Patent Application
20090175145
The invention relates to a device for scanning a record carrier having a track for recording information represented by marks, the device comprising scanning means comprising a radiation source and optical elements for generating a beam of radiation from the radiation source via a scanning spot on the track to a detector for detecting at least one scanning signal, a sensor for generating a sense signal from the beam, and power control means for setting the radiation power in a sequence of control periods of different lengths, each control period having one of a number of different power levels, while controlling the radiation power in dependence of a sampled sense signal.
The invention further relates to a method of adapting generating a sampled sense signal in the device.
A device and method for recording an optical record carrier are known from US 2005/0083828. A high-speed optical disc recording apparatus includes a laser diode for generating multi-pulse radiation and a photodiode outputting a sense signal indicative of the power of the radiation. An encoder generates a sequence of control periods of different lengths, each control period having one of a number of different power levels, while controlling the radiation power in dependence of a sampled sense signal. For calibrating the sampled sense signal the device provides a power control pattern according to a write strategy to a laser diode driver such that the laser diode outputs a multi-pulse having a fixed-duty ratio with two power levels. The measured power is averaged with a low-pass filter, is sampled and held, and is calibrated according to the fixed-duty ratio. The calibrated held average output of the measured power of the light pulses is compared with predetermined levels to control the laser diode driver output voltage. However, applying a dedicated power control pattern requires an area on the record carrier and a startup time available for calibration. Furthermore, the calibration based on averaging the sense signal appears to be inaccurate, and may in particular be dependent on operational conditions.
Therefore it is an object of the invention to provide a device and method for reliably generating a sampled sense signal in various operational conditions.
According to a first aspect of the invention the object is achieved with a device for recording information as described in the opening paragraph, the devices comprising sense means for generating the sampled sense signal by sampling the sense signal at a sampling time Ts in selected control periods that are selected on having at least a selected minimum length Lsel, Ts being located in a part of the selected control periods starting at Lsel, for determining a first sample value, further sampling the sense signal in the selected control periods for determining a second sample value on a detection time Tdet different from Ts in the selected control periods, determining a difference between the first and the second sample value and, in dependence on the difference, adapting said generating the sampled sense signal.
According to a second aspect of the invention the object is achieved with a method for generating a sense signal as described in the opening paragraph, which method comprises sampling the sense signal at a sampling time Ts in selected control periods that are selected on having at least a selected minimum length Lsel, Ts being located in a part of the selected control periods starting at Lsel, for determining a first sample value, further sampling the sense signal in the selected control periods for determining a second sample value on a detection time Tdet different from Ts in the selected control periods, determining a difference between the first and the second sample value and, in dependence on the difference, adapting said generating the sampled sense signal.
The effect of the measures is that the first and second sample value are sampled in selected control periods in the sequence that controls the radiation source, usually a multi-pulse drive signal for a laser diode. The control periods for sampling are selected to have at least the minimum length Lsel. By sampling beyond Lsel in the selected control periods, the sense signal has sufficiently stabilized to be reliably sampled. By determining the difference between two sample values taken at different instants in the selected control period it can be detected if transitory effects from applying a different, e.g. much higher, power level in the preceding control period have sufficiently decayed before the samples are taken, or remaining effects can be taken into account for adapting the generating of the sampled sense signal. Advantageously, such sampled sense signal is accurately representing the power of the radiation beam, and may be generated at any time, e.g. during operationally recording of data.
The invention is also based on the following recognition. As writing speed in various optical recording systems increases, the time intervals in the write strategy for various power levels become shorter. Traditionally, for multi-pulse write strategies, even at relatively low speeds, only the erase level can be sampled, but high speed recording using a blue laser, which also uses multi-pulse strategies, will have such short erase levels that sampling becomes very difficult and/or very expensive.
The inventors have seen that, when the selecting the longer control periods, samples can be taken in a relatively stable end part of such selected periods. However, in modern systems taking into account all possible sources of disturbance only the longest marks might be selected, which would very much limit the amount of sampled values and hence the reliability of the sampled sense signal, or might even require a dedicated calibration procedure. By detecting the difference between the first and second sample and monitoring the difference, and dynamically adapting the generation of the sampled sense signal, remaining instability and transitory effects can be taken into account based on a sufficient number of samples during operational use.
