WO2008139357A2

WO2008139357A2 - Efficient method for optimizing write strategies in optical recording

Info

Publication number: WO2008139357A2
Application number: PCT/IB2008/051729
Authority: WO
Inventors: Johannes J. H. B. Schleipen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-05-14
Filing date: 2008-05-05
Publication date: 2008-11-20
Anticipated expiration: 2009-11-14
Also published as: WO2008139357A3; TW200903463A

Abstract

A method is described for optimizing the writing strategy of an optical storage device. A set of codewords (CW(j)) is defined. Initial values are set for the writing parameters. A code sentence (CS) containing the set of codewords, and further comprising reference markers in between the codewords, is written to an optical storage medium. The written code sentence is read back. For each codeword (CW(j)), the readback signal is scaled on the basis of the reference markers, compared with a predetermined ideal standard shape, and a codeword error (CWE(j)) is calculated. A Strategy Quality Indicator (SQI) is calculated as the weighted summation of all codeword errors. The writing parameters are varied, and the above steps are repeated to find an optimum writing strategy where the Strategy Quality Indicator has an optimum value.

Description

Efficient method for optimizing write strategies in optical recording

FIELD OF THE INVENTION

The present invention relates in general to optical recording. Particularly, the present invention relates to a disc drive apparatus for writing information into an optical storage disc; hereinafter, such disc drive apparatus will also be indicated as "optical disc drive".

BACKGROUND OF THE INVENTION

As is commonly known to persons skilled in this art, 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. 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 scanning 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.

In the process of recording information, optical properties of the material of the disc are changed. For instance, the disc may comprise a dye material, and the writing process may partly destroy the dye. In another example, the disc may comprise a phase change material, and the writing process may partly change the material from a crystalline state to an amorphous state. In both cases, the reflective properties of the disc are changed.

On reading, the rotating disc is irradiated with a read beam, and the reflected light is captured by a sensor, which reflected light has an intensity fluctuating in accordance with the pattern written on the disc. The readout performance depends on how the data pattern was actually written. The writing process has multiple parameters that can be varied, and the quality of the writing process, expressed in terms of number of errors encountered during readout, depends on the precise value of the parameters of the writing process. For short, the collection of writing parameter values will be termed a "writing strategy".

For optimum readout performance, the writing process needs to be optimized. Since the optimum writing strategy may differ from disc drive to disc drive and from disc to disc, it is customary that, whenever a new disc is introduced into a disc drive for writing purposes, a disc drive performs an initialization procedure in which an optimum writing strategy is established, or at least an acceptable writing strategy.

SUMMARY OF THE INVENTION

In prior art, an initialization procedure involves writing a long random sequence of bits to the drive, using a specific writing strategy. The written sequence is read back, and the timing jitter is measured. Then, the writing strategy is amended, and a long random sequence of bits is written to the disc again, for one full revolution of the disc, and the jitter is measured again. It has been found that timing jitter is a parameter that is useful for analysing the writing quality: if the timing jitter is typically less than 7%, the chances are sufficiently high that any bit defects can be corrected by the error bits incorporated in the bit stream written to disc. Thus, the above steps of writing a random sequence, reading back, and measuring the jitter are repeated, amendments to the writing strategy are pursued such as to reduce the jitter, and the procedure is ended when the jitter is less than 7%.

A problem with prior art initialization procedures is that they consume a considerable amount of storage space and take a considerable amount of time. Especially when a factory prescribed write strategy does not seem to work properly, and a new custom write strategy has to be found from scratch, this write strategy optimisation process is a rather time consuming process. In this respect it is noted that jitter is a statistical parameter, so it can only be measured from a large amount of data written with the same writing strategy. Therefore, an object of the present invention is to provide a more efficient method for finding a good and preferably optimum writing strategy.

According to the present invention, a predefined sequence of code words is written to disc, and read back. The waveform of the actual read signal is compared to the expected waveform of an ideal theoretical read signal, as specified in the standard. A quantitative value representing the amount of discrepancy is determined, thereby explicitly taking into account the physics and chemistry of the writing process. This is repeated for a plurality of sequences of code words, it being noted that one 360° track portion can accommodate many code word sequences. The weighed summation of all quantitative values measured for all code words is a measure for the writing quality. Again, the above is repeated while varying the writing strategy parameters. An important advantage of the proposed procedure is that it results in an optimum writing strategy, or at least an acceptable writing strategy, very quickly.

