WO2008001296A2

WO2008001296A2 - A device for retrieving information and a method of compensating a tilt during said retrieving

Info

Publication number: WO2008001296A2
Application number: PCT/IB2007/052444
Authority: WO
Inventors: Yu Zhou
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-30
Filing date: 2007-06-25
Publication date: 2008-01-03
Also published as: WO2008001296A3; TW200818166A

Abstract

A device for retrieving information from a disc-like optical record carrier (11) using a radiation beam. The device comprises reading means (21, 22, 25, 30) for reading the information recorded on the record carrier and control means (20) for controlling the reading. Further, the device comprises a tilt compensation unit (32) for compensating a relative tilt of the record carrier to the radiation beam. This is done with use of at least one tilt compensation function predicted during initial calibration. During scanning of the record carrier, for reading or recording, the tilt compensation function is on the fly fine-tuned upon error conditions. Different algorithms are proposed for fine-tuning and selecting the compensation function.

Description

A device for retrieving information and a method of compensating a tilt during said retrieving

FIELD OF THE INVENTION:

The invention relates to a device for retrieving information from a disc-like optical record carrier, the record carrier comprising substantially spiral track with addressable locations, the information organized into information units comprising main data and error correction data, the device comprising: a rotation unit for rotating the record carrier; a reading head for scanning the track by a radiation beam and generating signals on the basis of optically readable effects along the track; a reading head control unit for controlling generation and positioning of the radiation beam using signals representing control parameters; a decoding unit for processing a high-frequency signal into the main data and the error correction data, the high-frequency signal being generated on the basis of effects as marks representing the information; and an error correction unit for finding and correcting data errors using the error correction data.

The invention further relates to a method of compensating a tilt in a disc-like optical record carrier system, the record carrier comprising substantially spiral track with addressable locations.

The invention also relates to a computer program product for use in compensating a tilt in a disc-like optical record carrier system.

BACKGROUND OF THE INVENTION:

On the market, there are large quantities of optical media that exhibit either tilt or skew. This type of defect not only affects disc playback performance but also greatly affects disc recording quality.

The United States Patent Application Publication US 2003/0198160 discloses a tilt servo control method based on a jitter of the high-frequency signal generated by a reading head of a device for reproducing DVD discs. Upon data read error, a procedure is started to find an angle of the objective lens leading to a minimum value of the jitter. This method can be used only during data reproduction.

Other methods include periodical or by disc regions calibrations in order to get minimal jitter or a rate of data errors (in case of playback), or maximal radial error push-pull signal (in case of recording). These methods are not so effective especially for recording, where the tracking performance is very sensitive to the disc tilt and skew. This may affect disc power calibration results for recording and performance also during recording.

SUMMARY OF THE INVENTION: Therefore, it is an object of the present invention to provide more flexible and efficient way of compensating a tilt.

For this purpose, according to a first aspect of the invention, the device for retrieving information, as described in the opening paragraph, further comprises a tilt compensation unit for compensating a relative tilt of the record carrier to the radiation beam by: selecting calibration locations at different radial positions; for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; - calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; storing the tilt compensation function in a memory; monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and in case of an error condition at an additional location during scanning, performing an adjustment procedure comprising: calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter; including the additional location optimal value of the tilt control parameter in the set of values and re-calculating the tilt compensation function; storing the tilt compensation function in the memory; and setting the tilt control parameter to a value of the tilt compensation function at the radial position of the additional location.

For this purpose, according to a second aspect of the invention, the method of compensating a tilt, as described in the opening paragraph, comprises steps of: a) rotating the record carrier and scanning the track by a radiation beam generated by a reading head; b) controlling positioning of the radiation beam using signals representing control parameters; c) generating a signal by the reading head on the basis of optically readable effects along the track; d) selecting calibration locations at different radial positions; e) for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; f) calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; g) storing the tilt compensation function in a memory; h) monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and i) in case of an error condition at an additional location during scanning, performing steps of: k) calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter;

1) including the additional location optimal value of the tilt control parameter in the set of values and re-calculating the tilt compensation function; m) storing the tilt compensation function in the memory; and n) setting the tilt control parameter to a value of the tilt compensation function at the radial position of the additional location.

For this purpose, according to a third aspect of the invention, a computer program product for use in compensating a tilt, as described in the opening paragraph, is provided, the computer program comprising program code means for causing a processor of the system, to perform the method as described in relation to the second aspect of the invention, when the computer program is run on the processor.

The measures according to the invention have the effect that the tilt compensation function, curve, predicted during initial calibration before the actual reproduction or recording is fine-tuned on the fly during playback or recording operations. This allows for more precise determination of the tilt compensation, and thus improves performance of the system by reducing probability of errors during said operations.

