The invention relates device for scanning an optical record carrier, the record carrier comprising a data layer having substantially parallel data tracks, the device comprising an optical head comprising a detector for receiving radiation reflected from a data track, the detector having sub-detectors arranged in a quadrant aligned in a direction corresponding to the track direction, and tilt means for generating a tilt signal representing a tilt angle between an optical axis of the optical head and a perpendicular of the data layer.
The invention further relates to a method of detecting tilt while scanning an optical record carrier, the record carrier comprising a data layer having substantially parallel data tracks, the method comprising generating a tilt signal representing a tilt angle between an optical axis of the optical head and a perpendicular of the data layer based on radiation reflected from a data track received on sub-detectors arranged in a quadrant aligned in a direction corresponding to the track direction.
In optical drives, the read-out performance is often degraded by tilt. Tilt is the angle between an optical axis of the optical head and a perpendicular of the data layer of the record carrier. Two types of tilt exist, called tangential tilt and radial tilt. With tangential tilt the spot is tilted in the track direction, which distorts the optical channel and causes severe inter-symbol interferences (ISI). With radial tilt the spot is tilted towards the neighbouring tracks, in which the neighbouring track data enter the target track read-out in the form of inter-track interference (ITI) or cross talk (XT). In order to increase the robustness of optical drives against tilt, a tilt estimator is needed with which the tilt can be corrected in either a mechanical or signal processing way.
A device and method for scanning an optical record carrier and detecting tilt are known from the document “New radial tilt detection method using only one beam and one four-quadrant detector” by Y. Wang et al. Japanese Journal of Applied Physics, Vol. 43, No. 11A, 2004, pp 7513-7518 (called doc1). In doc1 a four quadrant detector, having four sub-detectors denominated A, B, C and D, is used to generate a tilt error signal. The effects of disk radial tilt on a differential time detection (DTD) tracking error signal (TE) are calculated and measured. The method uses the difference between the offsets of two tracking methods due to tilt. The first tracking signal is the DTD signal, based on the time difference of the signal A+C and the signal B+D, illustrated by the formula DTDTE=τ(A+C)−τ(B+D). A second tracking error signal (TE) is based on a push-pull signal (PP) of the two detector halves A+B and C+D, illustrated by the formula PPTE=(A+B−C−D). The difference between the two tracking methods is analyzed and used to calculate a tilt signal representing the tilt angle.
However, the quality of the push-pull signal is in general lower than the quality of the DTD signal. Hence the tilt signal may be inaccurate and unreliable.
Therefore it is an object of the invention to provide a device and method for generating a reliable tilt signal.
According to a first aspect of the invention the object is achieved with a device as described in the opening paragraph, the tilt means being arranged for generating a diagonal push-pull signal based on a difference of a first signal of a first diagonally positioned pair of sub detectors and second signal of a second diagonally positioned pair of sub detectors, and processing the diagonal push-pull signal for generating the tilt signal.
According to a second aspect of the invention the object is achieved with a method as described in the opening paragraph, which method comprises generating a diagonal push-pull signal based on a difference of a first signal of a first diagonally positioned pair of sub detectors and second signal of a second diagonally positioned pair of sub detectors, and processing the diagonal push-pull signal for generating the tilt signal.
The effect of the measures is that the diagonal push-pull signal is generated as a single combined signal. Advantageously the diagonal push-pull signal comprises substantial signal elements representing the tilt angle. By processing the diagonal push-pull signal the tilt signal is generated.
The invention is also based on the following recognition. There are a few important requirements for a good tilt estimator. First, a tilt estimator should be able to detect the tilt on the fly during reading because in such a manner it enables a dynamic tilt correction that is necessary for achieving good drive playability. Secondly, use of extra optical components is not preferred, such as additional gratings for generating satellite spots or a second laser being active simultaneously with the main laser in a dual-wavelength method. Finally, the tilt estimation result, as a function of the tilt angle, must have a wide enough linear range (including sign) and high enough sensitivity around the nominal point (zero tilt) that can ease the proper working of the tilt correction.
