1 comprises radial actuator means 41 for controlling the radial position of the objective lens. While recording data on wobbled recording tracks of the record carrier 2, there may be situations where a pit is not formed at the center of a recording track on the record carrier 2. This may be due to substantial tolerances in optical, mechanical and electrical characteristics of the record carrier 2 and wavelength shift in a laser diode. Data is recorded at a position deviated from the center of the recording track. This deviation of the laser / light spot from the center of the recording track is referred to as radial-offset. The radial-offset correction value is the amount of offset to be used in order to move the laser / light spot radailly inward or outward with respect to the center of the recording track to bring back the radial-offset to a suitable position.

The record carrier 2 may also suffer from tilt. Tilt of the record carrier 2 is defined as a condition where the recording reference plane of the record carrier 2 is not perpendicular to the rotation axis. The record carrier 2 being tilted as a whole with respect to the laser beam (e.g. because the motor axle is tilted with respect to the frame and as a consequence the amount of tilt depends on the location on the record carrier) can cause tilt. Especially DVD systems, which have a relatively large numerical aperture (NA), are sensitive to disc tilt. Typically, in a recording device 1 having tilt correction, the objective lens 34 is pivotable, and the recording device 1 comprises tilt actuator means for controlling the tilt position of the objective lens 34. To this end, the objective lens 34 is mounted as to be pivotable about a pivot axis which is directed parallel to the Y-axis, such that an optical axis 36 of the objective lens 34 is located in the XZ- plane. Preferably, the pivot axis coincides with the optical center of the objective lens 34. A pivot angle Φ will be defined as the angle between the Z- axis and the optical axis 36 of the objective lens 34. Further, the actuator system 40 of the recording device 1 comprises a tilt actuator 43, arranged for pivoting the objective lens 34 with respect to the record carrier 2. Principally, the angle Φ should be zero. However, due to manufacturing process variations the record carrier 2 is generally not flat. The record carrier curves in both radial and circumferential directions. Therefore, the optical system 30 cannot scan the recording tracks with its optical axis 36 perpendicular to the recording surface of the record carrier 2. This deviation of the pivot angle Φ from the optical axis 36 of the objective lens 34 is referred to as tilt-offset.

The tilt-offset correction value is the amount of offset to be used in order to shift the pivot angle Φ with respect to the optical axis 36. Similar to focus-offset correction, the tilt-offset correction that gives good jitter performance in the read back signal is not necessarily the same as the correction that will result in good push-pull signal.

It is further noted that the radial actuator 41, the focus actuator 42, and the tilt actuator 43 may be implemented as one integrated 3D-actutator. The recording device 1 further comprises a control circuit 90 having a first output 92 connected to a control input of the motor 4, a second output 93 coupled to a control input of the radial actuator 41, a third output 94 coupled to a control input of the focus actuator 42, and a fourth output 95 coupled to a control input of the tilt actuator 43. The control circuit 90 is designed to generate:

1. at its first output 92, a control signal S_CM for controlling the motor 4,

2. at its second output 93, a control signal S_CR for controlling the radial actuator 41,

3. at its third output 94, a control signal S_CF for controlling the focus actuator 42, and

4. at its fourth output 95 a control signal S_CT for controlling the tilt actuator 43. The control circuit 90 further has a read signal input 91 for receiving a read signal S_R from the optical detector 35.

Fig. IB illustrates that the optical detector 35 comprises a plurality of detector segments, in this case four detector segments, 35a, 35b, 35c and 35d, capable of providing individual detector signals A, B, C, and D, indicating the amount of light incident on each of the four detector quadrants respectively. A centerline 37, separating the first and fourth segments 35a and 35d from the second and third segments 35ba and 35c, is oriented according to the Y-direction (track direction).

