US20060181975A1

US20060181975A1 - Cross-talk cancellation scheme for rll-based storage systems

Info

Publication number: US20060181975A1
Application number: US10/552,059
Authority: US
Inventors: Alexander Padiy; Bin Yin
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2003-04-07
Filing date: 2004-03-23
Publication date: 2006-08-17
Also published as: EP1467498A1; US7447514B2; DE60304448T2; DE60304448D1; ATE322770T1; US20040198370A1; EP1467498B1

Abstract

The invention relates to Run length Limited-codes storage systems. In modern storage systems, the inter-track spacing is chosen to be relatively small to allow for high storage densities. As a result, when reading a target track, data written on side tracks may appear in the recovered signal. This inter-track interference is called cross-talk. The invention proposes a cross-talk cancellation scheme based on the minimization of the mismatch between the actual (dm+1m) and the expected (exp) run length between two transitions (xm, xm+1) of the (dm+1,m) signal. The proposed solution significantly improves the ramp-up properties of the receiver and allows more efficient hardware implementation.

Description

FIELD OF THE INVENTION

The present invention relates to a cross-talk cancellation method, a computer program for implementing a cross-talk cancellation method, a signal processor comprising cross-talk cancellation means, and an apparatus for reading a signal stored along a track on a storage medium, said apparatus comprising cross-talk cancellation means.
The present invention relates to storage systems in which data are stored along tracks on a storage medium. In modern storage systems, the inter-track spacing is chosen relatively small to allow for high storage densities. As a result, when reading a target track, data written on side tracks may appear in the recovered signal. This inter-track interference is called cross-talk.
The invention is advantageously used in such storage systems to improve the recovered signal by removing the cross-talk. For example, the invention applies to optical storage systems (DVD, Blu-ray Disc, Small Form Factor Optical Disc . . . ), magnetic storage systems (hard disks notably), magneto-optical storage systems.
With optical storage systems, the cross-talk is even more severe when radial tilt is present in the system because then the optical spot extends more onto the side tracks.

BACKGROUND OF THE INVENTION

A cross-talk removing device is described in U.S. Pat. No. 6,134,211. This device has three reading elements simultaneously reading a main track and two adjacent tracks. The three signals that are read by the three reading elements are sampled so as to provide three sequences of samples. A cross-talk removing circuit applies adaptive signal processing (for example an LMS adaptive algorithm) to the three sequences of samples to produce a cross-talk-removed sequence of samples associated with the main track that is free of cross-talk components from the adjacent tracks.
The adaptive processing comprises an adaptive filtering, the filter coefficients being updated so as to converge to zero an error value present in the cross-talk-removed sequence of samples.
This convergence is achieved by using a reference sample extracting circuit. When the values of three successive samples transit from positive to negative or from negative to positive, the reference sample extracting circuit extracts the central sample value of three successive sample values. The extracted sample value is supplied to a subtractor that calculates the difference between the extracted sample value and a reference value. This difference is used as the error (e) that has to be converged to zero to update the filter coefficients.
In this scheme it is assumed that the central sample value is the sample value at ideal zero-crossing time. This assumption can only be made if the samples are bit-synchronous samples.
In U.S. Pat. No. 6,134,211 this is achieved by running the analog-to-digital converters and the cross-talk removing circuit on a clock that is driven by a time recovery circuit. As a consequence, the cross-talk cancelling scheme is only operational when the time recovery circuit has acquired both the frequency and the phase lock.
In U.S. Pat. No. 6,134,211, when the sample sequences remain in an asynchronous state (that is, when the time recovery circuit is not locked), the sequences are filtered on the basis of fixed predetermined coefficients. This helps to avoid divergence of the filter coefficients but leads to a ramp-up problem: if the time recovery circuit cannot converge because of strong cross-talk, the cross-talk cancellation scheme will remain inefficient and the system will be stuck.
Another problem of the prior art system is that it is hardly compatible with asynchronous receiver architectures.
In such asynchronous architectures, the analog-to-digital converters and the filters are run on a fixed clock. A transition from the fixed clock domain to the bit-synchronous domain is done at the output of the cross-talk cancellation circuit by means of a sample rate converter controlled by a time recovery circuit. An additional sample rate controller, locked to the time recovery circuit, would be needed for each adjacent track to produce the bit-synchronous samples that are needed to derive the above-described error (e).
Moreover, if the fixed clock (at which the filters are running) and the clock driven by the time recovery circuit (at which the filters coefficients are updated) differ substantially from each other, inverse sample rate converters would also be required to interpolate the filter coefficients from the domain of the clock driven by the time recovery circuit to the domain of the fixed clock.
This would lead to an increased complexity of the architecture.

