WO2007138539A1

WO2007138539A1 - A method for reducing noise in a radial tracking servo

Info

Publication number: WO2007138539A1
Application number: PCT/IB2007/051985
Authority: WO
Inventors: Bin Yin; Juil Lee
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-06-01
Filing date: 2007-05-25
Publication date: 2007-12-06
Also published as: CN101461000A; TW200809822A; JP2009539200A

Abstract

The present invention relates to a method reducing noise, such as inter-layer cross-talk, in the tracking error signal (TES) of the radial tracking servo (S) during closed loop operation. By subtracting the convolution of an updated filter (ƒ) and a noise signal (N) from the tracking error signal the noise is reduced. The optical apparatus includes a tracking servo (S) including a photo detector (DET) for generating a tracking error signal (TES). The method includes the steps of 1) updating a noise cancellation filter using a tracking actuator model (PM), 2) determining a noise cancellation signal using the noise cancellation filter and a noise signal, and 3) applying the noise cancellation signal to the tracking error signal (TES) for reducing noise in the tracking error signal. These steps are carried out during closed loop operation of the tracking servo. The method further includes switching between closed loop and open loop operation of tracking servo (S).

Description

A method for reducing noise in a radial tracking servo

The present invention relates to an optical recording apparatus capable of recording and/or reading information from/to an optical carrier wherein the recording apparatus performs radial tracking control. More particularly, the present invention relates to noise elimination in a radial tracking control system.

In an optical recording and/or reading apparatus, i.e. an optical drive, a radial tracking system is used for tracking the optical disc in a radial direction.

The tracking system may be based on so-called three-spot push-pull signals; however, the tracking system may also be based on other techniques, such as a one-spot push-pull signal. The three-spot push-pull signals are generated from three laser beams (a main scanning beam and two satellite beams), which after being reflected from the optical disc forms a main scanning spot and two satellite spots which are detected with segmented detectors. The outputs from the segmented detectors are processed to generate the three-spot push-pull signals. By further processing the three-spot push-pull signals a tracking error signal (TES) can be obtained. When the main scanning beam and the accompanying satellite beams are scanned across several tracks of the disc during e.g. a jumping, the tracking error signal provides information which enables e.g. counting the number of scanned tracks.

In multilayer discs having more than one information layer, such as double layer DVD discs and double or multilayer Blu-ray (BD) discs, laser beam reflections from different layers will cause interference on the segmented detector resulting in noise in the tracking error signal. Such interference noise, commonly known as inter-layer cross-talk, results in poor scanning performance of the tracking system. Noise, such as inter-layer crosstalk may be reduced by adding (or subtracting) a noise cancellation signal to the tracking error signal (TES). The noise cancellation signal may be obtained by estimating a filter by using the tracking error signal.

Generally, noise can be removed from a signal when the frequency spectrum of the noise is different than the spectrum of the signal. In tracking systems, the spectrum of the noise may be coincident, or at least overlapping, with the spectrum of the tracking error signal and, therefore, simple filtering is not useful for removing noise from the tracking error signal.

Moreover, during reading and/or recording from/to an optical disc the tracking system may be operated in closed loop control so that the tracking error signal is controlled to substantially zero. Since the tracking error signal is substantially zero elimination of inter- layer cross-talk by use of the above mentioned filtering results may not be effective.

WO 2005/0506630 discloses a method for suppressing cross-talk by providing an additional circuit for outputting improved satellite signals (S<+>, S< ->), which circuit suppresses cross-talk of the main track present in the satellite signals (S<+>,S<->) by minimizing a correlation between the satellite signals (S<+>,S<->) and the read signal (C), the improved satellite signals (S<+>, S< ->) being subsequently fed to the first circuit, which is arranged to suppress the cross-talk of the read signal (C) by minimizing a correlation between the improved read signal (C ) and the improved satellite signals (S<+>, S< ->). However, WO 2005/0506630 does not disclose solutions for reducing noise in the tracking error signal during closed loop operation of the tracking servo and is therefore not an optimal solution.

Hence, an improved optical recording and/or reading apparatus would be advantageous, and in particular a more efficient and/or reliable optical recording and/or reading apparatus would be advantageous.

Accordingly, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. In particular, it may be seen as an object of the present invention to provide a method for reducing or cancelling noise in a tracking error signal that solves the above mentioned problems of the prior art with removing noise in a tracking system operated in closed loop.