Fig. IB also illustrates that the read signal input 91 of the control circuit 90 comprises four inputs 91a, 91b, 91c, and 91d for receiving the individual detector signals A, B, C, and D respectively. The control circuit 90 is designed to process the individual detector signals A, B, C, and D, in order to derive data and control information, as will be clear to a person skilled in the art. For instance, a data signal S_D can be obtained by summation of all individual detector signals A, B, C, and D according to

Further, a one-spot push-pull tracking error signal S_TE is obtained by:

1. Summation of the signals A and D from all individual detector segments 35a and 35d on one side of the centerline 37,

2. Summation of the signals B and C from all individual detector segments 35b and 35c on the other side of the centerline 37, and 3. Taking the difference of the two summations as S_TE = (A+D) - (B+C).

Fig. IA shows a point P on the record carrier 2, having polar co-ordinates r and Φ . In an ideal case, a point P (r, Φ) on the record carrier surface is parallel to the Z- axis, but in the case where the record carrier 2 has a warped surface, as shown, the normal in point P(r, Φ) makes an angle θ (r, Φ) with the Z-axis. This angle θ (r, Φ) will be indicated as the tilt in point P(r, Φ). The tilt may vary over the surface of the record carrier, in other words the tilt (r, Φ) may be a function of radial co-ordinate r and angular co-ordinate Φ.

As a result of the record carrier tilt, the focus spot F is no longer circular, and this aberration may lead to cross talk, which may cause write errors. Further, servo and wobble signals are sensitive to tilt.

In order to avoid these problems, it is desirable that the optical beam 32b incident on the record carrier 2 is substantially perpendicular to the record carrier surface, which can be obtained by giving the objective lens 34 a pivot position such that the pivot angle Φ of the lens equals the tilt θ of the disc. Then, the net tilt of the record carrier 2 with respect to the optical beam 32b is zero. This offset value used to correct the tilt is referred to as tilt-offset correction value.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 2 shows an example flow chart 200 illustrating the steps of the method of recording. In step 202 a relationship between the focus offset and the radial error signal is found. In step 204 a write focus offset value is found at which the radial error signal amplitude is substantially maximum.

In the method disclosed, the system goes off-track and the radial error signal amplitude is measured for different focus-offset set values. The test writing operation that is carried out in Shirota's method (US 2006/0002250) is not necessary for finding the write focus-offset value. Dual layer discs require different servo-offset parameters for writing as compared to reading. Dual layer discs are much more sensitive to servo-offsets and they have poorer signal quality as compared to single layer discs. For such dual layer discs it is important to make sure that good signal quality is obtained resulting in different servo-offset parameters for reading and writing. Furthermore, using Shirota's method (US 2006 / 0002250) recording cannot be performed reliably for dual layer discs where the focus-offset value between reading and writing has large difference. The disclosed method overcomes this. In one possible embodiment, the relationship between a focus offset and a radial error signal is found as follows: obtaining an initial focus-offset value on the basis of a calibration procedure carried out during initial start-up of the recording device 1. This initial focus-offset value is obtained by reading the record carrier 2. The record carrier 2 can be one of the following types: 1. Blank recordable,

2. Blank rewritable,

3. Partly rewritable,

4. Partly recordable. Next, a step-size, a maximum allowable focus-offset value and a minimum allowable focus-offset value is obtained. The range of focus-offset for reliable calibration is determined based on optical pickup specification and experimental result. If focus-offset exceeds an allowable range, the signal quality from the optical pickup can deteriorate. The servo system can become unstable and the system can fail to follow the deviation of the disc. Normally, the allowable range is approximately in the range of about +/- 500 nm. The step- size is selected such that at least 8 data points can be found during calibration for a reliable curve fitting procedure. These can be determined empirically or calculated using the range of allowable focus-offset values. The initial focus-offset value used at the start of the calibration can be set to zero if electrical offset has been compensated. The other factor that is considered is the total time available for calibration. After having obtained the step-size, the minimum allowable focus-offset value and the maximum allowable focus-offset value, the push-pull signal amplitude values are determined. This involves the following steps, and a possible result of this procedure is schematically illustrated in Fig. 3: 1. Start with the initial focus-offset value. 2. Set focus-offset varying mode to ramping up mode.

