Optical apparatus

FIELD OF THE INVENTION

The present invention relates in general to an optical apparatus. As a particular example, the present invention relates to an optical scanning apparatus for writing/reading information into/from an optical information carrier. In the following, the invention will be specifically explained for the case of an optical disc drive for scanning an optical storage disc, wherein the disc is rotated and a write/read head is moved radially with respect to the rotating disc. The present invention is applicable in the case of optical as well as magneto- optical disc systems. Hereinafter, the wording "optical disc drive" will be used, but it is to be understood that this wording is intended to also cover magneto -optical disc systems. Further, it is to be noted that the invention is not restricted to optical disc drives; the present invention can also be applied in the case of, for instance, a microscope.

BACKGROUND OF THE INVENTION

As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writable type, where information may be stored by a user. Further, storage discs of different format types have been developed, such as for instance CD, DVD, BD.

For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in more detail.

For rotating the optical disc, an optical disc drive typically comprises a motor, which drives a hub engaging a central portion of the optical disc.

For optically scanning the rotating disc, an optical disc drive comprises a light beam generator device (typically a laser diode), an objective lens for focussing the light beam in a focal spot on the disc, and an optical detector for receiving the reflected light reflected from the disc and for generating an electrical detector output signal.

During operation, the light beam should remain focussed on the disc. To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focal actuator means for controlling the axial position of the objective lens. Further, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens.

In many disc drives, the orientation of the objective lens is fixed, i.e. its axis is directed parallel to the rotation axis of the disc. In some disc drives, the objective lens is pivotably mounted, such that its axis can make an angle unequal to zero with the rotation axis of the disc.

For any reason, it may be that the optical disc suffers from tilt. Tilt of the optical disc can be defined as a situation where the substrate or cover layer through which the beam is focussed onto the storage layer is not exactly perpendicular to the optical axis. Tilt can be caused by the optical disc being tilted as a whole, but is usually caused by the optical disc being warped, and as a consequence the amount of tilt depends on the location on disc. Tilt may cause a degradation of performance, especially readout performance; for instance, jitter increases with increasing tilt. The tolerance margins for disc tilt in optical disc drives are quite narrow, in the order of 0.5° or less. Therefore, tilt compensation mechanisms have been developed.

One prior art approach for tilt compensation involves tilting the objective lens, in which case a tilt actuator controls the tilt position of the objective lens. Such method may be suitable for disc drives using only one laser beam. However, with the development of different formats, it became desirable to design disc drives capable of handling two or more different disc format types, such as for instance CD, DVD, BD. Such disc-drives, which will hereinafter also be indicated by the phrase "combi-drive", comprise two or more different laser beams with mutually different wavelengths, each beam being associated with a specific format. Now a problem is that, for compensating disc tilt by tilting the objective lens,

different lens tilt angles are required for the different laser beams. In other words, if an objective lens common to two or more laser beams is tilted at a particular lens tilt angle which optimally compensates disc tilt for one of the different laser beams of a combi-drive, the disc tilt problem may still remain for the one or more other laser beams. This problem arises due to manufacturing errors, e.g. in the alignment of different optical components in the light path between the laser diodes and the objective lens.

US-2004/0114495 discloses a disc drive device comprising two different optical systems with two different objective lenses for two different laser beams. In such case, the two different objective lenses can be tilted independently from each other, to achieve optimum tilt compensation for both laser beams. In contrast, the present invention relates to a disc drive device comprising a single common objective lens for the two or more laser beams.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate or at least reduce the above- mentioned problems.

According to an important aspect of the present invention, an optical system for an optical disc drive apparatus comprises common components and individual components. The common components are for guiding two or more different laser beams, whereas the individual components are for guiding one single laser beam only. The common components include the objective lens. A common component, such as for instance the objective lens, may be tilted to optimally compensate for disc tilt for one of the laser beams; this will be indicated as "common tilt compensation". At least one individual component associated with one of the other beams may be tilted to compensate for disc tilt for such other beam; this will be indicated as "individual tilt compensation". In a preferred embodiment, the individual component tilted for individual tilt compensation is a pre-collimator lens.