In an embodiment of the device the sense means are arranged for adapting Lsel in dependence on the difference. By adapting Lsel the selection of selected control periods is adapted to select only the periods having sufficient length and stability for reliably sampling the sense signal while the stability is monitored by the difference. This has the advantage that the selection is adapted to the operational conditions which affect the difference.
In an embodiment of the device the sense means are arranged for increasing Lsel until the difference is below a predetermined threshold value, in a particular case until the difference is less than 1% of the first or second sample value.
The difference between the first and second sample is indicative of the stability of the sense signal representing the detected power level. Applying the threshold while increasing Tsel has the advantage that the stability is controlled to be at a predetermined level, while allowing sampling during any interval that fulfills the stability requirement.
In an embodiment, on the record carrier the marks in the track have lengths corresponding to an integer number of channel bit lengths T, the shortest marks having a length of a predefined minimum number Lmin of channel bit lengths T, and the sense means are arranged for adapting Lsel to a value larger than Lmin. In particular the marks may be selected that are created by selecting the selected control periods having a read power level or an erase power level. This has the advantage that control periods having a relatively large length are easily selected by monitoring the marks to be written.
In an embodiment of the device the sense means comprise processing means for generating the sampled sense signal in dependence of at least one correction parameter, and are arranged for determining the at least one correction parameter in dependence of the difference. By sampling the sense signal on two or more sample times in the selected control period, the way the sense signal varies can be detected. Advantageously a model of the transfer function of the sensor having one or more correction parameters may be used to apply when processing the sample values for accurately generating the sampled sense signal.
Further preferred embodiments of the device and method according to the invention are given in the appended claims, disclosure of which is incorporated herein by reference.
These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which
In the Figures, elements which correspond to elements already described have the same reference numerals.
The record carrier 4 comprises a radiation-sensitive recording layer which upon exposure to radiation of sufficiently high intensity is subjected to an optically detectable change, such as for example a change in reflectivity, for forming marks 8 representing information in a track 11. The track 11 may be indicated by a servo pattern for generating servo tracking signals for positioning an optical head opposite the track. The servo pattern may for example be a shallow wobbled groove, usually called a pre-groove, and/or a pattern of indentations, usually called pre-pits or servo pits.
The optical write head 6 is arranged opposite the rotating record carrier. The optical write head 6 comprises a radiation source, for example a solid-state laser, for generating a write beam 13. The intensity I of the write beam 13 can be modulated in conformity with a control signal Vs in a customary manner. The intensity I of the write beam 13 varies between a write intensity Iw, which is adequate to bring about detectable changes in the optical properties of the radiation-sensitive record carrier, and an intensity In for producing spaces.
The marks may be in any optically readable form, e.g. in the form of areas with a reflection coefficient different from their surroundings, obtained when recording in materials such as dye, alloy or phase change material, or in the form of areas with a direction of polarization different from their surroundings, obtained when recording in magneto-optical material.
During scanning a beam reflected from the record carrier is modulated in conformity with the information pattern being scanned. The modulation of the read beam can be detected in a customary manner by means of a radiation-sensitive detector which generates a read signal which is indicative of the beam modulation.
User data can be recorded on the record carrier by marks having discrete lengths in unit called channel bits, for example according to the CD or DVD channel coding scheme. The marks are having lengths corresponding to an integer number of channel bit lengths T. The shortest marks that are used have a length of a predefined minimum number d of channel bit lengths T for being detectable via the scanning spot on the track that has an effective diameter, usually being roughly equal to the length of the shortest mark.
For reading the radiation reflected by the data layer is detected by a detector of a usual type, e.g. a four-quadrant diode, in the head 22 for generating detector signals to be processed by read processing unit 30 of a usual type including a demodulator, deformatter and output unit to retrieve the information.
For writing the device is provided with recording means for recording information on a record carrier of a writable or re-writable type, for example CD-R or CD-RW, or DVD+RW or BD. The recording means cooperate with the head 22 for generating a write beam of radiation, and comprise write processing means for processing input information to generate a write signal to drive the head 22, which write processing means comprise an input unit 27, a formatter 28 and a power control unit 29. The power control unit 29 controls the power of a radiation source, usually a laser, in the head 22 to create optically detectable marks in the recording layer.