Known methods, like method based on jitter value, push-pull (like doc1 discussed above) or high frequency read signal (RF) amplitude based, cannot reliably detect tilt angle and/or tilt sign. Only part of the above requirements is met with existing methods. The problems mainly lie on that extra optical components have to be used, which increases the cost and introduces possible instability to the system, and that the estimation is static (e.g. in a special tilt detection procedure) and can not be done on the fly (during scanning the data tracks, e.g. for reading data).
The inventors have seen that from available optical elements and detectors, the diagonal push-pull signal is generated easily in the high frequency domain based on the sub-detector signals without requiring further filtering or time detection. Advantageously, the diagonal push-pull signal contains signal elements corresponding to the radial tilt. A tilt signal is conveniently generated by processing the diagonal push-pull signal, e.g. by an appropriate filter, while assuming that scanning spot is centered on the track by a tracking servo system.
In an embodiment of the device the tilt means are arranged for generating a channel data signal based on data from the data track and a channel response of a diagonal push-pull channel, and for processing the diagonal push-pull signal by cross-correlating the diagonal push-pull signal and the channel data signal for generating the tilt signal. The channel data signal represents the signal of an ideal diagonal push-pull channel, i.e. a signal based on the data marks in the track and the response of the elements constituting the diagonal push-pull channel. Cross-correlating the channel data signal with the diagonal push-pull signal has the advantage that the signal elements representing tilt are magnified.
In an embodiment of the device the tilt means comprise discrimination means for generating a difference signal for discriminating a tracking offset from a tilt based on a diagonal push-pull signal cross-correlated with a data read signal convolved with a first filter having a first impulse response based on the channel response of the diagonal push-pull channel in the event of tilt, and the diagonal push-pull signal cross-correlated with a data read signal convolved with a second filter having a second impulse response based on the channel response of the diagonal push-pull channel in the event of tracking offset. Using both filters has the advantage that the difference signal is generated from the same detector signals that are used for generating the tilt signal. If the difference signal indicates a tracking offset, the device may first correct the tracking offset. Hence it is prevented that tracking offset disturbs the tilt detection.
Further preferred embodiments of the device and method according to the invention are given in the appended claims, disclosure of which is incorporated herein by reference.
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
In the Figures, elements which correspond to elements already described have the same reference numerals.
The record carrier 11 may exhibit a tilt as schematically indicated by arrow 301. For example the tilt may result from a non-flat surface, a non perfect mechanical support, or scanning system offset, etc. A tilt angle 304 is defined at the position of the scanning spot 23, as the angle between an optical axis 302 of the head 22 and a perpendicular 303 of data layer of the record carrier. Note that in practice the tilt angle is about 1 degree or less, and the Figure is not drawn to scale.
The head, or the record carrier support system, may further include tilt actuators for adapting a tilt angle between a perpendicular to the data layer and an optical axis of the optical system of the head. The tilt actuators may be controlled based on the tilt signal generated as discussed below.
During 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 head 22 for generating detector signals coupled to a front-end unit 31 for generating various scanning signals, including a main scanning signal 33 and error signals 35 for tracking and focusing. The error signals 35 are coupled to the servo unit 25 for controlling said tracking and focusing actuators. The main scanning signal 33 is processed by read processing unit 30 of a usual type including a demodulator, deformatter and output unit to retrieve the information. The control unit 20 comprises control circuitry, for example a microprocessor, a program memory and control gates. The control unit 20 may also be implemented as a state machine in logic circuits.
The device may be provided with recording means for recording information on a record carrier of a writable or re-writable type. The recording means comprise an input unit 27, a formatter 28 and a laser unit 29 and cooperate with the head 22 and front-end unit 31 for generating a write beam of radiation. The formatter 28 is for adding control data and formatting and encoding the data according to the recording format, e.g. by adding error correction codes (ECC), synchronizing patterns, interleaving and channel coding. The formatted data comprise address information and are written to corresponding addressable locations on the record carrier under the control of control unit 20. The formatted data from the output of the formatter 28 is passed to the laser unit 29 which drives the laser and controls the laser power for writing the marks in a selected layer.