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. 1 schematically illustrates an optical disc drive apparatus;

Fig. 2 is a block diagram schematically illustrating steps in a process of recording data on an optical disc;

Fig. 3 schematically illustrates a track portion;

Fig. 4 is a graph illustrating the possible signal wave shapes; Fig. 5 is a graph illustrating writing parameters;

Fig. 6 schematically illustrates a basic aspect of the present invention;

Fig. 7 is a diagram schematically illustrating a portion of a code sentence.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 schematically illustrates an optical disc drive apparatus 1 , suitable for storing information on or reading information from an optical disc 2, typically a DVD, CD, 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 1, the optical system 30 comprises a light beam generating means 31, typically a laser such as a laser diode, arranged to generate a light beam 32. In the following, different sections of the optical path of light beam 32 will be indicated by a character a, b, c, etc added to the reference numeral 32.

The light beam 32 passes a beam splitter 33 and an objective lens 34 to reach (beam 32b) the disc 2. The light beam 32b reflects from the disc 2 (reflected light beam 32c) and passes the objective lens 34 and the beam splitter 33 (beam 32d) to reach an optical detector 35.

The objective lens 34 is designed to focus the light beam 32b in a focal spot F on a recording layer of the disc 2.

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. Since radial actuators are known per se, while the present invention does not relate to the design and functioning of such radial actuator, it is not necessary here to discuss the design and functioning of a radial actuator in great detail.

For achieving and maintaining a correct focusing on the desired recording layer, said objective lens 34 is mounted axially displaceable, while further the actuator system 50 also comprises a focal actuator 52 arranged for axially displacing the objective lens 34 with respect to the disc 2. Since axial actuators are known per se, while further the design and operation of such axial actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such focal actuator in great detail. It is noted that means for supporting the objective lens with respect to an apparatus frame, and means for axially and radially displacing the objective lens, are generally known per se. Since the design and operation of such supporting and displacing means are no subject of the present invention, it is not necessary here to discuss their design and operation in great detail. It is further noted that the radial actuator 51 and focal actuator 52 may be implemented as one integrated 2D-actuator.

The disc drive apparatus 1 further comprises a control circuit 90 having a first output 91 coupled to a control input of the radial actuator 51, having a second output 92 connected to a control input of the focal actuator 52, having a third output 94 coupled to a control input of the motor 4, and having a fourth output 96 coupled to a control input of the laser 31. The control circuit 90 is designed to generate at its first output 91 a control signal Sc_R for controlling the radial actuator 51 , to generate at its second control output 92 a control signal SQF for controlling the focal actuator 52, to generate at its third output 93 a control signal SQM for controlling the motor 4, and to generate at its fourth output 96 a control signal S(_X for controlling the laser 31.

The control circuit 90 further has a read signal input 95 for receiving a read signal SR from the optical detector 35. Figure 2 is a block diagram schematically illustrating steps in a process of recording data on an optical disc. First, a stream of data bits is provided (block 11), either by converting an analogue signal to a digital signal, or by directly providing a digital signal. It is noted that the nature of the signal (audio, video, data) does not play a role here. It is further noted that the digital signal is a sequence of 0-s and 1-s. Second, the bits are grouped in blocks, and error code correction bits are added

(block 12). This again yields a sequence of 0-s and 1-s; the resulting signal will be indicated as an ECC digital signal.

Third, the ECC digital signal is converted to a run-length coded digital signal (block 13). This again yields a sequence of 0-s and 1-s; the resulting signal will be indicated as an RLC digital signal.