In an embodiment of the device, the tilt compensation unit is adapted to calculate the tilt compensation function as the second order polynomial fitted to the set of values. This reflects the fact that the disc tilt usually has parabolic shape and allows for efficient calculations.

In another embodiment of the device, the effects being comprised in a pre- embossed track structure indicating the track, the tilt compensation unit is adapted to use a radial error signal as the quality parameter, a radial error signal based tilt compensation function as the tilt compensation function and the error condition requiring that the radial error signal is greater than a predefined radial error signal value. This allows for the tilt compensation at unrecorded areas of recordable discs.

Advantageously, the tilt compensation unit is adapted so that the error condition requires occurrence of an uncorrected data error in the main data, the quality parameter is a jitter or a rate of data errors and each location optimal value of the tilt control parameter corresponds to a minimal value of the quality parameter at said each location, the jitter representing time variations of the high-frequency signal. This allows for the tilt compensation for read-only discs or at recorded areas of recordable discs.

In a further embodiment of the device, the tilt compensation unit is adapted so that for said each calibration location, calibrating the tilt control parameter is performed separately for the jitter and for the rate of data errors, and two tilt compensation functions are calculated and stored as a jitter based tilt compensation function and as a rate of data errors based tilt compensation function, respectively, the tilt compensation unit is further adapted for, in case of the error condition at the additional location, performing before the adjustment procedure: measuring the jitter and the rate of data errors at the additional location; checking a first predefined condition requiring that the jitter is greater than a predefined jitter value; in case the first predefined condition is fulfilled, setting the jitter as the quality parameter and the jitter based tilt compensation function as the tilt compensation function, otherwise checking a second predefined condition requiring that the rate of data errors is greater than a predefined rate of data errors value; - in case the second predefined condition is fulfilled, setting the rate of data errors as the quality parameter and the rate of data errors based tilt compensation function as the tilt compensation function, otherwise not performing the adjustment procedure.

This provides selective way of compensating the tilt, improving its efficiency.

It is advantageous, if the tilt compensation unit is adapted so that at locations where the high-frequency signal is not present, the radial error signal based tilt compensation function is calculated or re-calculated, stored and used as the tilt compensation function for setting the tilt control parameter. This makes possible to apply the tilt compensation in case of discs having both, recorded and unrecorded areas, for example in case of so-called not finalized discs. In another embodiment of the device, the tilt compensation unit is adapted so that in case the second predefined condition is fulfilled and the tilt compensation function for setting the tilt control parameter is the jitter based tilt compensation function, the adjustment procedure is not performed and the tilt control parameter is set to the value of the rate of data errors based tilt compensation function at the radial position of the additional location. This facilitates switching from the compensation based on the jitter to the compensation based on the rate of data errors, without re-calculating the tilt compensation curve.

Further preferred embodiments of the device and the method according to the invention are given in the appended claims, disclosure of which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS:

These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which: Figure Ia shows an example of a disc-shaped optical record carrier,

Figure Ib shows a cross-section taken of the record carrier,

Figure Ic shows an example of a wobble of the track,

Figure 2 shows a reading device, in accordance with the invention, Figure 3 shows an example of initial calibrations and calculations of the tilt compensation functions based on selected calibration locations, in accordance with the invention.

Figure 4 shows an example of a procedure of fine-tuning the tilt compensation functions during playback, in accordance with the invention.

Figure 5 shows an example of dependence of the tilt angle versus radial position along a radius of a disc.

Figure 6 shows an example of a tilt compensation curve before and after on the fly fine-tuning, in accordance with the invention. Figure 7 shows dependence of the jitter and PI BLER on the tilt.

Figure 8 shows dependence of the radial error push-pull signal on the tilt. Corresponding elements in different Figures have identical reference numerals/symbo Is .

DETAILED DESCRIPTION OF EMBODIMENTS :

Figure Ia shows a disc-shaped record carrier 11 having a track 9 and a central hole 10. The track 9 is arranged in accordance with a spiral pattern of turns constituting substantially parallel tracks on an information layer. The record carrier may be optically readable, called an optical disc. Recorded information is represented on the information layer by optically detectable marks recorded along the track. The marks are constituted by variations of a physical parameter and thereby have different optical properties than their surroundings, e.g. variations in reflection. The marks on the information layer can be pre- embossed, like in read-only discs, such as CD-ROM or DVD. Alternatively, the information layer, or at least a part of it, can be of a recordable type, on which marks can be recorded. Examples of a recordable disc are the CD-R, CD-RW, and writable versions of DVD, such as DVD+RW, and the high-density writable optical disc called Blu-ray Disc, BD. The track 9 on the recordable type of record carrier is indicated by a pre-embossed track structure provided during manufacture of the blank record carrier, for example a pregroove.