This object and several other objects are obtained in a first aspect of the invention by providing a method for operating an optical apparatus capable of recording and/or reading information to/from an optical carrier, said optical apparatus comprising, a tracking servo comprising a tracking controller intended for controlling a tracking actuator via a control output and said tracking servo further comprising a photo detector intended for generating a tracking error signal, where said method comprises the steps of, 1) updating a noise cancellation filter using a tracking actuator model, 2) determining a noise cancellation signal using the noise cancellation filter and a noise signal, and

3) applying the noise cancellation signal to the tracking error signal for reducing noise in the tracking error signal, wherein the steps of 1) updating the noise cancellation filter, 2) determining the noise cancellation signal, and 3) applying the noise cancellation signal are carried out during closed loop operation of the tracking servo, and wherein said method further comprises switching between a) a first switch state wherein the noise cancellation filter is updated during the closed loop operation of the tracking servo, and b) a second switch state wherein the noise cancellation filter is updated during an open loop operation of the tracking servo and wherein the noise cancellation filter is updated on basis of a measured signal.

The invention is particularly, but not exclusively, advantageous for reducing noise in a tracking servo of an optical recording and/or reading apparatus. In particular, the apparatus according to the present invention provides a more precise and thus more reliable optical recording and/or reading apparatus.

It is an advantage that the invention provides a method for updating a noise cancellation filter during closed loop operation of the tracking servo because such a method may reduce noise in the feedback signal of the tracking servo.

Since the noise in the feedback signal of the tracking servo may be reduced, the performance of the tracking servo may be improved. Thus, improved performance may be provided when reading and/or recording information data from/to the optical carrier. The improved performance may also lead to a lower risk for erroneous reading and/or recording. Furthermore, the improved performance may lead to increased reading and/or recording speeds. Additionally, the improved performance may lead to increased insensitivity to disturbances, such as shocks and the improved performance may lead to increased robustness against poor quality of optical carriers, e.g. discs having specifications being outside of the relevant standards. The capability of the method for reducing noise in the tracking error signal means that noises from different noise sources may be reduced. One such noise source is caused by radiation beams being reflected from different layers in a multi-layer disc. The reflections from different layers cause interference of the radiation beams on the photo detector which results in noise in the tracking error system. Accordingly, it may be an advantage that the method of the present invention is capable of reading and/or recording data from/to multilayer optical carriers, such as two layer discs, three layer discs, four layer discs or discs having even more than four layers. Moreover, the method of the present invention may provide advantages during reading and/or recording data from/to single layer optical carriers.

It is an advantage of the present invention that the reduction of noise in the tracking error signal obtained by application of the noise cancellation signal to the tracking error signal works irrespective of the root cause of the of the noise contributions. As such it can suppress noise-induced tracking offsets that are due to a variety of physical causes. It is an advantage to use a tracking actuator model since this makes the method independent, or at least partially independent, of measured outputs from the actuator.

By providing a switch for switching between a first switch state for updating the noise cancellation filter during closed loop operation, and a second switch state for updating the noise cancellation filter during open loop operation of the tracking servo, the method of the present invention may be capable of reducing noise during reading and/or recording from/to the optical carrier, as well as reducing noise during a jump of the optical pick-up unit from one track to another track.

The noise signal may be determined as a function of a first satellite push-pull signal and a second satellite push-pull signal. The function may comprise a filtering function or an amplification function or other suitable functions. It is an advantage to use a filtering function or an amplification function since filtering or amplification of the satellite push-pull signals may improve the noise cancellation filter, thereby, obtaining a further reduction of noise in the tracking error signal.

The noise signal may comprise a difference between a first satellite push-pull signal and a second satellite push-pull signal. It is an advantage that the difference between a first satellite push-pull signal and a second satellite push-pull signal may be inherently independent of the radial information in the tracking error signal and of the beamlanding tracking offset. It is another advantage that the difference between a first satellite push-pull signal and a second push-pull signal may have significant cross-correlation with the noise term in the tracking error signal since the noise basically results from the same radiation beam where only the optical paths of the radiation beams are different.

The noise cancellation filter may be substantially continuously updated during closed loop operation of the tracking servo, which is an advantage because the noise cancellation filter may be continuously adapted to variations in the noise in the tracking error signal. The variations in the noise in the tracking error signal may be caused by changes in the noise source and/or noise channel.

The noise cancellation filter may be an inter-layer cross-talk cancellation filter. It is an advantage that the noise cancellation filter is capable of reducing noise caused by inter-layer cross-talk as well as other noises.