3. Set focus-offset = focus-offset + step-size.

4. Read push-pull signal amplitude value.

5. Repeat steps 3 to 4 until maximum focus-offset value is reached.

6. Reset focus-offset value to initial focus-offset value. 7. Set focus-offset = focus-offset - step-size.

8. Read push-pull signal amplitude value.

9. Repeat steps 7 to 8 until minimum focus-offset value is reached.

10. Curve-fit the values of the focus-offset and the push-pull signal amplitude and find the write focus-offset value at which the push-pull signal amplitude is substantially higher. Fig. 3 is an example graph, schematically illustrating results obtained using the above procedure. The push-pull signal amplitude (vertical axis) is plotted against the corresponding focus-offset (horizontal axis). The boxes indicate the measured values. As should be clear to a person skilled in the art, any function Y (X) having a maximum for X = X_m can, within a small range around this maximum X_m, be reasonably approximated by a quadratic function according to

Y(X) = C₀ + C₁ (X - X_1n) + C₂ (X - X_m)² wherein c₀, C₁ and C₂ are constants. Finding an optimum fit for measurements Y₁ (X₁) is equivalent to finding optimum values for X_m, and C₀, C₁ and C₂. Usually, this is done by the well-known least squares method. In any case, it should be clear to a person skilled in the art that it is possible to calculate, on the basis of several measurements around the maximum of such a function, an optimum parabolic fit, and consequently it is possible to calculate X_m and Y_1n (X_1n).

The focus-offset curve shown in Fig. 3 illustrates such a parabolic fit. With the calibration data of X (focus-offset values) and Y (push-pull signal amplitude), the coefficients C₀, C₁ and C₂ are calculated. After calculating the coefficients C₀, C₁ and C₂ , the point X_m is calculated with Y_m, as X_m = (- C₁/ (2 x C₂) ).

Then the write focus-offset is set to a value corresponding to the maximum push-pull signal amplitude value.

In a further embodiment, a focus-offset correction value suitable for recording is found. This guarantees robust write performance for multi-layer discs. The focus-offset correction value is incorporated into the existing optimum power calibration procedure in order to achieve a reliable write operation. Hence, the disclosed method not only determines the optimum recording power but also a suitable focus-offset value for recording. This involves the following steps and a possible result of this is schematically illustrated in Fig. 3:

1. Set focus-offset value to the focus offset value at which the push-pull signal amplitude is substantially higher.

2. Perform a normal writing test to determine optimum recording power.

3. Record test data with the optimum power found in step 2.

4. Start with the focus-offset value set in step 1.

5. Set focus-offset varying mode to ramping up mode. 6. Set focus-offset = focus-offset + step-size;

7. Read jitter value of recorded test data.

8. Repeat steps 6 to 7 until maximum focus-offset value is reached.

9. Reset focus-offset value to initial focus-offset value.

10. Set focus-offset = focus-offset - step-size. 11. Read jitter value of recorded test data.

12. Repeat steps 10 to 11 until minimum focus-offset value is reached.

13. Curve-fit the values of the focus-offset and the jitter value of the recorded test data and find the read focus-offset value at which the jitter value is substantially lower. The computation of the read focus offset value by curve fitting is carried out in a similar manner as described with respect to finding the write focus offset value.

It is to be noted that the measuring time is substantially equal to the rotational speed of the record carrier 2 so that the measured result of push-pull signal amplitude value and the jitter signal amplitude value is an average value over one record carrier revolution. Fig. 3 shows a clear difference between best focus-offset for reading (that is based on jitter) and best focus-offset for writing. If the focus-offset used during writing is optimised towards best jitter instead of best push-pull signal, the recording device 1 can have servo -tracking problem during writing. If the system goes off-track during recording, valuable data can be lost and the disc can be damaged. The disclosed method overcomes this. Further, the focus-offset correction value for writing is calculated as the difference of the write focus-offset value and the read focus-offset value and is stored in memory, EEPROM and/or a database of the recording device 1. The stored focus-offset correction value is read and used when writing is again performed on the record carrier 2. Thus it is possible to realize proper focusing easily which can be advantageous. In a still further embodiment, a corrected write focus offset value is found as the difference of the write focus offset value and the focus-offset-correction value. A recording power is determined as a function of the corrected write focus offset value and is used to record data on the record carrier 2.