Figure Ib is a cross-section taken along the line b-b of the record carrier 11 of the recordable type, in which a transparent substrate 15 is provided with a recording layer 16 and a protective layer 17. The track structure is constituted, for example, by a pregroove 14, which enables a read/write head to follow the track 9 during scanning. The pregroove 14 may be implemented as an indentation or an elevation, or may consist of a material having a different optical property than the material surrounding it. A track structure may also be formed by regularly spread sub-tracks, which periodically cause servo signals to occur. The record carrier may be intended to carry real-time information, for example video or audio information, or other information, such as computer data.

Figure Ic shows an example of a periodic variation of the transversal position of the track of the (recordable) disc, also called wobble. The variations cause an additional signal to arise in auxiliary detectors, for example in a push-pull channel generated by sub detectors or partial detectors in the central spot in a head of a scanning device. The wobble is, for example, frequency modulated and position information is encoded in the modulation. A comprehensive description of the prior art wobble as shown in Figure Ic in a writable CD system comprising disc control information encoded in such a manner can be found in US

4,901,300 and US 5,187,699. It is noted that other transversal variations are known which are intended to be detected by variations of reflected radiation by (sub) detectors in a scanning head, such as variations in the width of the track, prepits adjacent to the track, and other.

The marks and the transversal variations are examples of optically detectable effects on a record carrier.

Figure 2 shows an example of a device for retrieving information from a record carrier 11 such as CD-ROM, DVD, CD-R, CD-RW, DVD+RW or BD, according to the invention. The device is provided with scanning means for scanning the track of the record carrier 11, which means include a rotation unit 21 for rotating the record carrier 11, a reading head 22 for scanning the track by a radiation beam 24 and a reading head control unit 25 for controlling generation and positioning of the radiation beam. The head 22 comprises an optical system of a known type for generating the radiation beam 24 guided through optical elements to generate the radiation spot 23 on a track of the information layer of the record carrier. The radiation beam 24 is generated by a radiation source, e.g. a laser diode. The head further comprises (not shown) a focusing actuator for focusing the beam to the radiation spot 23 on the track by moving the focus of the radiation beam 24 along the optical axis of said beam, and a sledge and a tracking actuator for positioning the radiation spot 23 in a direction transverse to the scanning direction of the track on the center of the track. For a disc shaped medium the transverse direction is called radial direction and the tracking actuator is called radial actuator. The tracking actuator may comprise coils for radially moving an optical element; it may be arranged for changing the angle of a reflecting element. Analogously, the focusing actuator may comprise coils for moving the focus of the radiation beam 24. It should be noted that the focusing and radial actuators may be constructed in the form of one actuator for positioning an optical element such as a lens and performing functions of said actuators. The tracking and focusing actuators are driven by actuator signals from the reading head control unit 25. For reading, the radiation reflected by the information layer is detected by a detector of a usual type, e.g. a four-quadrant diode, in the reading head 22 for generating detector signals coupled to a front-end unit 31 for generating various scanning signals, including a main detector signal 33 and sub detector signals 35 for tracking and focusing. The main detector signal 33 is also called a high-frequency, HF, signal. The sub detector signals 35 are coupled to the reading head control unit 25 for controlling said focusing actuators. Examples of the sub detector signals 35 are a focusing error signal and a radial error signal. The main detector signal 33 is processed by a read processing unit 30 of a usual type including a decoding unit and an error correction unit and output unit to retrieve the information. The decoding unit is for processing the high-frequency signal into the main data and the error correction data comprised in the information. The error correction unit is for finding and correcting data errors using the error correction data. The read processing unit may comprise a memory buffer for transitionally storing the main data. Alternatively, the memory buffer can be a separate unit. Hence reading means for reading information include the rotation unit 21, the reading head 22, the reading head control unit 25 and the read processing unit 30.

The reading head 22 is also referred to as the Optical Pickup Unit, OPU. The device comprises a control unit 20, which is connected via control lines

26, e.g. a system bus, to the other units in the device, for controlling these units. The control unit 20 comprises control circuitry, for example a microprocessor, a program memory and interfaces for performing different control procedures and functions. The control unit 20 may also be implemented as a state machine in logic circuits. The control unit 20 controls the scanning, for example for recording or reading of information, and may be arranged for receiving commands from a user or from a host computer.