The noise cancellation signal may be an inter-layer cross-talk cancellation signal being applied to the tracking error signal for reducing inter-layer cross-talk in the tracking error signal. It is an advantage that the noise cancellation signal is capable of reducing noise caused by inter-layer cross-talk as well as other noises. The step of 1) updating a noise cancellation filter using a tracking actuator model may comprise using a least-mean-square algorithm for minimising a function comprising a convolution of the tracking actuator model with the control output. It is an advantage to use a least-mean-square algorithm for minimising the function since it may provide a simple and effective equation for updating the noise cancellation filter. Other algorithms than least-mean- square algorithms, such a gradient algorithms, may be used for minimising the function.

The tracking error signal may be determined on the basis of a differential time detection (DTD) method. It is an advantage that the tracking error signal may be determined on the basis of the DTD method instead of the method based on satellite push-pull signals. The second switch state comprises, applying the noise cancellation signal to the tracking error signal thereby obtaining a noise-reduced tracking error signal, wherein the noise cancellation filter is updated by minimizing cross-correlation between the noise signal and the noise-reduced tracking error signal. It is an advantage that the method may also be used for reducing noise during open loop operation of the tracking servo in the second switch state, since the method then is operable both during open loop operation and closed loop operation of the tracking servo.

In a second aspect, the invention relates to an optical apparatus capable of recording and/or reading information to/from an associated optical carrier, said optical apparatus comprising, a tracking servo comprising a tracking controller intended for controlling a tracking actuator via a control output and said tracking servo further comprising an photo detector intended for generating a tracking error signal, said optical apparatus further comprising, 1) means for updating a noise cancellation filter using a tracking actuator model, 2) means for determining a noise cancellation signal using the noise cancellation filter and a noise signal, and

3) means for applying the noise cancellation signal to the tracking error signal for reducing noise in the tracking error signal, wherein the means of 1) for updating the noise cancellation filter, 2) for determining the noise cancellation signal, and 3) for applying the noise cancellation signal are arranged for being operated during closed loop operation of the tracking servo, and wherein said optical apparatus further comprises a switch for switching between a) a first switch state wherein the noise cancellation filter is updated by means during the closed loop operation of the tracking servo, and b) a second switch state wherein the noise cancellation filter is updated by means during an open loop operation of the tracking servo and wherein the noise cancellation filter is updated on basis of a measured signal. In a third aspect, the invention relates to a computer program product being adapted to enable a computer system comprising at least one computer having data storage means associated therewith to control an optical recording apparatus according to the first aspect of the invention.

This aspect of the invention is particularly, but not exclusively, advantageous in that the present invention may be implemented by a computer program product enabling a computer system to perform the operations of the second aspect of the invention. Thus, it is contemplated that some known optical apparatus may be changed to operate according to the present invention by installing a computer program product on a computer system controlling the said optical recording apparatus. Such a computer program product may be provided on any kind of computer readable medium, e.g. magnetically or optically based medium, or through a computer based network, e.g. the Internet.

The first, second and third aspect of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

The present invention will now be explained, by way of example only, with reference to the accompanying figures, where Fig. 1 is a schematic illustration of an optical recording/reading apparatus and an optical carrier according to the present invention.

Fig. 2 is a schematic illustration of the tracks in an information layer of an optical carrier according to the present invention. The spots from the radiation of the main scanning beam and the two satellite beams comprised are also shown.

Fig. 3 is an illustration of a photo detector according to the present invention.

Fig. 4 is a diagram of a tracking servo operated in open loop according to the present invention.

Fig. 5 is a diagram of a tracking servo operated in closed loop according to the present invention.

Fig. 6 is a diagram of a tracking servo S, according to the present invention, capable of switching between open loop and closed loop operation where an actuator model

PM is used for updating the noise cancellation filter / during closed loop operation.

Fig. 7 is flow-chart, according to the present invention, for illustrating the method according to the invention.

Figure 1 is a schematic illustration of an optical recording/reading apparatus and an optical carrier 1 according to the invention. The optical carrier 1 is fixed and rotated by holding means 30.

The optical carrier 1 may be a type which is capable of both recording data information to it and reading data information from it. Alternatively, the optical carrier 1 may be a type which only allows reading data information from it since data has been prerecorded. The optical carrier 1 comprises a material suitable for recording information by means of a radiation beam 5. The recording material may be of, for example, the magneto-optical type, the phase-change type, the dye type, metal alloys like Cu/Si or any other suitable material. Information may be recorded in the form of optically detectable regions, also called marks for rewriteable media and pits for write-once media, on the optical carrier 1.