Most preferably, this individual component tilted for individual tilt compensation is an anastigmatic component. This has the advantage that such component does not produce any, or only a very limited amount, of astigmatism when tilted. Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with

reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Fig. IA schematically illustrates an optical disc drive;

Fig. IB is a block diagram illustrating schematically an optical detector connected to a signal processor;

Fig. 2 is a graph schematically showing jitter as a function of tilt angle;

Fig. 3 is a schematic block diagram illustrating components of an optical system according to the invention for an embodiment with three different laser beams;

Fig. 4 schematically shows a cross section of a pre-collimator lens; Figs. 5-10 are graphs illustrating relationships between different parameters of a pre-collimator lens.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure IA schematically illustrates an optical disc drive apparatus 1, suitable for storing information on or reading information from an optical disc 2, typically a DVD or a CD or a BD. For rotating the disc 2, the disc drive apparatus 1 comprises a motor 4 fixed to a frame (not shown for sake of simplicity), defining a rotation axis 5. For receiving and holding the disc 2, the disc drive apparatus 1 may comprise a turntable or clamping hub 6, which in the case of a spindle motor 4 is mounted on the spindle axle 7 of the motor 4. The disc drive apparatus 1 further comprises an optical system 30 for scanning tracks (not shown) of the disc 2 by an optical beam. More specifically, in the exemplary arrangement illustrated in figure IA, the optical system 30 comprises a first light beam generating means 31 and a second light beam generating means 41, each typically a laser such as a laser diode, each arranged to generate a first light beam 32 and a second light beam 42, respectively. In the following, different sections of the optical path of a light beam 32, 42 will be indicated by a character a, b, c, etc added to the reference numeral 32, 42, respectively.

The first laser 31 and the second laser 41 are different types of laser in that their respective laser beams 32, 42 have a different wavelength. For instance, a laser beam suitable for handling a CD has a wavelength in the order of about 780 nm, while a laser beam suitable for handling a DVD has a wavelength in the order of about 660 nm. The disc drive apparatus 1 is designed for handling two or more types of disc, i.e. CD as well as DVD for example. In that case, the first laser beam 32 will have the wavelength mentioned for use with CD or DVD, respectively, while the second laser beam 42 will have the wavelength

mentioned for use with DVD or CD, respectively. For implementing or explaining the present invention, it does not matter whether the first laser beam 32 is a CD-type beam while the second laser beam 42 is a DVD-type beam, or the opposite.

It is noted that, in case the disc drive apparatus 1 is designed for handling three types of disc, the optical system 30 would comprise a third laser; this is not illustrated in figure IA.

The first light beam 32 is reflected by a first beam splitter 33, and passes a collimator lens 39 and an objective lens 34 to reach (beam 32b) the disc 2. The beam splitter is schematically depicted as a cube, but may have other implementations. The first light beam 32b reflects from the disc 2 (reflected first light beam 32c) and passes through the optical components to finally reach an optical detector 35.

The second light beam 42 passes a pre-collimator lens 48, is reflected by a second beam splitter 44, passes the first beam splitter 43, and then follows an optical path comparable to the optical path of the first light beam 32, indicated by reference numerals 42b, 42c, 42d.

It is noted that, since the disc drive apparatus in practice handles only one disc at a time, only one of the laser beams will be active at a time, the other laser beam(s) being off. The objective lens 34 is designed to focus the active light beam 32b, 42b in a focal spot F on a recording layer (not shown for sake of simplicity) of the disc 2, which spot F normally is circular.