The device may be provided with recording means for recording information on a record carrier of a writable or re-writable type, for example CD-R or CD-RW, or DVD+RW or BD. In this case, the reading head 22 is adapted for recording marks on a record carrier. The recording means cooperate with the head 22 and the front-end unit 31 for generating a write beam of radiation, and comprise write processing means for processing the input information to generate a write signal to drive the head 22, which write processing means comprise an input unit 27, a formatter 28 and a modulator 29. For writing information the power of the beam of radiation is controlled by the modulator 29 to create optically detectable marks in the recording layer. The marks may be in any optically readable form, e.g. in the form of areas with a reflection coefficient different from their surroundings, obtained when recording in materials such as dye, alloy or phase change material, or in the form of areas with a direction of polarization different from their surroundings, obtained when recording in magneto -optical material. Writing and reading of information on/from optical discs and formatting, error correcting and channel coding rules are well-known in the art, for example from the CD and DVD systems.

In an embodiment the device is a storage system only, for example an optical disc drive for use in a computer. The control unit 20 is arranged to communicate with a processing unit in the host computer via a standardized interface (not shown). Digital data is interfaced to the formatter unit 28 and from the read processing unit 30 directly. In this case, the interface acts as an input unit and an output unit.

In an embodiment the device is arranged as a stand alone unit, for example a video playback/recording device for consumer use. The control unit 20, or an additional host control unit included in the device, is arranged to be controlled directly by the user. The device includes application data processing, for example audio and/or video processing circuits. The information presented to the input unit 27 may comprise analog audio and/or video, or digital uncompressed audio/video signals; in this case the input unit 27 may comprise compression means for these signals. The read processing unit 30 may comprise suitable audio and/or video decompression units.

In operation the reading head control unit 25 applies a set of beam control parameters for controlling various aspects of the radiation beam. A first example of a beam control parameter is related to achieving a correct focus, and is called a focus offset. The focus offset is used to provide an adjusted set point for the sub detector signals and/or actuator signals that control the focus. The focus offset may for example compensate deviations of the optical system or detector location in the head. A similar example of a beam control parameter is called a radial offset, and compensates the transverse position of the scanning spot. Another example of a beam control parameter is an actuator tilt, hereinafter also called a tilt control parameter, which is used to actively compensate for tilt in drive, for example in the reading head or the rotation unit, or disc tilt. Further beam control parameters may be related to the power of the beam, timing of certain signal elements in the beam, etcetera. In practice beam control may be (partly) implemented in a software or in other units, such as a laser power control unit or a signal pattern from a recording unit. It is noted that the beam control parameters may require calibration or measurements during manufacture of the device, or may be affected by ageing, temperature, or other actual operational conditions during use of the device.

The device comprises a tilt compensation unit 32 for compensating a relative tilt of the record carrier to the radiation beam. This relative tilt may be a result of a tilt in the rotation unit and/or the reading head, or a disc tilt.

The tilt compensation unit 32 controls the reading head control unit 25 by adjusting a tilt control parameter of the reading head control unit 25 to a value obtained from a tilt compensation function in dependence on a radial position of the radiation beam along a radius of the disc. A tilt/skew is first predicted based on initial passive calibration during start-up and subsequent active tilt compensation is done on the fly while the actuator is tracking across the disc.

Initially, for example during start-up of a disc, the tilt compensation unit 32 performs an initial calibration procedure in order to calculate the tilt compensation function. This procedure can be based for example on a jitter of the High-Frequency signal, HF jitter, which jitter represents time variations of this signal. For example, the jitter can be presented as a standard deviation of the time variation of the digitized data passed through the equalizer of the decoding unit; the jitter of the leading and trailing edges of the signal is measured relative to the Phase-Locked Loop (PLL) clock and normalized by the channel bit clock, as known in the art. A calibration procedure can be based also on a parameter measuring quality of data retrieval by means of rate of data errors in the information processed by the read processing unit 30. For example, so-called PI BLER, Parity of the Inner code BLock Error Rate, can be used as known from the DVD system. Here, a unit of information can be a single block or a group of blocks. PI BLER is also known as "PI Sum 8", which is a moving average sum of the Parity Inner errors over 8 Error Correction Code blocks. In case of CD discs, a corresponding parameter is so-called Cl BLER. It should be noted that hereinafter PI BLER can also denote Cl BLER, PI Sum 8 or a more general term - the rate of data errors.

The above two quality parameters, the jitter and the rate of data errors, can be measured only in presence of the high-frequency signal, which is generated on the basis of the marks in the track.

However, dependence of the radial error signal on the wobble of the track can be used at locations with no marks. Hence, calibrations based on the radial error signal can be used during Optimal Power Control procedures for recordable discs and during recordings on such discs. The initial calibration is done at selected locations at different radial positions on a disc. For example, three calibration locations can be chosen at the inner, middle and outer position along the disc radius from the center. At each selected location, an optimal value of the tilt control parameter is obtained by means of finding a minimum value of the quality parameter (for example, for the jitter and PI BLER) or a maximum value of the quality parameter (for example, for the radial error). This can been seen in Figure 7 and Figure 8.