The control unit 20 controls the scanning and retrieving of information and may be arranged for receiving commands from a user or from a host computer. The control unit 20 is connected via control lines 26, e.g. a system bus, to the other units in the device. The control unit 20 comprises control circuitry, for example a microprocessor, a program memory and interfaces for performing the procedures and functions as described below. The control unit 20 may also be implemented as a state machine in logic circuits. It is noted that the focus adjustment functions as described below may also be implemented as software functions in the control unit 20.
The input unit 27 may comprise compression means for input signals such as analog audio and/or video, or digital uncompressed audio/video. Suitable compression means are described for video in the MPEG standards, MPEG-1 is defined in ISO/IEC 11172 and MPEG-2 is defined in ISO/IEC 13818. The input signal may alternatively be already encoded according to such standards.
In an embodiment the recording device is a storage system only, e.g. an optical disc drive for use in a computer. The control unit 20 is arranged to communicate with a processing unit in the host computer system via a standardized interface. Digital data is interfaced to the formatter 28 and the read processing unit 30 directly.
In an embodiment the device is arranged as a stand alone unit, for example a video recording apparatus for consumer use. The control unit 20, or an additional host control unit included in the device, is arranged to be controlled directly by the user, e.g. to perform the functions of a file management system.
The power control unit 29 drives the radiation source by a power control signal. The laser is driven using a pulse sequence that contains higher frequency components that the channel rate itself. This has the form of a multi-level pulse whose purpose is to achieve a “mark” or a “space” of a given length of the optical medium so that it matches encoded data from the encoder in formatter 28. The conversion of encoded data to a pulse-train with higher time resolution and multiple levels is known as a Write Strategy. Thereto the power control unit 29 generates a sequence of control periods of different lengths, each control period having one of a number of different power levels, according to the write strategy. Usually the write strategy is adapted for the type of record carrier and the recording speed. For controlling the radiation power to each of the desired power levels the power control unit 29 receives a sampled sense signal 34 that is based on the sense signal 32.
The second curve shows a second power control signal 62 for the same rewritable type of record carrier, and indicates the pulsed sequence for writing a mark of 8 channel bits (18).
The third curve shows a third power control signal 63 for a write once type of record carrier like DVD+R at 2.4× nominal speed. The power levels change at even higher time resolution. The fourth curve shows a fourth power control signal 64 for a further write once type of record carrier like DVD-R or DVD+R up to 16× nominal speed. Note that further power levels C, W3 and W4 are used.
For performing the various write strategies for the power control signals shown in
The sense unit 31 generates the sampled sense signal as follows. The sense signal is sampled at a sampling time Ts in selected control periods that are selected on having at least a selected minimum length Lsel. The sampling time Ts is located in a part of the selected control periods starting at Lsel, for determining a first sample value. Furthermore the sense signal is sampled in the selected control periods for determining a second sample value on a detection time Tdet different from Ts in the selected control periods. Preferably the first and second sample values are taken from the same selected control periods. Subsequently a difference is determined between the first and the second sample values. Because the first and the second sample values have a predetermined difference in time, the difference between the sample values indicates the stability of the sense signal. It is to be noted that in a selected period the radiation power is assumed to be stable at a single, desired power level. Therefore, ideally, the sense signal should also be stable. However, due to noise and, in particular, due to known sources of disturbance, the sense signal will only become sufficiently stable at the end of a relatively long control period in the sequence, as discussed below in detail. Finally, the sense unit adapts the generating of the sampled sense signal in dependence on the detected difference.
In an embodiment of the device, the sense unit 31 is arranged for adapting the minimum length Lsel in dependence on the difference as detected by difference unit 74. For example Lsel may be set to a value in dependence of the type of the record carrier that is actually present in the device. Subsequently the difference is detected, and the value for Lsel may be adapted to a lower value as long as the difference remains small. Also the sense unit 31 may be arranged for increasing Lsel until the difference is below a predetermined threshold value. In a practical example Lsel may be increased until the difference is less than 1% of the first or second sample value.
Usually, on the record carrier, the marks in the track have lengths corresponding to an integer number of channel bit lengths T, while the shortest marks have a length of a predefined minimum number Lmin of channel bit lengths T. Lmin for CD and DVD is 3 channel bits, while the maximum length Lmax of the marks is usually 11 to 14. The sense unit 31 may be arranged for adapting Lsel to a value substantially larger than Lmin, but smaller than Lmax, for example between 8 and 10.