In operation, the control circuit 90 controls the focus actuator 42. Fig. 4 shows an example block diagram, schematically illustrating the control circuit for controlling the focus actuator. The control circuit comprises:

1. write focus-offset unit (450A) arranged to find a write focus-offset value for writing on the basis of a relationship between a focus-offset and a push-pull signal.

2. focus-offset correction unit (450B) arranged to find a focus-offset-correction value for writing.

The control circuit 90 sets the focus-offset value during writing. Further, the control circuit 90 controls the focus actuator 42 to shift the focus position of the optical beam from the recording reference plane of the record carrier 2 by the write focus-offset correction value. Furthermore, the control circuit 90 dynamically controls the focus actuator 42 to perform the writing on the basis of the corrected write focus-offset value.

In one embodiment, the tilt offset value suitable for recording is found that is different from the tilt offset value for reading. This guarantees robust write performance for multi-layer discs. A relationship between a tilt offset and a radial error signal is found. A write tilt offset value is found at which the radial error signal amplitude is substantially maximum.

In one possible embodiment, the relationship between the tilt offset and a radial error signal is found as follows: obtaining an initial tilt-offset value on the basis of a calibration procedure carried out during initial start-up of the recording device 1. This initial tilt-offset value is obtained by reading the record carrier 2. Next, a step-size, a maximum allowable tilt-offset value and a minimum allowable tilt-offset value is obtained. The initial tilt-offset value is obtained on the basis of a calibration procedure carried out during initial start-up of the recording device 1. The initial tilt-offset value is obtained by reading the record carrier 2. Next, a step-size, a maximum allowable tilt-offset value and a minimum allowable tilt-offset value is obtained. The range of tilt-offset calibration is determined based on optical pickup specification and experimental result. If tilt-offset exceeds an allowable range, the signal from the optical pickup can become so bad that the servo loops that keep the optical spot in focus can fail. Normally, the allowable range is approximately in the range of about +/- 20 milli-radians. The step-size is selected such that at least 8 data points can be found during calibration for a reliable curve fitting procedure. These can be determined empirically or calculated using the range of allowable tilt-offset values. The other factor that is considered is the total time available for calibration. After having obtained the step-size, the minimum allowable tilt-offset value and the maximum allowable tilt-offset value, the jitter signal amplitude values are determined. This involves the following steps, and a possible result of this procedure is schematically illustrated in Fig. 5:

1. Start with the initial tilt-offset value.

2. Set tilt-offset varying mode to ramping up mode.

3. Set tilt-offset = tilt-offset + step-size. 4. Read push-pull signal amplitude value.

5. Repeat steps 3 to 4 until maximum tilt-offset value is reached.

6. Reset tilt-offset value to initial tilt-offset value.

7. Set tilt-offset = tilt-offset - step-size.

8. Read push-pull signal amplitude value. 9. Repeat steps 7 to 8 until minimum tilt-offset value is reached.

10. Curve-fit the values of the tilt-offset and the push-pull signal amplitude and find the write tilt-offset value at which the push-pull signal amplitude is substantially higher.

The tilt-offset curve obtained by the above procedure is shown in Fig. 5. With the calibration data of X (tilt-offset values) and Y (push-pull signal amplitude), the coefficients C₀, C₁ and C₂ are calculated. After calculating the coefficients C₀, C₁ and C₂ , the point X_m is calculated with Y_m, as X_m = (- C₁/ (2 x C₂) ).

Then the write tilt-offset is set to a value corresponding to the maximum push-pull signal amp litude value .

In a further embodiment, a tilt-offset correction value suitable for recording is found. This guarantees robust write performance for multi-layer discs. The tilt-offset correction value is incorporated into the existing optimum power calibration procedure in order to achieve a reliable write operation. Hence, the disclosed method not only determines the optimum recording power but also a suitable tilt-offset value for recording. This involves the following steps and a possible result of this is schematically illustrated in Fig. 5:

1. Set tilt-offset value to the tilt offset value at which the push-pull signal amplitude is substantially higher.

2. Perform a normal writing test to determine optimum recording power. 3. Record test data with the optimum power found in step 2.