In an embodiment of the device, the optimal value is determined simply by taking this value of the tilt control parameter applied during measurements, for which a minimal value of HF jitter or PI BLER was measured; in case of the radial error, a maximal value.

However, as the measured values are only discrete points, taking the point with the minimal value might not always render the optimal value of the parameter. Therefore, in a further embodiment of the device, the tilt compensation unit 32 is adapted to determine the optimal value of the control parameter by calculating a minimum or a maximum, respectively, of a function fitted to the measured values. The function may be the second or higher order polynomial; the second order one requires simpler computations.

A set of these optimal values is used to calculate a required tilt compensation function. An example of measured optimal values of the tilt control parameter and a tilt compensation function fitted to those values is shown in Figure 5.

The simplest function that rather accurately represents experimental values is the second order polynomial. Therefore, the tilt compensation function is of the form: Tilt angle (tilt control parameter) = a*x² + b*x + c, where "x" represents the dimension in radial direction from the sledge home position and parameters "a", "b", and "c" are constants obtained from fit to the optimal values.

Figure 3 shows an example of an initial procedure of calculating tilt compensation functions based on different quality parameters.

A predefined error condition during scanning triggers the tilt compensation unit 32 to execute on the fly adjustment procedure to fine-tune or change a tilt compensation function.

For fine-tuning, additional calibration of the tilt control parameter is performed at this additional location when the error condition occurred. The resulting additional optimal value is included in the original set of optimal values found during the initial calibration and the compensation function is re-calculated. This improves quality of the compensation function so it describes in more reliable way the tilt control parameter. This improvement can be seen from an example in Figure 6, which shows the original, predicted, function fitted to three points, and the modified function resulting from inclusion of an additional point at the radial position of about 50 mm. The tilt compensation unit 32 performs the initial determination of the tilt function and its adjustment by: selecting calibration locations at different radial positions; for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; storing the tilt compensation function in a memory; - monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and in case of an error condition at an additional location during scanning, performing an adjustment procedure comprising: - calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter; including the additional location optimal value of the tilt control parameter in the set of values and re-calculating the tilt compensation function; - storing the tilt compensation function in the memory; and setting the tilt control parameter to a value of the tilt compensation function at the radial position of the additional location.

Thus, the tilt compensation unit 32 performs a method comprising steps of: d) selecting calibration locations at different radial positions; e) for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; f) calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; g) storing the tilt compensation function in a memory; h monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and i) in case of an error condition at an additional location during scanning, performing steps of: k) calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter;

In an embodiment of the device, the effects being comprised in a pre- embossed track structure indicating the track, the tilt compensation unit 32 is adapted to use a radial error signal as the quality parameter, a radial error signal based tilt compensation function as the tilt compensation function is and the error condition requiring that the radial error signal is greater than a predefined radial error signal value.

In case of the radial error quality parameter, depending on a disc type, the device may use for example a radial error push-pull signal, a radial error signal based on so- called sampled tracking, or other radial error signals known in the art.

In an embodiment of the device, the error condition is triggered when the radial error is greater than about 1/3 of the peak-to-peak radial error signal during open loop. In an embodiment of the device, the tilt compensation unit 32 is adapted so that the error condition requires occurrence of an uncorrected data error in the main data, the quality parameter is a jitter or a rate of data errors and each location optimal value of the tilt control parameter corresponds to a minimal value of the quality parameter at said each location.

In another embodiment of the device, the tilt compensation unit 32 is adapted so that for said each calibration location, calibrating the tilt control parameter is performed separately for the jitter and for the rate of data errors, and two tilt compensation functions are calculated and stored as a jitter based tilt compensation function and as a rate of data errors based tilt compensation function, respectively, the tilt compensation unit 32 is further adapted for, in case of the error condition at the additional location, performing before the adjustment procedure: measuring the jitter and the rate of data errors at the additional location; checking a first predefined condition; in case the first predefined condition is fulfilled, setting the jitter as the quality parameter and the jitter based tilt compensation function as the tilt compensation function, otherwise checking a second predefined condition; in case the second predefined condition is fulfilled, setting the rate of data errors as the quality parameter and the rate of data errors based tilt compensation function as the tilt compensation function, otherwise not performing the adjustment procedure.

The first predefined condition is related to HF jitter. It is known that the higher is the jitter, the worse is the quality of HF signal leading to data errors. Experimental data show that a jitter based corrective action should be triggered when the jitter is greater than substantially 13%.

Similarly, in case of the rate of data errors, experimental data show that the PI BLER corrective action should be triggered for PI BLER greater than 130, in case of DVD. This threshold value may be chosen different for different systems and/or definition of the rate of data errors.