For sake of completeness, figure IA shows that the disc drive apparatus 1 further comprises an actuator system 50, which comprises a radial actuator 51 for radially displacing the objective lens 34 with respect to the disc 2 (for track following), a focal actuator 52 arranged for axially displacing the objective lens 34 with respect to the disc 2 (for achieving and maintaining a correct focusing), and a pivot actuator or tilt actuator 53 for pivoting the objective lens 34 with respect to the disc 2 (for the purpose of tilt compensation). Since such actuators are known per se, while the present invention does not relate to the design and functioning of such actuators, it is not necessary here to discuss the design and functioning of an actuator in great detail. It is noted that the radial actuator 51, focal actuator 52, and pivot actuator 53 may be implemented as one integrated 3D-actuator.

The disc drive apparatus 1 further comprises a control circuit 90 having a first output 93 coupled to a control input of the radial actuator 51, having a second output 94 coupled to a control input of the focal actuator 52, and having a third output 95 coupled to a control input of the pivot actuator 53. The control circuit 90 is designed to generate control

signals SQR, SQF, SQT for controlling the radial actuator 51, the focal actuator 52, and the pivot actuator 53, respectively.

The control circuit 90 further has a read signal input 91 for receiving a read signal SR from the optical detector 35. Figure IB illustrates that the optical detector 35 comprises a plurality of detector segments, in this case four detector segments 35a, 35b, 35c, 35d, capable of providing individual detector signals A, B, C, D, respectively, indicating the amount of light incident on each of the four detector quadrants, respectively. A centre line 36, separating the first and fourth segments 35a and 35d from the second and third segments 35b and 35c, has a direction corresponding to the track direction. Since such a four-quadrant detector is commonly known per se, it is not necessary here to give a more detailed description of its design and functioning.

Ideally, the incident optical beam 32b, 42b produces a narrow unaberrated focus spot F. If the disc 2 has a warped surface, as shown in figure IA in an exaggerated manner, the incident optical beam 32b, 42b may not be directed perfectly perpendicular to the disc surface, in which case the focus spot F is no longer circular, and this aberration ("coma") may lead to write errors and/or read errors. Further, servo signals are sensitive to tilt. Generally, it can be said that performance parameters as a function of tilt angle follow a curve that is indicated as a "bathtub" curve. This is illustrated in figure 2 for the example of jitter. Figure 2 is a graph, schematically showing jitter J (vertical axis, in arbitrary units) as a function of tilt angle α (horizontal axis, in arbitrary units). The figure shows that the jitter J has a minimum Jj _nJn for a certain tilt angle α _min , and that the jitter rapidly increases as |α - α _min | increases. It appears that the jitter is more than linearly proportional to the tilt angle, resulting in a curved line with the concave part directed upwards, hence the name "bathtub curve". It is noted that the curve shown is idealised.

Theoretically, one might expect a curve that is symmetrical with respect to α=0. However, in practice such a bathtub curve is often asymmetric, and the minimum α _min may be offset with respect to zero, as shown. This is due to small comatic aberrations in the readout beam resulting from misalignment and/or manufacturing errors of the different optical components. As a consequence, an optimal tilt compensation is achieved with an optical beam that is not perfectly perpendicular to the disc surface but makes an angle of 90 ^o -a _min with the disc surface. In the case of a single-beam disc drive, this can be pre- compensated in the process of manufacturing a drive by giving the objective lens a preset tilt,

so that the optimum readout performance is found for zero disc tilt. In the case of a multi- beam disc drive (combi drive), however, an additional problem is that the optimal tilt angles α _m i _n for the different beams are mutually independent and are usually mutually different. Thus, such pre-compensation is only possible for one of the beams, while the tilt problem of the other beams may continue to exist to a lesser or worse degree.

It is noted that, for disc drives having a pivoting objective lens and a tilt actuator mechanism 53, this problem may be solved by actively tilting the objective lens, but this usually only applies to radial tilt.

The present invention provides a solution to this problem, that will be discussed with reference to figures IA and 3. Figure 3 is a schematic block diagram, illustrating components of an optical system 300 according to the invention, for an embodiment with three different laser beams, in a layout that is a variation on the layout of figure IA.

Comparable to figure IA, the optical system 300 of figure 3 comprises a first laser 31, with a first beam splitter 33 and an objective lens 34. In front of the objective lens 34, a collimator lens 39 is shown.