4. Start with the tilt-offset value set in step 1.

5. Set tilt-offset varying mode to ramping up mode.

6. Set tilt-offset = tilt-offset + step-size;

7. Read jitter value of recorded test data. 8. Repeat steps 6 to 7 until maximum tilt-offset value is reached.

9. Reset tilt-offset value to initial tilt-offset value.

10. Set tilt-offset = tilt-offset - step-size.

11. Read jitter value of recorded test data.

12. Repeat steps 10 to 11 until minimum tilt-offset value is reached. 13. Curve-fit the values of the tilt-offset and the jitter value of the recorded test data and find the read tilt-offset value at which the jitter value is substantially lower.

Fig. 5 shows a clear difference between best tilt-offset for reading (that is based on jitter) and best tilt-offset for writing. If the tilt-offset used during writing is optimised towards best jitter instead of best push-pull signal, the recording device 1 can have servo -tracking problem during writing. If the system goes off-track during recording, valuable data can be lost and the disc can be damaged. The disclosed method overcomes this. Further, the tilt-offset correction value for writing is calculated as the difference of the write tilt-offset value and the read tilt-offset value and is stored in memory, EEPROM and / or a database of the recording device 1. The stored tilt-offset correction value is read and used when writing is again performed on the record carrier 2. Thus it is possible to realize proper focusing easily which can be advantageous.

In a still further embodiment, a corrected write tilt offset vlaue is found as the difference of the write tilt offset value and the tilt-offset-correction value. A recording power is determined as a function of the corrected write tilt offset value and is used to record data on the record carrier 2.

In operation, the control circuit 90 controls the tilt actuator 43. Fig. 6 shows an example block diagram, schematically illustrating the control circuit for controlling the tilt actuator. The control circuit comprises:

1. write tilt-offset unit (660A) arranged to find a write tilt-offset value for writing on the basis of a relationship between a tilt-offset and a push-pull signal.

2. tilt-offset correction unit (660B) arranged to find a tilt-offset-correction value for writing.

The control circuit 90 sets the tilt-offset value during writing. Further, the control circuit 90 controls the tilt actuator 43 to shift the tilt position of the objective lens by the tilt-offset correction value. Furthermore, the control circuit 90 dynamically controls the tilt actuator 43 to perform the writing on the basis of the corrected write tilt-offset value.

Push-pull signal is rescaled after write focus-offset correction and write tilt- offset correction due to changes in the signal level from the optical head after completing the tilt and focus-offset calibration. The scaling of the push-pull signal (or radial error signal) is needed to ensure non-clipping of signal at the input of the servo radial proportional integral derivate (PID) control loop. The servo radial PID control loop is the mechanism that tracks the optical spot on the disc. The control loops get feedback from the radial error signal that is derived or calculated from signals coming from the optical pickup. The radial-offset can also affect the push-pull signal and hence a suitable radial-offset should be used during recording. Before recording can start on the record carrier 2, good push-pull signal amplitude is obtained by using the tilt-offset correction value and the focus-offset correction value to adjust the radial-offset to the best position for writing. In an embodiment, a radial offset value suitable for recording is found that is different from the radial offset value for reading. Different radial offset values are gradually set. For each radial-offset, the recording device 1 monitors if the light / laser spot still stays on track and measures the quality of the wobble signal. The quality of the wobble signal is measured from the amount of missing synchronization symbols. The missing synchronization symbols are obtained from the high frequency component of the push-pull signal. The high frequency component of the push-pull signal contains address information. Besides address information, this signal also contains synchronization symbols that are used in the phase locked loop and the decoder. The recording device 1 checks the frequency of missing synchronization symbols in the push-pull signal. The symbol is considered missing if the symbol is not present at the timing location where it is expected by the interpolator or flywheel. If the radial-offset is not correct, many missing synchronization symbols will be present. The missing synchronization rate = % of missing synchronization symbols. From the missing synchronization symbols, the recording device 1 determines the range of the radial-offset values where the laser/light spot can stay on track and the wobble quality is good. The write radial-offset for writing is selected as the value in the middle of the calibrated range of the radial-offset values. As can be seen from Fig. 7, the missing synchronization symbol is almost zero for large range of the radial-offset values. In order to exclude localized defects on the record carriers, the wobble quality is considered good if less than 5% of the synchronization symbols are missing.