SUMMARY OF THE INVENTION

One of the objects of the invention is to propose a solution for cross-talk cancellation that solves the above-mentioned problems.
This is achieved with a cross-talk cancellation method as claimed in claim 1, a program as claimed in claim 2, a signal processor comprising cross-talk cancellation means as claimed in claims 3 to 5, and an apparatus for reading a signal stored along a track on a storage medium as claimed in claims 6 to 8.
The cross-talk cancellation means according to the invention are intended for receiving a main signal associated with a target track and satellite signals associated with side tracks, said main signal showing transitions and runs of various lengths between two transitions. They comprise:
filtering means for filtering the satellite signals with adaptive filters, thereby generating filtered versions of the satellite signals,
updating means for updating the coefficients of the adaptive filters by minimizing the mismatch between the actual and the expected run length between two transitions of the main signal,
processing means for generating an improved main signal from said main signal by subtraction of said filtered versions of the satellite signals.
According to the invention the error that is to be minimized when updating the filter coefficients is the mismatch between the actual and the expected run length between two transitions of the main signal. Contrary to the prior art minimization scheme, the minimization scheme of the invention does not use the notion of ideal transition time. Therefore it does not require the use of bit-synchronous samples. The proposed minimization scheme requires frequency lock but not phase lock.
A first advantage of the proposed minimization scheme is that it resolves the above-mentioned ramp-up problem.
A second advantage of the proposed minimization scheme is that it can be implemented in an asynchronous architecture without any additional hardware complexity.
Advantageously, when used in an asynchronous receiver having a bit clock that is driven by a time recovery circuit, the cross-talk cancellation means of the invention are operated at a fixed clock that is asynchronous with respect to this bit-clock.
In such a case, if the bit clock frequency is different from the fixed clock frequency, additional time recovery means are provided to derive the ratio between the bit clock frequency and the fixed clock frequency, said ratio being used by said updating means for updating said coefficients.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be further described with reference to the following drawings:
FIGS. 1 and 2 are functional block diagrams of examples of an apparatus according to the invention for reading a storage medium;
FIGS. 3 and 4 are schematic representations of a first and a second configuration of tracks and light spots used in a 3-spot cross-talk cancellation scheme;
FIG. 5 is a functional block diagram of the cross-talk cancellation means according to the invention;
FIG. 6 is a schematic representation of a received signal showing two transitions and a run between the two transitions.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention applies to storage media having tracks each forming a 360° turn of a spiral line. Encoded data are recorded along the tracks. The encoding scheme that is used in optical recording system is a Run Length Limited encoding scheme (RLL). When the data recorded along the tracks are encoded with an RLL encoding scheme, the tracks exhibit marks corresponding to runs of a same value, and the edges of a mark correspond to a transition between two runs. The size of the mark corresponds to the length of the run. It is an integer multiple of a reference unit size mark.
FIGS. 1 and 2 show block diagrams of a first and a second example of an apparatus for reading such a disc. The apparatus shown in FIG. 1 carries reference number 6-1. The apparatus of FIG. 2 carries reference number 6-2. According to FIGS. 1 and 2, the apparatuses 6-1 and 6-2 comprise an optical unit 8 having three reading elements: a main reading element 12 for reading a main signal associated with a main track, and two satellite reading elements 11 and 13 for reading two satellite signals associated with the two tracks that are adjacent to the main track. In the subsequent description, one of these satellite signal is called upper satellite signal, and the other satellite signal is called lower satellite signal. The three reading elements transmit three light spots 21, 22 and 23.
FIGS. 3 and 4 show the locations of the three light spots 21, 22 and 23 with respect to the three tracks to be read 31, 32 and 33. The main light spot 22 is centered on the main track 32. The satellite light spots 21 and 23 may be centered either on the satellite tracks 31 and 33 as represented in FIG. 3, or between the main track 32 and the adjacent tracks 31 and 33 as represented in FIG. 4. The satellite signals read by the satellite light spots 21 and 23 in FIGS. 3 and 4 are said to be “associated with” the adjacent tracks because the light spots 21 and 23 overlap with at least part of the adjacent tracks.
The embodiment of FIG. 4 is advantageous for rewritable optical disc systems because it allows reusing the 3-spot push-pull radial tracking means which are currently available in all such systems (the signal read by the reading elements 11, 12 and 13 and the main light spots 21, 22 and 23 can be used both for tracking and for cross-talk cancellation).