The optical system 300 further comprises a second laser 131, with a second beam splitter 133 and a first pre-collimator lens 138 arranged between the second laser 131 and the second beam splitter 133. The function of the pre-collimator lens 138 is to slightly reduce the divergence of the second beam 132 produced by the second laser 131 (the magnification of the pre-collimator lens 138 is typically in the order of about 1.4), so that the fraction of the emitted laser power which eventually is focussed on the disc is increased, which is favourable for increasing the maximally possible writing speed.

The optical system 300 further comprises a third laser 231, with a third beam splitter 233 and a second pre-collimator lens 238 arranged between the third laser 231 and the third beam splitter 233.

Optical beam 32 from the first laser 31 is reflected by the first beam splitter 33, and passes the collimator lens 39 and the objective lens 34 to reach disc 2. After being reflected, the reflected beam passes the objective lens 34, the collimator lens 39, the first beam splitter 33, the second beam splitter 133, and the third beam splitter 233 to reach detector 35.

Optical beam 132 from the second laser 131 passes the first pre-collimator lens 138, is reflected by the second beam splitter 133, and passes the first beam splitter 33, the collimator lens 39 and the objective lens 34 to reach disc 2. After being reflected, the

reflected beam passes the objective lens 34, the collimator lens 39, the first beam splitter 33, the second beam splitter 133, and the third beam splitter 233 to reach detector 35.

Optical beam 232 from the third laser 231 passes the second pre-collimator lens 238, is reflected by the third beam splitter 233, and passes the second beam splitter 133, the first beam splitter 33, the collimator lens 39 and the objective lens 34 to reach disc 2. After being reflected, the reflected beam passes the objective lens 34, the collimator lens 39, the first beam splitter 33, the second beam splitter 133, and the third beam splitter 233 to reach detector 35.

Thus, the objective lens 34, the collimator lens 39, the first beam splitter 33, the second beam splitter 133, and the third beam splitter 233 are common components, which are passed by all laser beams 32, 132, 232. In contrast, the first pre-collimator lens 138 and the second pre-collimator lens 238 are individual components, individually passed by the second and third laser beams 132, 232, respectively. It is noted that, besides the first pre- collimator lens 138 and the second pre-collimator lens 238, the optical system may comprise more individual components.

It is noted that an individual pre-collimator lens may be associated with the first laser 31 as well, but this is not shown in figure 3.

According to the present invention, at least one of the individual components, preferably one of the pre-collimator lenses, is slightly tilted with respect to the optical path of the corresponding laser beam, in such a direction and at such a tilt angle as to introduce a coma effect compensating for the offset of the bathtub curve associated with this laser beam.

For eliminating the offset differences between the several laser beams, it is possible that one laser beam is considered as a master beam, which may be implemented without tilted individual components. In the example of figure 3, laser beam 32 is not associated with a pre-collimator lens or any other individual component that may be tilted. Further, all other laser beams are considered as slave beams, and each pre-collimator lens 138, 238 is tilted such that the corresponding offset α _m i _n of the corresponding laser beam is made equal to the offset α _m i _n of the master beam. Then, with tilting the objective lens 34, the remaining offset is reduced to zero. The tilting of a pre-collimator lens may introduce an astigmatic aberration into the corresponding beam. The amount of coma is a linear function of the tilt angle of the pre- collimator lens, whereas the amount of astigmatic aberration is a quadratic function of the tilt angle of the pre-collimator lens. For relatively small tilt angles, the amount of astigmatic aberration is relatively small and may possibly be neglected, but if the pre-collimator lens tilt

angle required is relatively large, the amount of astigmatic aberration may increase rapidly. In order to eliminate or at least reduce this problem, the tilted individual component preferably is an anastigmatic component, such as for instance an anastigmatic pre-collimator lens. An anastigmatic optical component is a component which, when tilted, does produce coma but does not produce any substantial astigmatism.