The wobble signal is modulated with information that can be used by the system for reliable write operation. Within the data structure of the wobble signal, synchronization symbols are present to aid the data retrieval process. For DVD+R DL format the information in the wobble signal is called Address in Pre-groove (ADIP). Each ADIP word consists of 1 synchronization symbol and 51 data symbols. The synchronization symbols occur periodically in the wobble signal. Once the phase locked loop in the system tracks the incoming wobble signal, the occurrence of the synchronization pattern in the wobble signal can be predicted. The mechanism that interpolates or predicts the occurrence of the synchronization pattern is called a flywheel or interpolator. If the system does not detect a synchronization symbol at the time when the flywheel predicts one, it is assumed that the system detects one missing synchronization symbol occurrence.

The procedure for finding the radial-offset value for writing comprises the following: obtaining an initial radial-offset value on the basis of a calibration procedure carried out during initial start-up of the recording device 1. This initial radial-offset value is obtained by reading the record carrier 2. Next, a step-size, a maximum allowable radial- offset value and a minimum allowable radial-offset value is obtained. The range of radial- offset for reliable calibration must be determined based on optical pickup specification and experimental result. If radial-offset applied is too large, the servo system may not be able to follow the deviation of the disc and can lose the track. Normally, the allowable range is approximately in the range of about +/- 50 nm. The step-size is selected such that at least 8 data points can be found during calibration for a reliable curve fitting procedure. These can be determined empirically or calculated using the range of allowable radial-offset values. The other factor that is considered is the total time available for calibration. After having obtained the step-size, the minimum allowable radial-offset value and the maximum allowable radial-offset value, the push-pull signal amplitude and the wobble signal are determined. This involves the following steps, and a possible result of this procedure is schematically illustrated in Fig. 7:

I . Enable the radial tracking loop. 2. Start with an initial radial-offset value.

3. Set radial-offset varying mode to ramping up mode.

4. Set radial-offset = radial-offset + step-size.

5. Read push-pull signal amplitude and wobble signal.

6. Calculate the amount of missing synchronization symbols from the wobble signal. 7. Repeat steps 4 to 6 until maximum radial-offset value is reached.

8. Reset radial-offset value to initial radial-offset value.

9. Set radial-offset = radial-offset - step-size.

10. Read push-pull signal amplitude and wobble signal.

I 1. Calculate the amount of missing synchronization symbols from the wobble signal. 12. Repeat steps 9 to 11 until minimum radial-offset value is reached.

13. Find a range of radial-offset values on the basis of the amount of missing synchronization symbols.

14. Select a write radial-offset value as the mean of the range of the radial-offset values. This is the radial-offset that is suitable for write operation. In a further embodiment, a radial-offset correction value for writing is found.

This involves obtaining a read radial offset value at which middle of peak-to-peak radial tracking error signal is substantially at zero level in off-track mode. A radial-offset correction value is found as the difference of the write radial offset value and the read radial offset value. Finding the read radial offset value involves the following steps: 1. Disabling the radial tracking loop for measuring the radial tracking error signal.

2. Determining the radial track error signal offset, Vref- (RτE_Peak₊ + RτE_Peak)/2.

3. Setting read radial-offset value to the value obtained in Step 2 that is suitable for read operation. Further, the radial-offset correction value for writing is calculated as the difference of the write radial-offset value and the read radial-offset value and is stored in memory, EEPROM and / or a database of the recording device 1. The stored radial-offset correction value is read and used when writing is again performed on the record carrier 2. Thus it is possible to realize proper focusing easily which can be advantageous. In a still further embodiment, a corrected write radial offset value is found as the difference of the write radial offset value and the radial-offset-correction value. A recording power is determined as a function of the corrected write radial offset value and is used to record data on the record carrier 2.