The optical carrier 1 may have a single layer wherein data is recorded to and/or read from. Alternatively, the optical carrier 1 may have a plurality of layers, such as two layers or three layers. The apparatus comprises an optical pick-up unit OPU, the optical pick-up unit OPU being displaceable by actuation means 21, e.g. an electric stepping motor. The optical pick-up unit OPU comprises a photo detection system 10, a radiation source 4, a beam splitter 6, an objective lens 7, and lens displacement means 9. The optical pick-up unit OPU also comprises beam splitting means 22, such as a grating or a holographic pattern that is capable of splitting the radiation beam 5 into at least three beams. For reason of the clarity the radiation beam 5 is shown as a single beam after passing through the beam splitting means 22. The radiation beam 5 is reflected from the recording medium 1 into the reflected radiation beam 8 which also comprises at least 3 beams. The three beams of the incident radiation beam 5 and the reflected radiation beam 8 are referred to as two satellite beams and a main scanning beam.

The function of the photo detection system 10 is to convert the radiation beam 8 into electrical signals. Thus, the photo detection system 10 comprises several photo detectors, e.g. photodiodes, charged-coupled devices (CCD), etc., capable of generating one or more electric output signals that are transmitted to a pre-processor 11. The photo detectors are arranged spatially to one another, and with a sufficient time resolution so as to enable detection of focus and radial tracking errors in the pre-processor 11. Thus, the pre-processor 11 transmits focus and radial tracking error signals to the processor 50. The photo detection system 10 can also transmit a read and/or recording signal or RF signal representing the information being read from and/or recorded to the optical carrier 1.

The read signal and the recording signal are processed in the processor 50 as well in the pre-processor 11.

The radiation source 4 for emitting a radiation beam 5 can for example be a semiconductor laser with a variable power, possibly also with variable wavelength of radiation. Alternatively, the radiation source 4 may comprise more than one laser if e.g. the optical pick-up unit OPU does not comprise beam splitting means 22. In particular, the radiation source 4 may comprise three lasers, one laser for the main scanning beam and two lasers for the satellite beams.

The optical pick-up unit OPU is optically arranged so that the radiation beam 5 is directed to the optical carrier 1 via a beam splitter 6, and an objective lens 7. Radiation beam 8 reflected from the medium 1 is collected by the objective lens 7 and, after passing through the beam splitter 6, falls on a photo detection system 10 which converts the incident radiation beam 8 to electric output signals as described above. The processor 50 receives and analyses output data signals from the preprocessor 11 and the processor 50 transmits data signal to the pre-processor 11. The processor 50 can also output control signals to the actuation means 21, the radiation source 4, the lens displacement means 9, the pre-processor 11, and the holding means 30, as illustrated in figure 1. Similarly, the processor 50 can receive data, e.g. information to be written, indicated at 61, and the processor 50 may output data from the reading process as indicated at 60.

Figure 2 is a schematic illustration of the tracks 201, 202, 203 in an information layer 204 of an optical carrier 1. The tracks 201, 202, 203 are separated by the pitch p. The main scanning beam and the two satellite beams comprised by the radiation beam 5 shown in figure 1 form three spots 205, 206, 207 in the information layer 204. Thus, the main scanning beam forms the main spot 206 and the two satellite spots forms the satellite spots 205 and 207. The data information is read and/or recorded from/to the track 201 via the main scanning beam and spot 206. The satellite spots 205 and 207 may be directed to positions between tracks or the satellite spots 205 and 206 may be directed to tracks neighbouring to the track 201. Figure 2 shows that the satellite spot 205 is located between tracks 201 and 202 whereas satellite spot 207 is located between tracks 201 and 203. Alternatively, satellite spot 205 could be located on track 202 and satellite spot 207 could be located on track 203. A purpose of the satellite spots 205 and 207 is to ensure that the main spot 206 maintains correct tracking during reading and recording and to avoid the so-called beamlanding tracking offsets as is well-known in the art.

Figure 3 is an illustration of the photo detector DET comprised by the photo detection system 10. The photo detector DET comprises photo detectors units DA, DB and DC. The radiation beam 8 being reflected from the optical carrier 1 forms spots SA, SB and SC on photo detectors DA, DB and DC, respectively. Spot SA is generated by the reflection of the main scanning beam whereas the spots SB and SC are formed by the reflections of the satellite beams.

The satellite detectors DB and DC are segmented into segments Bl, B2 and Cl, C2, respectively. Main detector DA is segmented into segments Al, A2, A3 and A4. Satellite detectors DB and DC generate satellite push-pull signals PPb and PPc, respectively. The push-pull signals PPb and PPc are equivalent to the difference of radiation power incident on segments Bl, B2 and C2, C2, respectively. Main detector DA generates signals that are equivalent to the sum of power incident on the respective segments Al, A2 and A3, A4, and by subtracting these sums the push-pull signal PPa is established. Thus, push-pull signal PPa corresponds to the push-pull signals PPb and PPc.