In an embodiment the recording device is a storage system only, e.g. 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 system via a standardized interface. Digital data is interfaced to the formatter 28 and the read processing unit 30 directly.
In an embodiment the device is arranged as a stand alone unit, for example a video recording apparatus 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, and to perform the functions of the file management system. The device includes application data processing, e.g. audio and/or video processing circuits. User information is presented on the input unit 27, which may comprise compression means for input signals such as analog audio and/or video, or digital uncompressed audio/video. Suitable compression means are for example described for audio in WO 98/16014-A1 (PHN16452), and for video in the MPEG2 standard. The input unit 27 processes the audio and/or video to units of information, which are passed to the formatter 28. The read processing unit 30 may comprise suitable audio and/or video decoding units.
The device has a tilt detection unit 32 for detecting a tilt and, in dependence thereon, generating a tilt signal based on a diagonal push-pull signal. The tilt signal may be coupled to the servo unit 25, providing a tilt error signal for adjusting the tilt servo. Alternatively, or additionally, the tilt signal may be used elsewhere, e.g. to adjust a recording process or to adapt the processing of the read signal in read unit 30, e.g. by compensating an amount of inter track cross-talk which is related to the amount of tilt represented by the tilt signal. The tilt signal is determined as discussed in detail below with reference to
As can be seen in
Based on the signals from each sub-detector a diagonal push-pull (DPP) signal is generated based on a difference of a first signal of a first diagonally positioned pair of sub detectors and second signal of a second diagonally positioned pair of sub detectors. The signal indicative of tilt can be detected from the diagonal push-pull signal.
Diagonally positioned pairs with respect to the center of the detector, i.e. the crossing of both arrows 200,201, are pairs A,C and B,D. Note that different shape and ordering of the sub-detectors may also be used. The diagonal push-pull signal contains tilt signal elements related to the deformed shape of the scanning spot, which is caused by the tilt. The diagonal push-pull signal is subsequently processed to isolate the tilt signal elements for generating the tilt signal, as explained further below. The diagonal push-pull signal may be based on the formula
I
k
(DPP)
=I
k
(A)
+I
k
(C)
−I
k
(B)
−I
k
(D) (1)
where Ik(X) (X=A, B, C and D) denotes a radiation intensity on each sub-detector as a function of scanning position or time instant k, the first signal of the first diagonally positioned pair of sub detectors A,C being denoted by Ik(PP1)=Ik(A)+Ik(C), and the second signal of the second diagonally positioned pair of sub detectors B,D being denoted by Ik(PP2)=Ik(D)+Ik(B).
In a nominal situation where the spot 15 is perfectly symmetric in radial direction, the diagonal push-pull signal Ik(DPP) is zero, implying that no light intensity variation difference exists between A, C and B, D; while with radial tilt, the resultant Ik(DPP) becomes nonzero. For the sake of simplicity, we assume the channel is free of non-linearity and noise. Then one can write the DPP channel readout signal as follows:
C
k
(DPP)=(a*h(DPP))k (2)
where ak represents the channel data sequence (alphabet {−1, 1}), hk(DPP) the DPP channel symbol response (CSR) of the diagonal push-pull channel and * a linear convolution.