Fourth, the controller 90 controls the laser 31 on the basis of the RLC digital signal (block 14), generally known as channel modulation. More particularly, the controller 90 generates its control signal SQL for the laser 31 such that the laser power is switched to result in a series of marks written in the track of the storage layer. The result is illustrated in figure 3, where a track is schematically indicated at 15. Marks 16 written by the laser beam are shown here schematically as more or less elongate ovals. The widths of the marks 16 are mutually substantially equal, the lengths of the marks 16 (measured in the longitudinal direction of the track) depend on the durations of the corresponding laser writing pulses, and on the physical and chemical properties of the recording layer. The intervals of unaffected storage layer material between successive marks 16 are indicated as spaces 17. The lengths of the spaces 17 depend on the duration of the pauses between successive laser writing pulses.

The marks 16 and spaces 17 can not have any arbitrary length, but these lengths L satisfy the relationship L = n-Lcu, in which LQJ is the length travelled by the laser spot within a time unit defined by a system clock, and in which n is an integer, e.g. having a value in the range from 2 to 8 for the Blu-ray Disc (BD) system. Thus, LQJ can be considered to be a unit length, generally known as the channel bit length. In figure 3, vertical broken lines indicate a scale with divisions of one unit length LQJ- It can clearly be seen that the figure shows marks 16 having length 2LQU, 3LQU, and 4LQU, and shows spaces 17 having length 2Lcu, 3L_Cu, and 4L_Cu-

It is noted that in this explanation the lengths L are expressed in length units (for instance μm); it is also possible to express these lengths in time units. The absolute value of this length unit is standardized and is being part of the CD, DVD or BD standard.

In the case of reading, the controller 90 generates its control signal SQL for the laser 31 such that the laser power is relatively low and constant. When the laser spot of the read beam hits a space 17, the output signal of the sensor 35 is relatively high due to the fact that the reflection coefficient of the space 17 material is relatively high, whereas when the laser spot of the read beam hits a mark 16, the output signal of the sensor 35 is relatively low due to the fact that the reflection coefficient of the mark 16 material is relatively low. When the radius of the laser spot would be much smaller than the length unit Leu, the output signal of the sensor 35 would practically be a digital signal. Reality is, however, that the radius of the laser spot is larger than the length unit Leu, while further the laser spot does not have a constant intensity over its surface area. As a consequence, the output signal of the sensor 35 is a much more complicated signal, determined by the optical transfer function of the optical channel. When the laser spot approaches a mark 16, the sensor output signal starts to decrease as soon as the edge of the laser spot meets the mark. As the laser spot continues, the sensor output signal continues to decrease, and reaches a minimum when the centre of the spot coincides with the centre of the mark. A similar description applies when the laser spot approaches a space 17, but now the increasing overlap of laser spot and space leads to an increase of the sensor output signal while the sensor output signal reaches a maximum when the centre of the spot coincides with the centre of the space. All in all, the sensor output signal has a sine-like shape, of which the height and the distance between slicer-crossings depend on the length of the mark or space, respectively.

Figure 4 is a graph illustrating the possible signal wave shapes, superimposed over each other, for the exemplary case of BR-R recording. Curve 42 has a wavelength of 4-Lcu_:. and corresponds to marks and spaces having a length equal to 2-LQJ- It is noted that the signal is always positive; the zero level is indicated by a horizontal line 40. Horizontal line 41 indicates a reference level indicated as "slicer", which is chosen halfway between the maximum values and the minimum values of the signal of curve 42. A signal of this type, corresponding to one single mark or space, i.e. one wave hill or one wave valley between two successive slicer-crossings, is indicated as an 12 or 2T signal.

Curve 43 corresponds to marks and spaces having a length equal to 3-LQJ; its distance between slicer-crossings is equal to 3 -Leu, ^and its amplitude is higher than that of curve 42. A signal of this type, corresponding to one single mark or space, is indicated as an 13 or 3T signal.

Curve 44 corresponds to marks and spaces having a length equal to 4-LQJ; its distance between slicer-crossings is equal to 4-LQJ- A signal of this type, corresponding to one single mark or space, is indicated as an 14 or 4T signal.

Curve 45 corresponds to marks and spaces having a length equal to 5-LQU; its distance between slicer-crossings is equal to 5-LQU- A signal of this type, corresponding to one single mark or space, is indicated as an 15 or 5T signal.