As can be seen from the write strategies in
The sense unit may comprise an averaging unit 76, and a further averaging unit 77, for determining the first and/or second sample value based on a sequence of sample values of the sense signal. By such averaging units, well known as such and for example including integration or summation of large numbers of samples, the effective resolution of digital sampled values, which in practice at the input may be 5 or 6 bits, can be increased to 10 or 12 bits.
The actual power control may be carried out using the sampled level with required sampling start delay, based on the averaged sample signal in either unit 76 or 77 as the radiation power feedback signal. In addition, the difference signal from unit 74 or the processed difference signal from unit 75 can be used to correct either the light power feedback signal or the power setpoint signal /value and in so doing eliminate or reduce the influence of the remaining “slow-tail” in the laser power control loop.
The sense unit 31 has the following purpose. High speed, multi-pulse write strategies have such short erase levels that direct sampling becomes very difficult and/or very expensive. This is because the time available for the amplifier of the forward sense (FS) signal to settle becomes shorter which implies either special FS design and/or very fast amplifiers are required in combination with higher gains as the laser wavelength decreases from CD to DVD to BD. Also the sense diode may be inherently slow. The speed of the sense diode may depend on the wavelength of the radiation and the depth the radiation penetrates into the semi-conductive layers. For example, lower wavelength red or infra-red radiation will penetrate deeper, and may reach the depletion layer of the FS diode, which reduces the response speed. The settling effect to a new sense level, which succeeds a higher sense level in a preceding period, is called the FS “slow-tail”. It is noted that in particular for a low level (e.g. read power level in write-once recording) following a high write level the slow tail is relatively strong with respect to the sense level to be detected.
In an embodiment of the sense unit 31 the processing unit 75 is arranged for generating the sampled sense signal in dependence of at least one correction parameter. The sense unit 31 is arranged for determining the at least one correction parameter in dependence of the difference. A correction parameter may be a slope of a decaying effect (slow tail) that is present in the sense signal due to a (higher) power level in a preceding period. The slope may be calculated from the two samples taken at different instants in time. A more complex model having several correction parameters may be implemented also, while a resulting curve from the model may be fitted based on two or more sample values. Further examples for a model of the sense process are, for example, described in US 2005/0083828 mentioned in the introductory part.
In this invention, the issue of how to set the minimum selection length Lsel for FS sampling is addressed. The current system, which dynamically adjusts the generation of the sampled sense signal, does take the behavior of individual drives into account (eg FS path gain spreads). In a calibration procedure the difference in sampled FS output between sampling with N channel periods run length rejection and N+1 channel periods run-length rejection is measured for N=0,1,2 . . . as shown in bottom traces 88,89 in
During operational use, after initially setting the minimum selection length Lsel, in order to maintain accurate power control only an N−1 (or N+1) to N comparison is required. For example, the value of N−1 or N+1 can be maintained on a second (parallel) sample channel as check.
Furthermore, the difference of the first and second sample value at a known distance in the selected periods can be used to calculate the power scaling effect on FS. Thereto further parameters, such as a known difference for a given starting power, are to be determined and taken into account in a model of the sense channel. A calculating based on a model of the slow tail, the known power in the preceding period and the sample values in the current period allow correction of the sample sense signal by subtracting the remaining slow tail contribution. The model may also depend on the radiation source or the wavelength, or on type of the record carrier or the recording layer that is actually scanned, which may affect the operation of the sense channel. The disc type may be detected from disc information retrieved from the record carrier, e.g. a prerecorded area or from a modulated, wobbled pregroove.
It is noted, that in this document the word ‘comprising’ does not exclude the presence of other elements or steps than those listed and the word ‘a’ or ‘an’ preceding an element does not exclude the presence of a plurality of such elements, that any reference signs do not limit the scope of the claims, that the invention may be implemented by means of both hardware and software, and that several ‘means’ or ‘units’ may be represented by the same item of hardware or software. Further, the invention is not limited to the embodiments, and the lies in each and every novel feature or combination of features described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06115553.7 | Jun 2006 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB07/52099 | 6/5/2007 | WO | 00 | 12/15/2008 |