Returning to FIGS. 1 and 2, the three signals that are read by the three reading elements 11, 12 and 13 are input to a signal processor 40 comprising cross-talk cancellation means 42 and decoding means 44. The signal produced by the decoding means 44 is input to a reproduction circuit 46 that generates an output signal (for example an audio or a video signal).
FIG. 5 is a functional representation of the cross-talk cancellation means 42. The cross-talk cancellation means 42 comprise three analog-to- digital converters 51, 52 and 53 for sampling the main signal, the upper satellite signal and the lower satellite signal. The three analog-to- digital converters 51, 52 and 53 operate at a fixed clock 55 and generate a sequence of main samples 62, a sequence of lower satellite samples 61, and a sequence of upper satellite samples 63. The sequences of lower and upper satellite samples 61 and 63 are processed by a lower adaptive filter 71 and an upper adaptive filter 73, respectively, which generate a filtered version 81 of the sequence of lower satellite samples and a filtered version 83 of the sequence of upper satellite samples. The sequence 62 of main samples is processed by an optional equalizer 90, which generates an equalized sequence of main samples 92. Then a subtractor 93 subtracts the filtered version 81 of the sequence of lower satellite samples and a filtered version 83 of the sequence of upper satellite samples from the equalized sequence 92 of main samples, thereby generating an improved sequence of main samples 102.
Alternatively, if the equalizer 90 is omitted, the improved sequence of main samples 102 is generated by subtraction of the filtered version 81 of the sequence of lower satellite samples and a filtered version 83 of the sequence of upper satellite samples from the sequence 62 of main samples.
The improved sequence of main samples 102 is input to a sample rate converter 120 driven by a time recovery circuit 130 (for example a Phase Lock Loop circuit). The output of the sample rate converter 120 is the input of the decoding means 44.
The improved sequence of main samples 102 and the sequences of lower and upper satellite samples 61 and 63 are processed by lower and upper coefficient updating means 111 and 113. The lower and upper coefficient updating means 111 and 113 update the respectively coefficients used by the lower filter 71 and by the upper filter 73.
The behaviour of the cross-talk cancellation means 42 can be formalized by the following mathematical expression: $\begin{matrix} {\tilde{C}}_{m} = C_{m} - \sum_{k} f_{k}^{+} S_{m - k}^{+} - \sum_{k} f_{k}^{-} S_{m - k}^{-} & (equation 1) \end{matrix}$
where:
S_m ⁺ is the sample m of the upper satellite signal;
S_m ⁻ is the sample m of the lower satellite signal;
C_mis the sample m of the main signal;
f_k ⁺ are the coefficients of the upper filter and f_k ⁻ are the coefficients of the lower filter;
{tilde over (C)}_mis the improved main sample m obtained at the output the subtractor 93.
Advantageously, the algorithm used to update the filter coefficients is the LMS algorithm (Least Mean Square). According to the invention the driving term Z_mof the algorithm (that is the term to be minimized) is the mismatch between the actual and the expected run length between two transitions of the main signal.
This means that: $\begin{matrix} {(f_{k}^{\pm})}_{m + 1} = (1 - μ) {(f_{k}^{\pm})}_{m} - μ \frac{\partial}{\partial f_{k}^{\pm}} {(Z_{m})}^{2} & (equation 2) \end{matrix}$
The cross-talk minimization scheme of the invention will be described below with reference to FIG. 6. As will be apparent from this description, for the proposed scheme to operate properly, the ratio α between the PLL-driven bit clock and the fixed clock that runs the analog-to- digital converters 51, 52 and 53 must be available. In FIG. 5, two arrows 141 and 143 indicate that the frequency ratio α is supplied to the first and second coefficient updating means 111 and 113. The arrows 141 and 143 are represented in dashed lines because they may be omitted if the frequency ratio α is equal to 1.
The ratio α is advantageously supplied by a time recovery circuit external to the cross-talk cancellation means 42 and specifically designed for fast approximate recovery of the bit frequency. Such an external time recovery circuit is already present in most reading apparatuses.
For example, in some systems (mostly in writable/rewritable systems), the wobble clock can be conveniently used for estimating the ratio α. FIG. 1 gives an example of an implementation that will be advantageously used in such systems: in FIG. 1, the external time recovery circuit carries reference number 50-1 and is connected In the path between the main reading element 12 and the cross-talk cancellation means 42. In other systems (mostly in ROM systems), average run length measurements can be used for the same purpose. FIG. 2 gives an example of an implementation that will be advantageously used in such systems: in FIG. 2, the external time recovery circuit carries reference number 50-2 and is connected In the path between the cross-talk cancellation means 42 and the decoding means 44.
FIG. 5 is a schematic representation of the received main signal. Two successive transitions X_mand X_m+1, are represented.
{tilde over (C)}_(m,L)is the improved main sample on the left of the transition X_m;
{tilde over (C)}_(m,R)is the improved main sample on the right of the transition X_m;