The optical power of the satellite beams are a factor g smaller than the optical power of the main scanning beam. The factor g may be within the range from 1/5 to 1/50, such as 1/15. In order to compensate for lower power of the satellite beams as compared to the main scanning beam the push-pull signals PPb and PPc are amplified with a factor l/2g via amplifiers 310 and 311. The amplified push-pull signals PPb and PPc and the push-pull signal PPa are subtracted to generate a tracking error signal TES. The tracking error signal TES enables the main scanning spot 206 to follow the track 201 during reading and/or recording from/to the optical carriers. Similarly, the tracking error signal TES enables a jump of the main scanning spot from one track to another track by counting the numbers of so- called S-curves.

Instead of the push-pull detection method, the differential time (or phase) detection (DTD or DPD) method may be used. The DTD detection method is based on the difference of radiation power incident on segments A2, A4 and Al, A3. The push-pull signals PPa, PPb, PPc can be written as

PP_a = Aύn§ + B

PP_b = -gAsm$ + gB (eq. 1)

PP_C = -gA sin § + gB

where A is the amplitude, B is the tracking off-set known as beamlanding and where φ =

2πy/p with y being the radial position on the optical carrier 1 and p the track pitch. Thus, the tracking error signal TES is given as

TES = PP_a - {PP_b + PP_c)l2g

= 2 A sin φ

which shows that all effects of beamlanding are cancelled in the three-spot push-pull method.

A problem arises when additional noise sources are present that are not equal for all detector parts DA, DB, and DC, i.e. if: PP_a = Asin$ + B + N_a

PP_b = -gA sin § + gB + gN_b (eq. 3)

PP_C = -gA sin § + gB + gN_c

where N_a ≠ Nb ≠ N_c. The situation with different noises N_a ≠ Nb ≠ N_c can occur in optical carriers with more than one information layer. Radiation beams 5 reflected from one or more out-of-focus information layers can end up at the detectors DA, DB and DC and give rise to interference there. The interference, or so-called inter-layer cross-talk, results in noises N₀, Nb and Nc in the push-pull signals PPa, PPb and PPc. The noises N₀, Nb and N_c are different for detectors DA, DB and DC which gives a tracking error signal with an offset:

TES = 2Asin$ + N_a - (N_b + N_c )/2 (eq. 4)

Since the offset N_a - (N_b + N_c)/2 is of low frequency and normally within the servo working bandwidth, it is difficult or even impossible to eliminate by a simple filter, such as a high pass filter. Figure 4 is a diagram of a tracking servo SOL operated in open loop, such as during a jump of the main spot 206 from one track to another track. Furthermore, the diagram in figure 4 illustrates a method for reducing noise, such as inter-layer cross-talk, during open loop control of a tracking servo.

The push-pull signals PPa, PPb, PPc are combined in the combination unit 401 to create the tracking error signal TES. The noise signal N is created by determining the difference between the satellite push-pull signals PPb and PPc in the subtraction unit 402.

Thus, the noise signal N has the form

N = PP_b - PP_c = g N_b - g N_c (eq. 5)

Alternatively, the noise signal N may be determined as a function of the satellite push-pull signals PPb and PPc, such as a filter function h of the difference between PPb and PPc:

N = h * (PP_b - PP_c) = h * (g N_b - g N_c) (eq. 5a) where * denotes the convolution operator and where the filter h may be a FIR filter, a low pass filter, a high pass filter or a notch filter. The noise signal N may also be determined as an amplification function k of the difference between PPb and PPc:

N = kx(PP_b - PP_c) = kx(g N_b - g N_c) (eq. 5b)

where x denotes a scalar multiplication and where k is a scalar.

A least-mean-square-error (LMS) algorithm is used to iteratively find the coefficients of the noise cancellation filter / . When the noise cancellation filter / is time varying due to, for example, that the noise channel characteristics are not constant within one carrier revolution, the algorithm in the LMS-based adaption unit 403 is able to follow the variation of / by its adaptive nature. To enable the adaptation, a target function (or cost function) needs to be defined that, in this case, can be:

J(/) = (TESXTC x N)² (eq. 6)

the instantaneous form of the correlation between TES_XTC and the noise signal N. TES_XTC represents the phase-corrected version of TES_XTC that is filtered in the phase correction unit 404 by the sensitivity transfer function of the underlying tracking servo loop. This is to align the signal TES_XTC in phase with the noise signal N, which is necessary for the stability of the adaptation. According to the gradient descending rule, the update of the filter / (filter unit 406) in the discrete domain becomes

Λk + l) = ftk) + μ x(~ \_k ) (eq. 7)

where μ is a positive constant controlling the update speed and the stability. The filtered noise signal is then combined with the tracking error signal TES in a combination unit 405 to create TES_XTC- The method for reducing noise during open loop operation of the tracking servo SOL work properly only under the assumption that in the noise signal N there is not any component that has correlation with the track error signal Asinύ? . The assumption is essential since otherwise the filter / used for cross-talk cancellation will get updated towards the direction that part of the useful tracking error signal y4sinφ in tracking error signal TES is eliminated.