Curve 46 corresponds to marks and spaces having a length equal to 6-LQJ; its distance between slicer-crossings is equal to 6-LQU- A signal of this type, corresponding to one single mark or space, is indicated as an 16 or 6T signal.

Curve 47 corresponds to marks and spaces having a length equal to 7-LQJ; its distance between slicer-crossings is equal to 7-LQJ- A signal of this type, corresponding to one single mark or space, is indicated as an 17 or 7T signal. Curve 48 corresponds to marks and spaces having a length equal to 8-LQU; its distance between slicer-crossings is equal to 8-LQJ- A signal of this type, corresponding to one single mark or space, is indicated as an 18 or 8T signal.

Figure 4 further shows some parameters of the data signal, in this case for the BD standard. The standard determines some requirements of these parameters. For instance, the modulation depth of the 18 signal, indicated as I8pp, should be at least equal to 0.4 times the signal maximum I8H- The modulation depth of the 12 signal, indicated as I2pp, should be at least equal to 0.04 times I8pp. Since the standard is known to or at least available to persons skilled in the art, while further the precise standard requirements may differ for different standard, it is not necessary here to repeat all standard requirements. Suffice it to say that each of the signals 12 to 18 has a well-defined ideal shape, determined by the optical transfer function of the optical channel, which shape will hereinafter be indicated as the standard shape. For understanding the present invention, it is not necessary to know all shape details of these standard shapes.

In practice, it appears that the actual wave shape of the actual data read signal SR differs from the standard shape, which difference depends on the precise way the laser beam was controlled during the writing process. Deviations between standard shape and actual shape may lead to reading errors. As already mentioned, the writing process has multiple parameters that can be varied, and the quality of the writing process, expressed in terms of number of errors encountered during readout, depends on the precise value of the parameters of the writing process. For short, the collection of writing parameter values will be termed a "writing strategy".

Figure 5 is a graph illustrating some of the parameters of the writing process, in this case for a "castle-type" or "dog-bone" write strategy that may be applied for recordable discs (note: for re-writable discs, using phase change recording media, a pulsed write strategy has to be used). The horizontal axis represents time, and the horizontal divisions correspond to system clock units; one system clock unit will be indicated as T. The vertical axis represents laser power. The figure also shows marks and spaces to be written. In the left-hand side of the graph it can be seen that the laser power is set to a read level when no marks are to be written. When the shortest mark, having duration 2T, is to be written, the laser power is increased to a writing power level Pw(2T) for a duration t(2T). The start of the power pulse may be advanced a little bit with respect to the system clock, the advance being indicated as dt(2T). The advance dt may be negative, indicating a delay of the pulse with respect to the system clock. Thus, the laser power has a block shape which can be defined by three parameters.

Likewise, for a mark of duration 3T, the laser power has a block shape which can be defined by three parameters dt(3T), t(3T), Pw(3T).

For marks of duration 4T and longer, the laser power pulse has a more complicated form, which is indicated as a castle form. Initially, the laser power is set to a relatively high level indicated as Pi; the duration of this initial high level Pt is indicated as ti. Then, the laser power is set to a writing level Pw. At the end of the laser pulse, the power is increased again, to a level indicated as Pe, for a duration te. The entire duration of the power pulse is indicated as t. Again, there is an advance with respect to the system clock, the advance being indicated as dt. Again, the advance dt may be negative, indicating a delay of the pulse with respect to the system clock. Further, before a laser writing pulse, the laser power is set at a level suitable for reading; this is indicated as a reading level Pr. But, after termination of a laser writing pulse, there is a cooling-off period during which the laser power is set to a lower level (yet above zero) Pc. The duration of this cooling-off period is indicated as tc. The above-defined parameters may be different for marks of different lengths.

So, for a mark having length nT, n being an integer in the range from 4 to 8, there are parameters Pr(nT), Pi(nT), Pw(nT), Pe(nT), dt(nT), ti(nT), te(nT), t(nT), tc(nT). Together with the three parameters for the marks 2T and the three parameters for the marks 3T, plus the parameters Pc (assuming that this parameter does not depend on mark length or space length), this leads to 52 parameters for defining a writing strategy.