{tilde over (C)}_(m+1,L)is the improved main sample on the left of the transition X_m+1;
{tilde over (C)}_(m+1,R)is the improved main sample on the right of the transition X_m+1;
{tilde over (C)}_(m,L)+1is the improved main sample that precedes sample {tilde over (C)}_(m,L);
{tilde over (C)}_(m,L)+1is the improved main sample that follows sample {tilde over (C)}_(m,L);
{tilde over (C)}_(m,R)−1is the improved main sample that precedes sample {tilde over (C)}_(m,R);
{tilde over (C)}_(m,R)+1is the improved main sample that follows sample {tilde over (C)}_(m,R);
φ_mis the time interval between the ideal time of the transition X_mand the actual time of the transition X_m(in FIG. 5, φ_m<0);
φ_m+1is the time interval between the ideal time of the transition X_m+1and the actual time of the transition X_m+1(in FIG. 5, φ_m+1<0);
d_m+1,mis the actual run length between the two transitions X_mand X_m+1.
In the following it is assumed for simplification purposes, without loss of generality, that:
the time interval between two samples is equal to 1,
the transition moment m=1 corresponds to a rising transition,
and the time interval φ_mtakes values from the interval $[- \frac{1}{2}, + \frac{1}{2}] .$
A first implementation of the updating scheme of the invention will now be described. This first implementation is applicable when the fixed system clock is (nearly) equal to the PLL driven bit clock (that is when the ratio α is close to 1), but there is no phase lock between the two clocks.
The LMS driving parameter to be minimized Z_mis chosen to be equal to the difference between the actual run length d_m+1,mand the expected run length d_m+1,m ^(exp). Taking into account that an integer number of clock intervals should ideally fit between the transitions in the RLL encoded signal when there is no inter-symbol interference and no clock frequency variations, d_m+1,m ^(exp)can be approximated as d_m+1,m ^(exp)=round(d_m+1,m) where round(x) is defined as the integer number that is closest to the real number x.
Thus:
Z _m=ζ(d _m+1,m)
where ζ(x)=x−round(x)
with d _m+1,m=[(m+1, L)−(m,L)]+φ_m+1−φ_m (equation 3)
where [(m+1,L)−(m,L)] denotes the integer number of sampling intervals between the samples {tilde over (C)}_(m,L)and {tilde over (C)}_(m+1,L).
In the following it is assumed that the cross-talk is not extremely large, so that for small variations of the filter coefficients $\langle ζ (d_{m + 1, m}) \rangle < \frac{1}{2} .$
With this assumption, Z_mcan be approximated as follows:
Z_m=ζ(d_m+1,m)≈φ_m+1−φ_m+E where E is an integer independent of the filter coefficients. $\begin{matrix} \Rightarrow \frac{\partial Z_{m}}{\partial f_{k}^{\pm}} = \frac{\partial (φ_{m + 1} - φ_{m})}{\partial f_{k}^{\pm}} & (equation 4) \end{matrix}$
The time interval φ_mcan be computed approximately as a function g_mof the improved main samples:
φ_m ≈g _m({tilde over (C)} _(m,L) ,{tilde over (C)} _(m,L)−1 , . . . ,{tilde over (C)} _(m,L)−N _L ,{tilde over (C)} _(m,R)−1 , . . . ,{tilde over (C)} _(m,R)−N _R)·(−1)^m
The general form of a linear approximation is: $φ_{m} \approx (\sum_{k = 0}^{N_{L}} η_{k, L} {\tilde{C}}_{(m, L) - k} + \sum_{k = 0}^{N_{R}} η_{k, R} {\tilde{C}}_{(m, R) - k}) \cdot {(- 1)}^{m}$
A simple 2-term linear approximation may be used, which gives:
φ_m≈η·({tilde over (C)} _(m,L) +{tilde over (C)} _(m,R))·(−1)^m
with η_k,L=η_k,R=η>0
and φ_m+1≈η·({tilde over (C)} _(m+1,L) +{tilde over (C)} _(m+1,R))·(−1)^m+1
Based on this simple 2-term linear approximation and on equation 3 above:
Z _m≈ζ(ηR _m·(−1)^m+1+[(m+1,L)−(m,L)])=ζ(ηR _m)·(−1)^m+1
where R _m ={tilde over (C)} _(m,L) +{tilde over (C)} _(m,R) +{tilde over (C)} _(m+1,L) +{tilde over (C)} _(m+1,R)
The term $\frac{\partial}{\partial f_{k}^{\pm}} {(Z_{m})}^{2}$
in equation 2 can be computed as follows: $\frac{\partial}{\partial f_{k}^{\pm}} {(Z_{m})}^{2} = 2 \cdot Z_{m} \cdot \frac{\partial Z_{m}}{\partial f_{k}^{\pm}} \approx 2 \cdot ζ (η R_{m}) \cdot {(- 1)}^{m + 1} \cdot \frac{\partial (φ_{m + 1} - φ_{m})}{\partial f_{k}^{\pm}} \Rightarrow \frac{\partial}{\partial f_{k}^{\pm}} {(Z_{m})}^{2} \approx 2 \cdot η \cdot ζ (η R_{m}) \cdot [S_{(m, L) - k}^{\pm} + S_{(m, R) - k}^{\pm} + S_{(m + 1, L) - k}^{\pm} + S_{(m + 1, R) - k}^{\pm}]$
Eventually the expression for updating the filters coefficients is:
(f _k ^±)_m+1=(1−μ)(f _k ^±)_m−2·μ·η·ζ(ηR _m)·(S _(m,L)−k ^± +S _(m,R)−k ^± +S _(m+1,L)−k ^± +S _(m+1,R)−k ^±) (equation 5)
A second implementation of the updating scheme of the invention will now be described that can be used when the fixed system clock (under which the filters are running) is not equal to the PLL driven bit clock (that is when the ratio α≠1).
In this second implementation, the LMS driving parameter to be minimized Z_mis also chosen to be equal to the difference between the actual run length d_m+1,mand the expected run length d_m+1,m ^(exp), but the mathematical formulae used for computing d_m+1,m,φ_mand φ_m+1have to be modified so as to take into account the frequency ratio α.
Namely, in order to measure the run length in bit intervals, the number of samples between two transitions has to be multiplied by α, which means that:
d _m+1,m=α·[(m+1,L)−(m,L)]+φ_m+1−φ_m
The transition phases φ_malso have to be multiplied by α. This means that the general form of the linear approximation of φ_mis: $φ_{m} \approx α \cdot (\sum_{k = 0}^{N_{L}} η_{k, L} {\tilde{C}}_{(m, L) - k} + \sum_{k = 0}^{N_{R}} η_{k, R} {\tilde{C}}_{(m, R) - k}) \cdot {(- 1)}^{m}$
and the simple 2-term expression of the linear approximation is:

φ_m≈α·η·({tilde over (C)} _(m,L) +{tilde over (C)} _(m,R))·(−1)^m
Eventually the expression for updating the filters coefficients is:
(f _k ^±)_m+1=(1−μ)(f _k ^±)_m−2·α·μ·η·ζ(ηR _m)·└S _(m,L)−k ^± +S _(m,R)−k ^± +S _(m+1,L)−k ^± +S _(m+1,R)−k ^±┘ (equation 6)
It will be noted from equations 5 and 6 that the minimization scheme of the invention does not use the notion of ideal transition time.
With respect to the described cross-talk cancellation method, signal processor and reading apparatus, modifications or improvements may be proposed without departing from the scope of the invention. The invention is not limited to the examples provided. In particular:
The first and second implementations that were described are based on a simple 2-term linear approximation for the calculation of the time intervals φ_mand φ_m+1. This is not restrictive. Other approximations can be used. For example, a linear approximation using more than 2 terms may be used. The LMS updating scheme for these other approximations can be derived in a similar fashion as for the 2-term linear approximation.
The minimization algorithm used in the above described implementations is the LMS algorithm. This is not restrictive. Other minimization algorithms may be used to minimize Z_m. The corresponding coefficient updating equations may be readily derived by using the same principles as those described above for the LMS algorithm.
In the cross-talk cancellation means described with reference to FIG. 5, the main signal is equalized. In an alternative embodiment, the main signal may be processed by an adaptive filter in a similar fashion as the lower and upper satellite signal.
The functions described above may be implemented either in hardware or in software. FIGS. 1, 2 and 5 are functional representations of an apparatus and a signal processor according to the invention. A hardware implementation thereof may differ from this functional block representation.
The word “comprising” does not exclude the presence of elements or steps other than those listed.