In an embodiment of the device, the tilt compensation unit 32 is adapted so that at locations where the high-frequency signal is not present, the radial error signal based tilt compensation function is calculated or re-calculated, stored and used as the tilt compensation function for setting the tilt control parameter.

In yet another embodiment of the device, the tilt compensation unit 32 is adapted so that in case the second predefined condition is fulfilled and the tilt compensation function for setting the tilt control parameter is the jitter based tilt compensation function, the adjustment procedure is not performed and the tilt control parameter is set to the value of the rate of data errors based tilt compensation function at the radial position of the additional location.

In an embodiment of the device, the tilt compensation unit 32 is adapted for calibrating the tilt control parameter only one time at the additional location, for every quality parameter. This measure avoids performing redundant calculations. In particular, the tilt compensation unit 32 can be adapted to use the first predefined condition in addition requiring that no calibration based on the jitter was done before at the additional location.

In an embodiment of the device, the tilt compensation unit 32 is adapted to perform a procedure as shown in Figure 4. This embodiment is a combination of above embodiments related to tilt compensation functions based on HF jitter and PI BLER. In this example, the jitter based tilt compensation function is used as a default tilt compensation function. However, upon occurrence of uncorrected data error, "read error", in case the jitter at the additional location is lower than the predefined value of 13% or in case a jitter based calibration was already done at this location, the PI BLER based tilt compensation function is set as the tilt compensation function. So, the tilt compensation is based mainly on HF jitter with additional corrections based on PI BLER in case of persisting read errors.

In an embodiment of the device, the tilt compensation unit 32 is included in the control unit 20.

The tilt compensation unit 32 can be implemented in firmware. Embodiments of the method according to the invention correspond to functions/ procedures performed by the tilt compensation unit 32 as described above in reference to different embodiments of the device.

In particular:

In an embodiment of the method, the tilt compensation function is the second order polynomial fitted to the set of values.

In an embodiment of the method, the effects are comprised in a pre-embossed track structure indicating the track, the quality parameter is a radial error signal, the tilt compensation function is a radial error signal based tilt compensation function and the error condition requires that the radial error signal is greater than a predefined radial error signal value.

In an embodiment of the method, the radial error signal is a radial error push- pull signal and each location optimal value of the tilt control parameter corresponds to a maximal value of the radial error signal at said each location.

In an embodiment of the method, the effects are marks representing information organized into information units comprising main data and error correction data, the signal is a high-frequency signal generated on the basis of the marks, the method comprising steps of: processing the high-frequency signal into the main data and the error correction data; and finding and correcting data errors using the error correction data, and wherein the error condition is occurrence of an uncorrected data error in the main data, the quality parameter is a jitter or a rate of data errors and each location optimal value of the tilt control parameter corresponds to a minimal value of the quality parameter at said each location, the jitter representing time variations of the high-frequency signal.

In an embodiment of the method, for said each calibration location, calibrating the tilt control parameter is performed separately for the jitter and for the rate of data errors, and two tilt compensation functions are calculated and stored as a jitter based tilt compensation function and as a rate of data errors based tilt compensation function, respectively, and wherein the step k) is preceded by steps of: measuring the jitter and the rate of data errors at the additional location; checking a first predefined condition requiring that the jitter is greater than a predefined jitter value; - in case the first predefined condition is fulfilled, setting the jitter as the quality parameter and the jitter based tilt compensation function as the tilt compensation function, otherwise checking a second predefined condition requiring that the rate of data errors is greater than a predefined rate of data errors value; in case the second predefined condition is fulfilled, setting the rate of data errors as the quality parameter and the rate of data errors based tilt compensation function as the tilt compensation function, otherwise omitting remaining steps of the method.

In an embodiment of the method, wherein at locations where the high- frequency signal is not present, the radial error signal based tilt compensation function is calculated or re-calculated, stored and used as the tilt compensation function for setting the tilt control parameter.

In an embodiment of the method, in case the second predefined condition is fulfilled and the tilt compensation function for setting the tilt control parameter is the jitter based tilt compensation function, steps k), 1) and m) are not executed.

In an embodiment of the method, for every quality parameter, calibrating the tilt control parameter is done only one time at the additional location.

In an embodiment of the method, the jitter is the standard deviation of time variations of the high-frequency signal and the predefined jitter value is substantially 13%.

In an embodiment of the method, the first predefined condition in addition requires that no calibration based on the jitter was done before at the additional location. In an embodiment of the method, the predefined rate of data errors is substantially 130 data errors per one information unit.

Different embodiments of a computer program for use in compensating a tilt in a disc-like optical record carrier system, according to the invention, are operative to cause a processor of the system, and in particular the control unit 20, to perform functions/procedures as described in reference to embodiments of the device and/or the method presented above, when the computer program is executed by the system.