Finding an optimum writing strategy means that one first has to define a measurable parameter that is indicative of the writing quality (or, conversely, to the amount of errors occurring on reading the written signal). This parameter will hereinafter be indicated as a Strategy Quality Indicator SQL Then, one has to perform the writing process, read back the written data, measure SQI as a function of the 52 parameters, repeat the above for different values of the parameters, and find the minimum of SQI (or maximum, depending on the definition of SQI) in the 52-dimensional parameter space. In practice, one is not necessarily looking for the absolute minimum of SQI in the 52-dimensional parameter space; rather, one is satisfied if a point in the 52-dimensional parameter space is found where SQI is lower than a predefined threshold. Thus, mathematical techniques have been developed for varying one or more of the 52 parameters in an intelligent way, analysing the influence of such variation, finding a direction of steepest descent, and following such direction in next steps. Basically, these techniques involve travelling through the 52-dimensional parameter space in an intelligent manner, always measuring SQI, adapting the direction of travel on the basis of the measuring results, and terminating the quest when SQI has an acceptable value. In a device designed in accordance with the present invention, this part of the optimization procedure can remain the same: the present invention is not directed to improving the travelling instructions. Therefore, it is not needed to explain these mathematical travelling techniques in more detail. Basically, the present invention involves a different definition of SQL In the prior art, jitter is taken as SQL But jitter is a statistical parameter, and can only be measured by repeating the "same" writing procedure many times. In contrast, as will be explained in more detail, the present invention proposes a more directly measurable SQI, which can be measured by performing a certain writing procedure only once. A basic aspect of the present invention is illustrated in figure 6. The figure shows a sequence of a mark of length 8LQU, a space of length 8LQU, a mark of length 4LQU, a space of length 2Lc_U, and a mark of length 2LQJ- The figure also shows the expected, ideal waveform 101 of the corresponding sequence 18, 18, 14, 12, 12. If the writing process is perfect, the shown waveform 101 would result if the mentioned mark/space sequence would be written and read back. However, in practice the writing process is not perfect, and the sensor output signal would differ from the ideal waveform, as illustrated by an exemplary actual waveform 102. The present invention proposes to quantify this deviation and to take this value as a quality indicator. In a first step, the sensor output signal is scaled such that the amplitudes of the two 18 signals correspond to the ideal amplitudes. Basically, the scaling step involves multiplying all points of the sensor output signal by a same multiplying factor f.

In a second step, the scaled 14, 12, 12 signals of the sensor output signal are sampled. The sampling frequency will typically be equal to the mentioned system clock frequency, but a higher sampling frequency is also possible. In the case of the sampling frequency being equal to the mentioned system clock frequency, the sampling moments are preferably phase-shifted over 180° with respect to the clock timing, where the slicer- crossings are taken as phase=0° reference. The sample values will be indicated as X{, i being an index ranging from 1 to 8 in this example. It should be clear that the order of the scaling and sampling steps can be inverted. So, it is possible to first sample the non-scaled 14, 12, 12 signals of the sensor output signal to obtain measured signal samples si (indicated as black dots in the figure), to define a scaling factor f by scaling the two 18 signals, and to calculate the scaled samples X{ as X{ = fSi. In a third step, each scaled sample X[ is compared with the expected value wi of the ideal waveform at the corresponding timing, and the absolute value of the difference is indicated as sample error ej = |wi - xi|.

In a fourth step, all sample errors are summed to find a code word error CWE according to CWE = ∑e_t

It is noted that this code word error CWE is not a statistical parameter which can only be measured by writing and reading a certain codewords many times, but it is a directly measurable parameter which can be measured by writing and reading a certain codewords only once.

In the above example, the sequence 14, 12, 12 is considered to be a codeword, while the sequence 18, 18 is considered to be a reference marker for the purpose of scaling. The reference marker may be a different sequence, but preferable the reference marker consists of two signals of equal length, preferably the largest length possible.