Claims

1. A cross-talk cancellation method using a main signal (62) associated with a target track (32) and satellite signals (61, 63) associated with side tracks (31, 33), said main signal showing transitions (X_m) and runs of various lengths (d_m+1,m) between two transitions (X_m,X_m+1), said cancellation method comprising the steps of:

filtering said satellite signals with adaptive filters (71, 73), thereby generating filtered versions (81, 83) of said satellite signals,

updating the coefficients of said adaptive filters by minimizing the mismatch between the actual and the expected run length between two transitions of the main signal, and

processing said main signal, thereby generating an improved main signal (102), said processing including a subtraction of said filtered versions of said satellite signals.

2. A program comprising instructions for implementing a cross-talk cancellation method as claimed in claim 1 when said program is executed by a processor.

3. A signal processor (40) comprising cross-talk cancellation means (42) for receiving a main signal (62) associated with a target track (32) and satellite signals (61, 63) associated with side tracks (31, 33), said main signal showing transitions (X_m) and runs of various lengths (d_m+1,m) between two transitions (X_m,X_m+1), said cross-talk cancellation means comprising:

filtering means (71, 73) for filtering said satellite signals with adaptive filters, thereby generating filtered versions (81, 83) of said satellite signals,

updating means (111, 113) for updating the coefficients of said adaptive filters by minimizing the mismatch between the actual (d_m+1,m) and the expected (d_m+1,m ^(exp)) run length between two transitions of the main signal, and

processing means (93) for generating an improved main signal (102) from said main signal by subtraction of said filtered versions of the satellite signals.

4. A signal processor as claimed in claim 3, comprising a fixed clock (55), time recovery means (130), and a bit clock (120) driven by said time recovery means, said fixed clock being asynchronous with respect to said bit clock, wherein said cross-talk cancellation means are operated at said fixed clock.

5. A signal processor as claimed in claim 4, wherein said bit clock has a bit clock frequency and said fixed clock has a fixed clock frequency that is substantially different from said bit clock frequency such that the ratio between said bit clock frequency and said fixed clock frequency is substantially different from 1, said signal processor further comprising time recovery means (50-1, 50-2) for estimating said ratio and providing said ratio to said updating means, said updating means being designed to take said ratio into account for updating said coefficients.

6. An apparatus (6-1, 6-2) for reading a signal stored along a track on a storage medium (1) comprising a signal processor as claimed in claim 3.

7. An apparatus for reading a signal stored along a track on a storage medium comprising a signal processor as claimed in claim 4.

8. An apparatus for reading a signal stored along a track on a storage medium comprising a signal processor as claimed in claim 5.