Figure 4 schematically shows a cross section of a pre-collimator lens 400, having an optical axis 404 which will be taken as a Z-axis. An optical beam directed along the optical axis 404 will enter and leave the lens 400 via refractive lens surfaces 401 and 402 or vice versa. The shape of each of these lens surfaces 401 and 402 can be described by describing the z-coordinate thereof as a function of radial distance r from the optical axis 404, assuming that these surfaces have the shape of a conical section, according to the formula:

r ²

R + -jR ² - (\ + rήr ²

1 + K ^L wherein:

R is the radius of curvature of the surface; K is the so-called conical parameter, which is less than -1 for a hyperbolic surface, equal to -1 for a parabolic surface, between -1 and 0 for a prolate ellipsoid surface, equal to 0 for a spherical surface, and larger than 0 for an oblate ellipsoid surface. It is noted that this formula assumes z=0 per definition at the section with the optical axis 404. In the following, parameters specifically associated with the first surface 401 or the second surface 402 will be distinguished by addition of index 1 or 2, respectively.

Further, the lens has a thickness d measured along the optical axis 404, and the lens material has a refractive index n. Thus, assuming that the lens is made of one homogenous material, there are 6 parameters describing the lens, apart from the lens diameter which is assumed to be "large enough" and therefore not relevant for the optical properties. It is noted that the pre-collimator lenses typically have a numerical aperture NA < 0.1.

Although the refractive index n does depend on the choice of material, the refractive index of injection moulded plastics typically has a value of 1.5, and variation of the material will only slightly vary the refractive index.

Further, given the distance v between the object point and the first surface, and given the required magnification M, the distance b between the image point and the second

surface is fixed. For these conjugate points in the nominal configuration without tilt of the pre-collimator lens, one of the two radii of curvature and one of the conical parameters must be adjusted in order to obtain an aberration- free image.

Further, for practical and manufacturing reasons, the thickness d can only be chosen in a relatively narrow range between about 0.6 mm (or only slightly smaller) and 1.2 mm (or only slightly larger).

Thus, there remain only two main free parameters suitable for variation of the design of the pre-collimator lens to obtain the required anastigmatic property: for instance one radius parameter and one conical parameter. Assume that radius parameter R ₂ is fixed: in that case radius parameter Ri or R ₁ /R ₂ is free for variation. Assume that conical parameter K ₂ is fixed: in that case conical parameter Ki or K ₄ /K ₂ is free for variation.

EMBODIMENT 1

In a first embodiment, the lens 400 has a positive magnification M in the range 1 < M < 2. In that case, the focal length is positive. The inventors have found that the required anastigmatic property is obtained when the ratio R ₁ /R ₂ is chosen smaller than 1, more preferably in the range 0.5 < R ₁ /R ₂ < 1. The optimal value for the ratio R ₁ /R ₂ depends on the precise values of M, v, n, d and κl .

For a specific embodiment of the lens, with M = 1.4, n = 1.5, κl = 0, the lens having an entrance numerical aperture NA = 0.10, figure 5 is a graph illustrating the calculated relationship between ratio R ₁ /R ₂ , object distance v, and lens thickness d, for the case when the lens is anastigmatic. The vertical axis of the graph represents the ratio R ₁ /R ₂ , the horizontal axis of the graph represents the object distance v. Calculated points are indicated by a solid circle; the solid curves connecting these points are an interpolation. The graph shows three such curves, for three different values of the lens thickness d. It can be seen that in all cases 0.5 < R ₁ /R ₂ < 1 applies.

For this specific embodiment, figure 6 is a graph illustrating the relationship between the coma-creativity, object distance v, and lens thickness d, for the case when the lens is anastigmatic (see figure 5 for the corresponding values of the ratio R ₁ /R ₂ ). The phrase "coma-creativity" here means the amount of coma created per degree lens tilt, expressed in mλ/deg. Since it generally is desirable that the coma-creativity is as large as possible, it can

be seen that it is advantageous to have the object distance v be as large as possible, and that it is advantageous to have the thickness d be as small as possible.