The measured code word error in the above example only relates to the codeword [14-12-12], which is indicated as CWE(422). The codeword will hereinafter be abbreviated as [422]. A true data sequence obviously does not only comprise codewords [422] but also comprises different codewords. According to a further aspect of the present invention, the CWEs corresponding to a plurality of codewords are measured.

Thus, the invention proposes to define a set of codewords CW. The set may be restricted to codewords having a length of 3 signals, as in the above example, but may also include codewords with length 1, 2, 4 or more signals. It is possible that the set includes all possible combinations of signals, in which case the set would contain 7 codewords of length

₄ 1, 49 codewords of length 2, 343 codewords of length 3, 7 codewords of length 4, etc., assuming that 18 is the longest signal.

In practice, in a random data signal, not all codewords have the same probability or appearance frequency α. Here, α(abc) can be defined as being equal to n(abc)/N, in which n(abc) indicates the number of times a certain codeword [abc] appears in a random sequence of N codewords. It is noted that the appearance frequency α is a statistically determined value, but it is possible to perform such determination in advance and to consider the appearance frequency α to be a predetermined constant value. In determining the quality of the writing process, the quality with which a single codeword is written has increasing importance with increasing appearance frequency of such codeword. Thus, it is possible that the above-mentioned set of codewords is defined by including only those codewords having an appearance frequency above a certain predefined limit. It is possible that the value of such limit is chosen in such a way that the number of codewords included in the set has a predefined value M, for instance 1000. Now, the invention proposes that all codeword errors of all codewords in the set are measured by the above four steps. To that end, a sequence of codewords is written to disc, which sequence contains all codewords of the set. This sequence will hereinafter also be indicated as a code sentence CS. Figure 7 is a diagram schematically illustrating a portion of such code sentence CS. Individual codewords are indicated as CW(J), j being an index ranging from 1 to M, in which M indicates the number of codewords included in the set. In the entire code sentence, it is sufficient if each codeword CW(J) appears only once. It is noted that the order of codewords in the sentence is not essential. Apart from the codewords CW, the code sentence CS contains a plurality of reference markers RM. The reference markers are not necessarily mutually equal, but preferably they are. In figure 7, it is shown that the reference markers RM are alternated with the codewords CW, i.e. a reference marker RM is always written in between two successive codewords CW, but the number of codewords CW between two successive reference markers RM may be two or more. It should be clear that the number of disc revolutions needed for writing the entire code sentence CS is much less than in prior art; even one revolution may be sufficient. After having written the entire code sentence CS, the entire code sentence CS is read back, and the above-described first to fourth steps are executed for each individual codeword CW(J), resulting in a set of individual codeword errors CWE(J). In the above- mentioned first step, i.e. the scaling step, it is possible that for each individual codeword only one reference marker is taken into account, for instance the reference marker appearing directly before that specific individual codeword in the code sentence, but it is also possible that the two reference markers appearing before and after that specific individual codeword in the code sentence are taken into account. Also, if the number of codewords between successive reference markers is equal to 2 or more, the distance from that specific individual codeword to the two reference markers may be taken into account.

Then, all codeword errors CWE(J) are summed, weighted by their respective appearance frequency α(j), to find a sentence error SE according to

∑a(j) - CWE(J)

SE = - j It is this sentence error SE that is defined as Strategy Quality Indicator SQI according to the present invention. It should be clear that SQI may be multiplied by any suitable constant, as desired, without changing the principles of the invention. Therefore, since ∑α(j) is constant, dividing by this value may be omitted.

As already explained above, SQI is a value which depends on the writing parameters of the writing strategy. Thus, the above procedure is repeated, with different values of the writing parameters, in order to find a writing strategy where the sentence error SE is minimal or at least is smaller than a predetermined acceptable value. Summarizing, the present invention provides a method for optimizing the writing strategy of an optical storage device.

A set of codewords CW(J) is defined.

Initial values are set for the writing parameters. A code sentence CS containing the set of codewords, and further comprising reference markers in between the codewords, is written to an optical storage medium.

The written code sentence is read back.

For each codeword CW(J), the readback signal is scaled on the basis of the reference markers, compared with a predetermined ideal standard shape, and a codeword error CWE(J) is calculated.