Whilst the invention has been described with reference to preferred embodiments thereof, it is to be understood that these are not limitative examples. Thus, various modifications may become apparent to those skilled in the art, without departing from the scope of the invention, as defined by the claims and the embodiments. Further, the invention lies in each and every novel feature or combination of features described above. It is noted, that the invention may be implemented by means of a general purpose processor executing a computer program or by dedicated hardware or by a combination of both, and that in this document the word "comprising" does not exclude the presence of other elements or steps than those listed and the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements, that any reference signs do not limit the scope of the claims, that "means" may be represented by a single item or a plurality and that several "means" may be represented by the same item of hardware.

Claims

CLAIMS:

1. A method of compensating a tilt in a disc-like optical record carrier system, the record carrier comprising substantially spiral track with addressable locations, the method comprising steps of: a) rotating the record carrier and scanning the track by a radiation beam generated by a reading head; b) controlling positioning of the radiation beam using signals representing control parameters; c) generating a signal by the reading head on the basis of optically readable effects along the track; d) selecting calibration locations at different radial positions; e) for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; f) calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; g) storing the tilt compensation function in a memory; h) monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and i) in case of an error condition at an additional location during scanning, performing steps of: k) calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter;

2. A method as claimed in claim 1, wherein the tilt compensation function is the second order polynomial fitted to the set of values.

3. A method as claimed in claim 1, wherein the effects are comprised in a pre- embossed track structure indicating the track, the quality parameter is a radial error signal, the tilt compensation function is a radial error signal based tilt compensation function and the error condition requires that the radial error signal is greater than a predefined radial error signal value.

4. A method as claimed in claim 3, wherein the radial error signal is a radial error push-pull signal and each location optimal value of the tilt control parameter corresponds to a maximal value of the radial error signal at said each location.

5. A method as claimed in claim 1, wherein the effects are marks representing information organized into information units comprising main data and error correction data, the signal is a high-frequency signal generated on the basis of the marks, the method comprising steps of: processing the high-frequency signal into the main data and the error correction data; and finding and correcting data errors using the error correction data, and wherein the error condition is occurrence of an uncorrected data error in the main data, the quality parameter is a jitter or a rate of data errors and each location optimal value of the tilt control parameter corresponds to a minimal value of the quality parameter at said each location, the jitter representing time variations of the high-frequency signal.

6. A method as claimed in claim 5, wherein for said each calibration location, calibrating the tilt control parameter is performed separately for the jitter and for the rate of data errors, and two tilt compensation functions are calculated and stored as a jitter based tilt compensation function and as a rate of data errors based tilt compensation function, respectively, and wherein the step k) is preceded by steps of: measuring the jitter and the rate of data errors at the additional location; checking a first predefined condition requiring that the jitter is greater than a predefined jitter value; in case the first predefined condition is fulfilled, setting the jitter as the quality parameter and the jitter based tilt compensation function as the tilt compensation function, otherwise checking a second predefined condition requiring that the rate of data errors is greater than a predefined rate of data errors value; in case the second predefined condition is fulfilled, setting the rate of data errors as the quality parameter and the rate of data errors based tilt compensation function as the tilt compensation function, otherwise omitting remaining steps of the method.

7. A method as claimed in claims 4 and 6, wherein at locations where the high- frequency signal is not present, the radial error signal based tilt compensation function is calculated or re-calculated, stored and used as the tilt compensation function for setting the tilt control parameter.

8. A method as claimed in claim 6, wherein in case the second predefined condition is fulfilled and the tilt compensation function for setting the tilt control parameter is the jitter based tilt compensation function, steps k), 1) and m) are not executed.

9. A method as claimed in claim 6, wherein for every quality parameter, calibrating the tilt control parameter is done only one time at the additional location.

10. A method as claimed in claim 6, wherein the jitter is the standard deviation of time variations of the high-frequency signal and the predefined jitter value is substantially

13%.

11. A method as claimed in claims 9 and 10, wherein the first predefined condition in addition requires that no calibration based on the jitter was done before at the additional location.

12. A method as claimed in claim 6, wherein the predefined rate of data errors is substantially 130 data errors per one information unit.