Figure 5 is a diagram of a tracking servo S operated in closed loop where the closed loop is provided with a reference R equal to zero. The closed loop tracking servo comprises a controller C, such as a PID controller. The controller generates a control signal u being supplied to the actuator P. The actuator P comprises a radial tracking actuator 21 for coarse adjustment of the optical pick-up unit OPU in radial direction. The actuator P also comprises lens displacement means 9 for fine radial adjustment of the objective lens 7. The output of the actuator P is in the form of a radial displacement of the radiation beam 5, which radial displacement is generate by the tracking actuator 21 and the lens displacement means 9. The radial displacement of the optical pick-up unit displaces the radiation beam 5 radiated out from the optical pick-up unit OPU. The outputted laser beams are reflected from any of the layers 204 of the optical carrier 1 and generates an input, in the form of the two satellite spots and the main scanning spot, to the photo detector DET. The photo detector DET comprises segmented detectors DA, DB, DC for measuring the main scanning spot and the satellite spots and processing units, such as adding units, subtraction units and amplifiers 310,311, for generating the tracking error signal TES.

In the signal path of the laser beams between the output of the actuator P and the input to the detector DET, the laser beam signal is affected by real noise NR, such as inter-layer cross-talk. The real noise NR is modelled with noise signal N being inputted to the filter f which is equivalent to the convolution f * N of the noise signal N with the filter f. The real noise NR is added to the tracking error signal TES via adding unit 510. The tracking error signal is also affected by other noises sources d which are independent of the noise signal N. In order to eliminate or reduce the real noise NR, the noise signal N can be inputted to an estimated noise cancelling filter / and subtracted from the tracking error signal TES as illustrated by the subtracting unit 511 so as to obtain a tracking error signal TES_XTC having reduced or eliminated content of real noise NR. The noise-reduced tracking error signal TES_XTC is subtracted from the reference R and inputted to the controller C with the result that TES_XTC is controlled to substantially zero.

Clearly, the photo detector units DA, DB, DC of the photo detector DET can have other forms than shown in figure 3. For instance the photo detector units DA, DB and DC need not be separate units, but could be combined into a single unit. Also, any of the photo detector units DA, DB and DC could be a SI photodiode, a PSD detector, a CCD chip or other diode or image detectors. The adding unit represented by adding unit 510 can also be a subtracting unit and, similarly, the subtracting unit represented by subtracting unit 511 can also be an adding unit, in case of opposite polarities of the relevant signals. Since TES_XTC is controlled to zero or substantially zero the cross-correlation in the cost function J(/) = (TES_x^ xN)² used for updating/ , as described in connection with the open loop operation of the tracking servo S in figure 4, will also be zero or substantially zero. Since J(/) is zero or substantially zero it is impossible to get the estimate / of the noise channel, such as a cross-talk channel. Accordingly, the noise-reduced tracking error signal TES_XTC is not useful for updating the estimated filter / during closed loop operation of the tracking servo (S).

From the diagram in figure 5 the following relation can be found

TES_XK = P * u + N * (f - f) + d , (eq. 8)

and since TES_XTC is controlled to zero the following approximation can be established

P * u ~ N * {f - f) - d (eq. 9)

From the above equation (eq. 9) it is seen that the noise, such as cross-talk information, indicated by N, exists in the output of the actuator P * u even when the tracking servo loop is closed. Thus, the cost function J{f) can be reconstructed as

J(f) = [(P * u)x N]² (eq. 10)

or

Af) = (P * U)² (eq. 11)

By using J{f) from the above equation (eq. 11) the estimated filter / can updated by as follows L_+ι = L - v^χ(P* u)_k ^χN_k (eq. 12)

where the above equation (eq. 12) is derived by minimizing J{f) = {P * u)² (eq. 11) according to a least-mean-square (LMS) algorithm. The estimated filter / denotes a vector of estimated noise channel coefficients, such as cross-talk channel coefficients, at moment k , N__k a vector of corresponding noise signal samples corresponding to

N = PP_b - PP_C = g N_b - g N_c (eq. 5), and μ a positive factor controlling the update speed and stability. Substituting eq. 9 into eq. 12 and using the fact of d being independent of N , one has