A Strategy Quality Indicator SQI is calculated as the weighted summation of all codeword errors.

The writing parameters are varied, and the above steps are repeated to find an optimum writing strategy where the Strategy Quality Indicator has an optimum value. An important advantage achieved by the present invention as compared to prior art methods operating on the basis of measuring jitter is the increased speed and therefore reduced time required for the optimization procedure. The reason behind this is the use of an optimisation control signal SQI that has more physical and/or chemical relevance, as compared to jitter, which is a statistical parameter not taking into account the physics and chemistry of the writing process. Consequently, the optimisation process will converge faster and the found minimum will be closer to the global minimum, resulting in broader system margins.

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, it is possible to use a varying set of codewords. In the above example, all possible codewords of interest are included in the set. It is however possible that some codewords are more important than other codewords, expressed in terms of appearance frequency. Thus, it is possible to first perform an optimization procedure with a preliminary set of codewords, for instance the codewords of length 2 only, before performing the optimization procedure with the final set of codewords. Further, the above example describes that a code sentence is written, the code sentence is read back, the sentence error is calculated, one or more writing parameters are amended, and then the above is repeated. However, it is also possible that a writing sequence involves writing the code sentence, amending one or more writing parameters, again writing the code sentence, and repeating this for a plurality of writing strategies before reading back the plurality of code sentences. This is especially efficient in cases where the length of track needed for writing the code sentence is less than 360°, so that 2 or more code sentences can be written in one disc revolution.

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. 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.

Claims

CLAIMS:

1. Method for optimizing the writing strategy of an optical storage device(l), the method comprising the steps of: a) defining a set of codewords (CW(J)); b) defining a set of writing parameters describing the writing strategy; c) setting initial values for the writing parameters; d) writing a code sentence (CS) containing the set of codewords to an optical storage medium (2); e) reading back the written code sentence and obtaining a readback signal; f) analysing the readback signal to determine a Strategy Quality Indicator (SQI) indictative of the quality of the writing strategy of the storage device; g) amending the value of one or more of the writing parameters; h) repeating steps d) to g) until the Strategy Quality Indicator (SQI) has an optimum value or at least has a value within an acceptable range; wherein the Strategy Quality Indicator (SQI) is a non-statistical parameter directly measurable in the readback signal obtained from performing steps d) and e) once.

2. Method according to claim 1, wherein for each codeword (CW(J)) the readback signal is compared with a predetermined ideal standard shape to obtain a codeword error (CWE(J)); wherein each codeword error is multiplied by the statistical appearance frequency (α(j)) of the corresponding codeword; and wherein the Strategy Quality Indicator (SQI) is calculated as the weighted summation of all codeword errors according to the formula:

∑a.(j) - CWE⁽J) SQI = ^J _v _ j

3. Method according to claim 2, wherein for each codeword (CW(J)) the codeword error (CWE(J)) is calculated by sampling the readback signal at a predetermined sampling frequency to obtain sample values X[, comparing each sample value X[ with the expected value wi of the ideal waveform at the corresponding timing, calculating the absolute value of the difference to obtain a sample error ei = |wi - xi|, and summing all sample errors to obtain codeword error CWE according to the formula: CWE = ∑e_t

4. Method according to claim 1, wherein reference markers are included in the code sentence, subsequent reference markers being separated by a predetermined number of codewords.

5. Method according to claim 4, wherein said predetermined number is equal to 1.

6. Method according to claim 4, wherein a reference marker comprises one mark (16) and one space (17).

7. Method according to claim 6, wherein the mark and space of the reference marker have mutually equal lengths.

8. Method according to claim 6, wherein the mark and space of the reference marker have the longest length possible (18).

9. Method according to claim 3 and claim 4, wherein at least one reference marker is used to obtain a scaling factor f, and wherein the sample values xi are calculated as Xi = f-Si, in which si are the measured signal samples.

10. Method according to claim 9, wherein the scaling factor f is calculated such that the amplitudes of the readout signals of the reference marker correspond to the ideal amplitudes.

11. Optical disc drive ( 1 ), adapted to perform the method of any o f claims 1-10.