In operation, the control circuit 90 controls the radial actuator 41. Fig. 8 shows an example block diagram, schematically illustrating the control circuit for controlling the radial actuator, The control circuit comprises:

1. write radial-offset unit (870A) arranged to find a write radial-offset value for writing.

2. radial-offset correction unit (870B) arranged to find a radial-offset-correction value for writing. The control circuit 90 sets the radial-offset value during writing. Further, the control circuit 90 controls the radial actuator 41 to displace the objective lens with respect to the recording tracks of the record carrier by the write radial-offset correction value. Furthermore, the control circuit 90 dynamically controls the radial actuator 41 to perform the writing on the basis of the corrected write radial-offset value. Once the corrected write focus-offset value, the corrected write tilt-offset value and the corrected radial-offset value have been determined, the recording device 1 conducts the write test to determine the optimum recording power which involves writing test data on the record carrier (e.g. on power calibration area of the record carrier 2). Determining optimum recording power using the corrected focus-offset value, the corrected tilt-offset value, and the corrected radial-offset value ensures that the recording power obtained is generally accurate. Using the determined optical recording power, data is recorded on the record carrier 2. Unreliable recording on multi-layer discs that are generally caused by servo off-track and wrong address readout can be overcome by the disclosed method. During the process of obtaining the servo-offset correction values, a read focus-offset value is obtained based on jitter. This read focus-offset value is suitable for reading data from the record carrier 2. Further, a read tilt-offset value is obtained based on jitter. This read tilt-offset value is suitable for reading data from the record carrier 2. Furthermore, a read radial- offset is calibrated at read power and in off-track condition such that the middle of the peak-to-peak error signal is at zero level. This read radial-offset value is suitable for reading data from the record carrier 2. In case of an optical recorder having a playing (reading) functionality, it is possible to switch the focus-offset value, the tilt-offset value and the radial-offset value to the value suitable for reading the record carrier 2, i.e. to the read focus-offset value, the read tilt-offset value and to the read radial-offset value. Before recording starts, the optical recording device 1 can switch these values to the level suitable for recording data, i.e. the corrected write focus-offset value, the corrected write tilt-offset value and the corrected write radial-offset value.

Although the invention has been explained by embodiments using DVD dual layer discs, the invention is applicable to all types of optical disc media e.g. write-once media and write-many recordable types (CD-RW, DVD-RW, DVD+RW, Blu-ray discs). It is not limited to a two-layer one side, i.e. a dual layer, and to a two-layer double side, i.e. a dual layer double side. Moreover, the recording layer is not limited to the two layers, as described above. In the embodiments, the recording device is capable of recording data. However, the recording device may perform at least a recording operation among recording, reproducing and erasing operations. In the device claims enumerating several units, several of these units can be embodied by one and the same item of computer readable software or hardware. A person skilled in the art may change the units to get the same effects as intended by the present invention. A person skilled in the art can implement the described embodiments of the method in software or in both hardware and software. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. Use of the verb "comprise" does not exclude the presence of elements other than those stated in a claim or in the description. Use of the indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps. The Figures and description are to be regarded as illustrative only and do not limit the invention.

In summary, a method of recording is described. A relationship between a focus offset and a radial error signal is found. A write focus offset value is found at which the radial error signal amplitude is substantially maximum. This is useful for all optical recording devices.

Claims

CLAIMS:

1. A recording method (200) comprising: determining a relationship between a focus offset and a radial error signal; and finding a write focus offset value at which the radial error signal amplitude is substantially maximum.

2. The method of claim 1 further comprising: determining a focus-offset-correction value for writing.

3. The method of claim 2 wherein determining the focus-offset-correction value for writing further comprises: recording a test data on a record carrier using the write focus offset value; determining a relationship between the focus offset and a jitter signal measured from the recorded test data; finding a read focus offset value at which the jitter signal is substantially minimum; and finding the focus-offset-correction value as the difference of the write focus offset value and the read focus offset value.

4. The method of claim 3 further comprising: finding a corrected write focus offset value as the difference of the write focus offset value and the focus-offset-correction value; determining a recording power as a function of the corrected write focus offset value; and recording data on the record carrier using the determined recording power.