13. A device for retrieving information from a disc-like optical record carrier, the record carrier comprising substantially spiral track with addressable locations, the information organized into information units comprising main data and error correction data, the device comprising: - a rotation unit 21 for rotating the record carrier; a reading head 22 for scanning the track by a radiation beam and generating signals on the basis of optically readable effects along the track; a reading head control unit 25 for controlling generation and positioning of the radiation beam using signals representing control parameters; - a decoding unit 30 for processing a high-frequency signal into the main data and the error correction data, the high-frequency signal being generated on the basis of effects as marks representing the information; an error correction unit 30 for finding and correcting data errors using the error correction data; - a tilt compensation unit 32 for compensating a relative tilt of the record carrier to the radiation beam by: selecting calibration locations at different radial positions; for each calibration location, calibrating a tilt control parameter by measuring a quality parameter at different values of the tilt control parameter and determining a location optimal value of the tilt control parameter, the quality parameter being related to the signal; calculating a tilt compensation function representing dependence of the tilt control parameter on a radial position based on a set of values comprising the location optimal values of the tilt control parameter at the calibration locations; storing the tilt compensation function in a memory; - monitoring the radial position of the radiation beam during scanning and setting the tilt control parameter to a value of the tilt compensation function at the radial position; and in case of an error condition at an additional location during scanning, performing an adjustment procedure comprising: - calibrating the tilt control parameter by measuring the quality parameter at different values of the tilt control parameter and determining an additional location optimal value of the tilt control parameter; including the additional location optimal value of the tilt control parameter in the set of values and re-calculating the tilt compensation function; storing the tilt compensation function in the memory; and setting the tilt control parameter to a value of the tilt compensation function at the radial position of the additional location.

14. A device as claimed in claim 13, wherein the tilt compensation unit 32 is adapted to calculate the tilt compensation function as the second order polynomial fitted to the set of values.

15. A device as claimed in claim 13, the effects being comprised in a pre- embossed track structure indicating the track, wherein the tilt compensation unit 32 is adapted to use a radial error signal as the quality parameter, a radial error signal based tilt compensation function as the tilt compensation function and the error condition requiring that the radial error signal is greater than a predefined radial error signal value.

16. A device as claimed in claim 15, the radial error signal being a radial error push-pull signal, wherein the tilt compensation unit 32 is adapted so that each location optimal value of the tilt control parameter corresponds to a maximal value of the radial error signal at said each location.

17. A device as claimed in claim 13, wherein the tilt compensation unit 32 is adapted so that the error condition requires occurrence of an uncorrected data error in the main data, the quality parameter is a jitter or a rate of data errors and each location optimal value of the tilt control parameter corresponds to a minimal value of the quality parameter at said each location, the jitter representing time variations of the high-frequency signal.

18. A device as claimed in claim 17, wherein the tilt compensation unit 32 is adapted so that for said each calibration location, calibrating the tilt control parameter is performed separately for the jitter and for the rate of data errors, and two tilt compensation functions are calculated and stored as a jitter based tilt compensation function and as a rate of data errors based tilt compensation function, respectively, the tilt compensation unit 32 is further adapted for, in case of the error condition at the additional location, performing before the adjustment procedure: measuring the jitter and the rate of data errors at the additional location; checking a first predefined condition requiring that the jitter is greater than a predefined jitter value; in case the first predefined condition is fulfilled, setting the jitter as the quality parameter and the jitter based tilt compensation function as the tilt compensation function, otherwise checking a second predefined condition requiring that the rate of data errors is greater than a predefined rate of data errors value; in case the second predefined condition is fulfilled, setting the rate of data errors as the quality parameter and the rate of data errors based tilt compensation function as the tilt compensation function, otherwise not performing the adjustment procedure.

19. A device as claimed in claims 16 and 18, wherein the tilt compensation unit 32 is adapted so that at locations where the high-frequency signal is not present, the radial error signal based tilt compensation function is calculated or re-calculated, stored and used as the tilt compensation function for setting the tilt control parameter.

20. A device as claimed in claim 18, wherein the tilt compensation unit 32 is adapted so that in case the second predefined condition is fulfilled and the tilt compensation function for setting the tilt control parameter is the jitter based tilt compensation function, the adjustment procedure is not performed and the tilt control parameter is set to the value of the rate of data errors based tilt compensation function at the radial position of the additional location.

21. A device as claimed in claim 18, wherein the tilt compensation unit 32 is adapted for calibrating the tilt control parameter only one time at the additional location, for every quality parameter.

22. A device as claimed in claim 18, wherein the jitter is the standard deviation of time variations of the high-frequency signal and the predefined jitter value is substantially 13%.

23. A device as claimed in claims 21 and 22, wherein the tilt compensation unit 32 is adapted to use the first predefined condition in addition requiring that no calibration based on the jitter was done before at the additional location.

24. A device as claimed in claim 18, wherein the predefined rate of data errors is substantially 130 data errors per one information unit.

25. A computer program product for use in compensating a tilt in a disc-like optical record carrier system, the computer program comprising program code means for causing a processor of the system, to perform the steps of the method as claimed in any of claims 1 - 12, when the computer program is run on the processor.