For this specific embodiment, figure 7 is a graph illustrating the relationship between K ₂ , object distance v, and lens thickness d, for the case when the lens is anastigmatic (see figure 5 for the corresponding values of the ratio R ₄ /R ₂ , and figure 6 for the corresponding values of the coma-creativity). It can be seen that in all cases K ₂ < 0 applies.

For a specific embodiment of the lens, with M = 1.4, n = 1.5, thickness d = 0.8 mm, the lens having an entrance numerical aperture NA = 0.10, figure 8 is a graph illustrating the relationship between ratio R ₄ /R ₂ , object distance v, and Ki, for the case when the lens is anastigmatic. It can be seen that the influence of Ki on the ratio R ₄ /R ₂ is relatively small.

For this specific embodiment, figure 9 is a graph illustrating the relationship between the coma-creativity, object distance v, and Ki, for the case when the lens is anastigmatic. It can be seen that Ki has a large influence on the coma-creativity: with Ki being negative, the absolute value of Ki should preferably be chosen as high as possible. In any case, it is preferred that Ki is lower than -1. Further, for practical reasons, it is preferred that absolute value of Ki, K ₂ is smaller than 5, more preferably smaller than 2.

For this specific embodiment, figure 10 is a graph illustrating the relationship between K ₂ , object distance v, and Ki, for the case when the lens is anastigmatic. It can be seen that Ki has a large influence on K ₂ : with increasing magnitude of Ki, also the magnitude of K ₂ increases.

Summarizing, from the above it can be concluded that an anastigmatic lens of this embodiment for instance has the following parameters: M = 1.4, R1/R2 = 0.84, n = 1.5, d = 0.8mm, Ki = 0, v = 3mm, K ₂ = -0.11, in which case the coma creativity will be equal to about 4 mλ/deg. The figures show how a variation of one parameter influences the required settings of the other parameters as well as the coma creativity obtained.

EMBODIMENT 2

In a second embodiment, the lens 400 has a positive magnification in the range 1 < M < 2. In that case, the focal length is positive. In contrast to the first embodiment, κl is now larger than +0.5, more specifically Ki > 1. For these conditions, it can be shown that

anastigmatic configurations can be found, but such configurations are more sensitive to small deviations (for instance caused by tolerances) from the optimum parameter settings.

EMBODIMENT 3 In a third embodiment, the lens 400 has a negative magnification in the range

1 < -M < 2. In that case, the focal length is negative, which is advantageous in some cases, for instance for application in a situation with space limitations, such as in a notebook. It can be shown that in this case an anastigmatic property of the pre-collimator lens can be obtained under the same conditions as embodiment 1, with the exception that the sign of κl should be positive.

Summarizing, the present invention provides an optical scanning apparatus 1 for writing/reading information into/from an object 2, comprising an optical system 30 for optically scanning such object. The system comprises at least two light beam generators 31; 131; 231 for generating corresponding light beams 32; 132; 232, and common optical components 33; 34 and individual optical components 138; 238 for guiding the light beams to an object, receiving reflected light and guiding the reflecting light beams to an optical detector 35; the common optical components are for guiding at least two of the light beams and the individual optical components are for guiding one single light beam. A common optical component is tilted with respect to the optical axis in order to compensate for disc tilt for one of the light beams, and at least one individual optical components is tilted to compensate for disc tilt for a corresponding other light beam.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

For instance, in the above it has been mentioned only that the pre-collimator lens is tilted. Instead of a tilt, it is also possible that the optical centre of the lens is shifted away from the optical axis to introduce coma. Also a combination of such shift with tilt is possible. Both possibilities will be covered by the wording that the lens has an alignment offset with respect to the optical axis.

Further, in the above description it is assumed that the two refractive surfaces 401, 402 of the lens 400 are aligned with each other. It is also possible that these two refractive surfaces 401, 402 are displaced and/or tilted with respect to each other.

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. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.