5. The method of claim 4 further comprising: determining a relationship between a tilt offset and the radial error signal; and finding a write tilt offset value at which the radial error signal amplitude is substantially maximum.

6. The method of claim 5 further comprising: determining a tilt-offset-correction value for writing.

7. The method of claim 6 wherein determining the tilt-offset-correction value for writing further comprises: recording the test data on the record carrier using the write tilt offset value; determining a relationship between the tilt offset and the jitter signal measured from the recorded test data; finding a read tilt offset value at which the jitter signal is substantially minimum; and finding the tilt-offset-correction value as the difference of the write tilt offset value and the read tilt offset value.

8. The method of claim 7 further comprising: finding a corrected write tilt offset value as the difference of the write tilt offset value and the tilt-offset-correction value; determining the recording power as a function of the corrected write focus offset value and the corrected write tilt offset value; and recording data on the record carrier using the determined recording power.

9. The method of claim 8 further comprising: varying a radial offset over a range of allowable radial offset values and recording the test data on the record carrier; determining a relationship between the radial offset and quality of wobble signal measured in the form of missing synchronization pattern; finding a range of radial offset values on the basis of quality of wobble signal; and finding a write radial offset value for writing as the mean of the range of radial offset values.

10. The method of claim 9 further comprising: determining a radial-offset-correction value for writing.

11. The method of claim 10 wherein determining the radial-offset-correction value for writing further comprises: obtaining a read radial offset value at which middle of peak-to-peak radial tracking error signal is substantially at zero level in off-track mode; and finding the radial-offset-correction value as the difference of the write radial offset value and the read radial offset value.

12. The method of claim 11 further comprising: finding a corrected write radial offset value as the difference of the write radial offset value and the radial-offset-correction value; determining the recording power as a function of the corrected write focus offset value, the corrected write tilt offset value and the corrected write radial offset value; and recording data on the record carrier using the determined recording power.

13. The method of claim 3 or claim 7 or claim 9, wherein recording the test data comprises: recording the test data on a DVD.

14. The method of claim 3 or claim 7 or claim 11 further comprising: storing the focus-offset-correction value, the tilt-offset-correction value and the radial-offset-correction value in a memory.

15. A recording device ( 1 ) comprising : an optical system (30) for scanning recording tracks (T₁, T₂, ..T_n) of a record carrier (2), which optical system comprises light beam generating means (31), an objective lens (34) for focusing a light beam on the record carrier (2), an optical detector (35) for detecting a reflected light beam, controllable focus actuator (42) for axially displacing the objective lens with respect to a recording reference plane of the record carrier; and a control circuit having an input for receiving an output signal from the optical detector and having an output coupled to a control input of the focus actuator, wherein the control circuit further comprises: write focus offset unit (450A) arranged to find a write focus offset value at which the radial error signal amplitude is substantially maximum; and focus offset correction unit (450B) arranged to find a focus-offset-correction value for writing.

16. The recording device of claim 15 further comprising: a controllable tilt actuator (43) for pivoting the objective lens with respect to the record carrier; and a control circuit having an input for receiving an output signal from the optical detector and having an output coupled to a control input of the tilt actuator, wherein the control circuit further comprises: write tilt offset unit (660A) arranged to find a write tilt offset value at which the radial error signal amplitude is substantially maximum; and tilt offset correction unit (660B) arranged to find a tilt-offset-correction value for writing.

17. The recording device of claim 16 further comprising: a controllable radial actuator (41) for radially displacing the objective lens with respect to the recording tracks (T₁, T₂, ...T_n) of the record carrier; and a control circuit having an input for receiving an output signal from the optical detector and having an output coupled to a control input of the radial actuator, wherein the control circuit further comprises: write radial offset unit (870A) arranged to find a write radial offset value; and radial offset correction unit (870B) arranged to find a radial-offset-correction value for writing.

18. The recording device of any one of the claims 15 - 17 further comprising: a memory unit arranged to store the focus-offset-correction value, the tilt- offset-correction value and the radial-offset-correction value.

19. The recording device of any one of the claims 15 - 18, wherein the recording device is a DVD recorder.