A set of curves 41 provides examples of hk(DPP) at various radial tilt (RT) angles. The examples are based on scalar diffraction with a Blu-ray Disc set-up at 25 GB capacity. The curves are in general anti-symmetric around origin and tap amplitude increases with the tile angle θ. To a first order approximation, formula (2) can be rewritten as:
I
k
(DPP)(θ)≈θ×(a*h(DPP))k (3)
In reality, the signal Ik(DPP)(θ) suffers from noise and may get nonzero due to other light path imperfections that are irrelevant to radial tilt. Hence, (3) can be generalized to:
To extract the information of radial tilt θ, one needs ideally cross-correlate Ik(DPP)(θ) with (a*h(DPP))k to get the tilt estimate:
In principle, hk(DPP) is not exactly known in the receiver. However, we can produce a rough version of (a*h(DPP))k by convolving the estimated bit sequence âk with an FIR filter, whose impulse response sk is a stylized approximation of hk(DPP), e.g., sk=[1, 0, 0, 1, 0, 0, 0−1, 0, 0, 1] for 25 GB. Then, (5) becomes:
Moreover, it is also preferably not to use bit estimate âk explicitly to avoid the deterioration of tilt estimation due to bit errors as well as speed limitation due to a delay between Ik(DPP) and reconstructed âk in the possible use of a tilt corrector. For this reason, one can consider to replace âk in (6) with its synchronous central aperture signal Ik(CA) that is always immediately available, according to the formula:
Compared to âk, Ik(CA) takes some extra disturbances into estimation, like noise, cross talk and ISI. Cross talk impact can be limited to certain extent by the nature of cross-correlation assuming the cross talk appearance in Ik(DPP) is much weaker than that of the target track data. ISI will not give any influence as long as the central aperture channel symbol response keeps symmetric, which is the case with radial tilt, as well as constant, which is roughly true within the radial tilt range of interest ([−1°, 1°]).
For good and stable radial tilt estimation, the term Δh(DPP) in (4) caused by other light path imperfections should ideally be orthogonal to the selected signature filter sk, which is in fact the case with normally considered aberrations like tangential tilt, defocus and spherical aberration.
A tracking offset will cause an anti-symmetric DPP CSR, but the middle two lobes 43,44 have much higher amplitude than two outer lobes 42,45 shown in
where sk(O)32 [−1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1] and sk(M)=[1, 0, 0, 0, −1] is able to approximately get the energy ratio between the middle two lobes and outer two lobes of a DPP CSR. Note that the filter parameters in the example have been set for 25 GB Blu-ray Disc, but have to be adjusted for the specific read-out channel. When Δχ is larger than a preset threshold, instead of a radial tilt a tracking offset is identified and no tilt correction will be executed.
The digital signals are processed in a tilt calculation unit 50 for generating a tilt signal χθ. The tilt calculation unit 50 includes a channel response unit 501 having a response function sk as described above for generating the channel data signal based on the read signal Ik(Ca) representing data from the data track convoluted with the response function sk. In calculation unit 502 the channel data signal is multiplied with the diagonal push-pull signal Ik(DPP), and the result is integrated in integrating unit 503 to generate the tilt signal χθ as described above.
The tilt signal generating device may include a tilt discrimination unit 56 for processing the digital signals Ik(CA) and Ik(DPP) for generating a tilt discrimination signal χθ, for example based on formula (8) above. A tilt judging unit 57 compares the tilt discrimination signal Δχ with a predetermined threshold and generates a tilt control signal for activating the tilt calculation unit 50. The output of the tilt judging unit 57 acts as an enabling signal for the tilt estimation. When the output is “N”, χθ is set to zero. Additionally a tracking servo may be activated to correct the position of the scanning spot with respect to the center of the track.
In the simulation, four quadrant data signals of a BD disc are measured from a BD experimental tester with various radial tilt settings and then processed according to Equation (7) to get the tilt estimate. The results are shown in
Although the invention has been mainly explained by embodiments using BD optical discs, the invention is also suitable for other record carriers such as rectangular optical cards, magneto-optical discs, multilayer high-density discs or any other type of information storage system that has a tilt sensitive scanning system.
It is noted, 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 the invention may be implemented by means of both hardware and software, and that several ‘means’ or ‘units’ may be represented by the same item of hardware or software. Further, the scope of the invention is not limited to the embodiments, and the invention lies in each and every novel feature or combination of features described above.
Number | Date | Country | Kind |
---|---|---|---|
05301047.6 | Dec 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2006/054566 | 12/4/2006 | WO | 00 | 6/10/2008 |