L₊i = L - V^x\-^N* (f -f)]_k xK_k , (eq- 13)

which basically tells that the update stops when f = f , meaning that the noise component / * TV is eliminated from the tracking error signal TES before it is fed back into the controller C. In practice the noise cancellation filter / is substantially continuously updated. The noise cancellation filter / is continuously updated by using analog electronic circuits, however when digital electronic circuits are used for updating the noise cancellation filter / the filter is substantially continuously updated at discrete sampling points k. In practice, the signal P * u used in /_i+i = f _k - μx(P * u)_k x N_k (eq. 12) for

updating the noise cancellation filter / is normally unavailable, however, the noise cancellation filter / can be updated by using a tracking actuator model PM. The tracking actuator model PM is a model of the physical actuator P. The tracking actuator model PM can be modelled using a second order model or other suitable mathematical modelling methods.

Figure 6 is a diagram of a tracking servo S operated in closed loop where an actuator model PM is used for updating the noise cancellation filter / . Thus, a modelled equivalent PM * u of the signal P * u is constructed by convolving the output signal u of the controller C with a model of the actuator PM. The dotted rectangle indicated by XTC encloses features used for updating the noise cancellation filter.

The modelled equivalent PM * u and the noise signal N is inputted to the Least Mean Square based updating unit LMS via switch state 615 and input 613. The updating unit LMS updates / according to f _k+γ = /_t - μx(P* «)_i xiV_i (eq. 12) and

outputs an updated noise cancellation filter / , such as an inter-layer cross-talk cancellation filter. The capability of the filter / to be updated is indicated by arrow 612. The updated noise cancellation filter / is convolved with the noise signal N and subtracted from the tracking error signal TES via subtracting symbol 611 so that a noise-reduced or a noise- eliminated tracking error signal TES_XTC is obtained.

Since the feedback signal FS is free of real noise NR or at least has a reduced content of real noise NR the performance of the closed loop servo is not degraded as compared to the case where no noise cancellation signal is applied to the tracking error signal TES during closed loop operation. Clearly, if the real noise NR in the tracking error signal TES is correctly modelled by the convolution f * N of the noise signal N with the filter f, the real noise NR is completely eliminated given that estimated noise cancellation filter / is equal to filter f. The diagram in figure 6 is supplied with a switch 632 having a first switch state 615 and a second switch state 616. When the noise cancellation filter / is updated during the closed loop operation of the radial tracking servo (S) using the tracking actuator model (PM) the first switch state 615 is used.

However, when open loop operation of the tracking servo is required, for instance by so-called jumping between tracks where the optical pick-up unit OPU is scanned across several tracks, the second switch state 616 is used. In order to update the noise cancellation filter / during open loop operation, the noise reducing method described in connection with figure 4 has to be used. Therefore, in the second switch state 616 the Least Mean Square based updating unit LMS is provided with the noise-reduced tracking error signal TES_XTC and the noise signal N via switch state 616 and input 613, respectively. Thus, when the switch 632 is in the second switch state 616, the updating unit LMS updates / (eq. 7) and the method described in connection

with figure 4.

When switching from the closed loop operation in the first switch state 615 to the open loop operation in second switch state 616, the feedback signal FS must be inactivated or the feedback connection 621 must be opened. This may be done electronically by disabling the feedback signal FS or opening an electronic switch in the feedback connection 621. Alternatively, the feedback may be disabled by opening a switch placed in the feedback connection 621.

When switching from the closed loop operation in the first switch state 615 to the open loop operation in second switch state 616 the input to the controller C may set to zero and the actuator P may be supplied with a drive signal u, where the drive signal u is provided from a signal generator (not shown), which signal generator may also be responsible for generating the reference R. For instance, the actuator P may be provided with a drive signal u during open loop operation, where the drive signal u is equal to or proportional to the reference R.

Figure 7 is flow-chart for illustrating the method according to the invention. In a first decision step Dl, it is determined if the radial tracking servo (S) is operated in closed loop or in open loop. If the radial tracking servo (S) is operated in closed loop then proceed with step Sl. If the radial tracking servo (S) is operated in open loop then proceed with step S4.

In step S 1 , the noise cancellation filter ( / ) is updated using the tracking actuator model PM on basis of / = / - μ x (P * u)_k x N__k (eq. 12). Proceed with step S2.

In step S2, the noise cancellation signal / * N is determined using the noise cancellation filter / and the noise signal N. Proceed with step S3. In step S3, the noise cancellation signal / * N is applied to the tracking error signal TES for reducing noise in the tracking error signal TES. Go to decision step Dl. In step S4, the noise cancellation filter (/) is updated on basis of

signal (N) and the noise-reduced tracking error signal (TES_XTC). GO to step S2. Although the present invention has been described in connection with the specified embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term "comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to "a", "an", "first", "second" etc. do not preclude a plurality. Furthermore, reference signs in the claims shall not be construed as limiting the scope.

Claims

CLAIMS:

1. A method for operating an optical apparatus capable of recording and/or reading information to/from an optical carrier, said optical apparatus comprising, a tracking servo (S) comprising a tracking controller (C) intended for controlling a tracking actuator (P) via a control output (u) and said tracking servo further comprising a photo detector (DET) intended for generating a tracking error signal (TES), where said method comprises the steps of,

1) updating a noise cancellation filter (/ ) using a tracking actuator model (PM),

2) determining a noise cancellation signal (f * N ) using the noise cancellation filter (/) and a noise signal (N), and 3) applying the noise cancellation signal ( / * N ) to the tracking error signal

(TES) for reducing noise in the tracking error signal (TES), wherein the steps of 1) updating the noise cancellation filter (/), 2) determining the noise cancellation signal (f * N ), and 3) applying the noise cancellation signal (TES) are carried out during closed loop operation of the tracking servo (S), and wherein said method further comprises switching between a) a first switch state (615) wherein the noise cancellation filter (/) is updated during the closed loop operation of the tracking servo (S), and b) a second switch state (616) wherein the noise cancellation filter (/) is updated during an open loop operation of the tracking servo (S) and wherein the noise cancellation filter is updated on basis of a measured signal.

2. A method according to claim 1, wherein the noise signal (N) is determined as a function of a first satellite push-pull signal (PPb) and a second satellite push-pull signal (PPc).

3. A method according to claim 1, wherein the noise signal (N) comprises a difference between a first satellite push-pull signal (PPb) and a second push-pull signal (PPc).

4. A method according to claim 1 or 2, wherein the noise cancellation filter (/ ) is substantially continuously updated during closed loop operation of the tracking servo (S).

5. A method according to claims 1 or 4, wherein the noise cancellation filter (/ ) is an inter- layer cross-talk cancellation filter (/).

6. A method according to claim 1 , wherein the noise cancellation signal (f * N ) is an inter-layer cross-talk cancellation signal being applied to the tracking error signal (TES) for reducing inter-layer cross-talk in the tracking error signal (TES).

7. A method according to claim 1, wherein the step of 1) updating a noise cancellation filter ( / ) using a tracking actuator model (PM) comprises using a least-mean- square algorithm for minimising a function comprising a convolution of the tracking actuator model (PM) with the control output (u).

8. A method according to claim 1, wherein the tracking error signal (TES) is determined on basis of a differential time detection (DTD) method.

9. A method according to claim 1, wherein the second switch state (616) comprises, applying the noise cancellation signal (/ * N ) to the tracking error signal (TES) thereby obtaining a noise-reduced tracking error signal (TES_XTC), wherein the noise cancellation filter (/) is updated by minimizing cross-correlation between the noise signal

(N) and the noise-reduced tracking error signal (TESXTC).

10. An optical apparatus capable of recording and/or reading information to/from an associated optical carrier (1), said optical apparatus comprising, a tracking servo (S) comprising a tracking controller (C) intended for controlling a tracking actuator (P) via a control output (u) and said tracking servo further comprising an photo detector (DET) intended for generating a tracking error signal (TES), said optical apparatus further comprising, 1) means (630) for updating a noise cancellation filter (/ , 631) using a tracking actuator model (PM),

2) means (631) for determining a noise cancellation signal (f * N ) using the noise cancellation filter (/ , 631) and a noise signal (N), and 3) means (611) for applying the noise cancellation signal ( / * N ) to the tracking error signal (TES) for reducing noise in the tracking error signal (TES), wherein the means of 1) (630) for updating the noise cancellation filter (/), 2)

(631) for determining the noise cancellation signal (f * N ), and 3) (611) for applying the noise cancellation signal (TES) are arranged for being operated during closed loop operation of the tracking servo (S), and wherein said optical apparatus further comprises a switch (632) for switching between a) a first switch state (615) wherein the noise cancellation filter (/ , 631) is updated by means (630) during the closed loop operation of the tracking servo (S), and b) a second switch state (616) wherein the noise cancellation filter (/ , 631) is updated during an open loop operation of the tracking servo (S) and wherein the noise cancellation filter (/ , 631) is updated on basis of a measured signal.

11. A computer program product being adapted to enable a computer system comprising at least one computer having data storage means associated therewith to control an optical recording and/or reading apparatus according to the method as claimed in claim 1.