Radiation power and/or field control of domain expansion recording medium
The present invention relates to a method, an apparatus and a record carrier for controlling radiation power and/or field strength during a reading operation from a magneto-optical recording medium comprising a storage layer and a read-out layer. An expanded domain leading to a pulse in a reading signal is generated in said read-out layer by copying a mark region from said storage layer to said read-out layer upon heating by said radiation power and by applying an external magnetic field. A pulse pattern in the reading signal is analyzed. The analyzing result is compared with run length characteristics of the data stored in said storage layer. The radiation power and/or the magnetic field strength are controlled on the basis of the comparison result. Thus, much less or no disc capacity has to be reserved for radiation power and/or magnetic field calibration, since the user data can be used for this purpose.
The present invention relates to a method and to an apparatus for controlling radiation power and/or field strength during reading of a magneto-optical recording medium comprising a recording or storage layer and an expansion or read-out layer, such as a MAMMOS (Magnetic AMplifying Magneto-Optical System) disk. The invention also relates to a magneto-optical recording medium for use by the method and in the apparatus according to the invention.
In magneto-optical storage systems the minimum width of the recorded marks is determined by the diffraction limit, that is, by the Numerical Aperture (NA) of the focussing lens and by the radiation wavelength. Therefore, a reduction of this width is generally based on shorter wavelength radiation and/or higher NA focussing optics. During magneto-optical recording the minimum bit length can be reduced to below the optical diffraction limit by using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM the bit transitions are determined by the switching of the field and the temperature gradient induced by the switching of the radiation source such as, for example, a laser. For read-out of the small crescent shaped marks recorded in this way, Magnetic Super Resolution (MSR) or Domain Expansion (DomEx) methods have to be used. These technologies are based on recording media with several magneto-static or exchange-coupled RE-TM layers. According to MSR, a read-out layer on a magneto-optical disk is arranged to mask adjacent bits during reading while, according to domain expansion, a domain in the centre of a spot is expanded. Because of the advantage of the domain expansion technique over MSR, bits with a length below the diffraction limit can be detected with a similar signal-to-noise ratio (SNR) as bits with a size comparable to the diffraction limited spot. MAMMOS is a domain expansion method based on magneto-statically coupled storage and read-out layers wherein a magnetic field modulation is used for expansion and collapse of expanded domains in the read-out layer.
In the above mentioned domain expansion techniques, like MAMMOS, a written mark from the storage layer is copied to the read-out layer upon laser heating by a radiation beam and by applying an external magnetic field. Due to the low coercitivity of this read-out layer, the copied mark will expand to fill the area of the optical spot and can be detected with a saturated signal level which is independent of the mark size. Reversal of the external magnetic field collapses the expanded domain. On the other hand, a space in the storage layer will not be copied and no expansion occurs.
The resolution of the MAMMOS read-out process, that is the smallest bit size that can be reproduced without interference from neighbouring bits, is limited by the spatial extent of the copy process (copy window) which is determined by the overlap of the temperature-induced coercivity profile and the stray field profile of the bit pattern which depends on the strength of the external magnetic field. The radiation power that is used in the read-out process should be high enough to enable copying. On the other hand, a higher radiation power also increases the overlap due to the fact that the coercivity Hc decreases and the stray field increases with increasing temperature. When this overlap becomes too large, correct read-out of a space is no longer possible, because false signals are generated by neighbouring marks. The difference between the maximum and the minimum allowed radiation power determines the power margin. This power margin decreases strongly with decreasing bit length. Experiments have shown that with the current read-out methods bit lengths of 0.10 μm can be correctly detected, i.e. at an extremely small power margin (in the range of 1 bit of a 16 bit Digital-to-Analog Converter). Hence, a good balance of the radiation power and the intensity of the external magnetic field is an important factor when selecting optimum operating conditions.
However, even if optimum conditions have been set during an initial stage of a reading operation, the initial balance may be disturbed during reading due to environmental changes. These environmental changes comprise field blurring, disk tilt, temperature changes, non-uniformities of the thickness of the protective coat of the disk, and influences of the slider movement on the magnetic head. Thus, controlling the radiation power and the magnetic field strength during read-out is essential.
JP-A-2000-215537 discloses a method and an apparatus for controlling the radiation power and/or the field strength of the external magnetic field. In the method disclosed in JP-A-2000-215537 information defining a prescribed section on the disk and pulse information defining a prescribed pulse number are read from a specific area on the disk. Next, the number of pulses contained in the information read from the prescribed section is counted and compared with the prescribed pulse number. Based on the result of the comparison, the radiation power or the field strength is adjusted. However, this solution requires a specific type of recording record with specific pre-recorded information in specific prescribed regions. Furthermore, the control can only be performed for the prescribed regions with a given number of pulses (that is marks).
It is an object of the present invention to provide a method, an apparatus and a record carrier for providing improved power and/or field control which enables adjustment during the whole read-out process.
This object is achieved by a method as claimed in claim 1 or 2, by an apparatus as claimed in claim 13 or 14, and by a record carrier as claimed in claim 17 or 19.
Accordingly, the use of the run length characteristics for evaluation of a misbalance between the external magnetic field and the radiation power, resulting in, for example, an excessive copy window size, provides the advantage that the radiation power and/or the field strength control can be performed based on normal user data recorded on the recording medium, that is, as long as the run-length constraints of the coding of the normal user data are known. Thus, a continuous control function is provided without requiring a modified or specially pre-recorded recording medium. Moreover, the control according to the invention has the additional advantage that it is a so called “running-control”, that is the radiation power and/or the field strength control is performed while reading the user data without the need to perform a separate control step which would interrupt the reading of the user data.
According to an advantageous embodiment, a copy window size is determined in said comparison step, on the basis of which a control signal for said controlling step is generated. The copy window size is determined based on a detection of run length violations which may be determined by a pulse counting function or by a timer function. The copy window size determined may then be used to correct errors in the reading signal.
Preferably, the pulse pattern corresponds to the user data recorded on said recording medium. As an alternative, the pulse pattern corresponds to a test pattern with pre-defined mark and space run lengths recorded on said recording medium.
According to a further embodiment, the comparison step is performed based on a look-up table linking the copy window size to a corresponding number of false peaks or missing peaks in the pulse pattern.
According to a further embodiment, the control step comprises outputting a first control signal for coarse adjustment by radiation power control and a second control signal for fine adjustment by field strength control.
According to a further embodiment, a control information used in said controlling step is provided on the recording medium. This control information defines a medium-related characteristic between copy window size and radiation power data.
Other advantageous embodiments are defined in the dependent claims.
The present invention will be described hereinafter on the basis of preferred embodiments and with reference to the accompanying drawings in which:
A preferred embodiment will now be described on the basis of a MAMMOS disk player as indicated in
It is to be noted that in
The magnetic head 12 is connected to a head driver unit 14. It receives, at the time of recording, code-converted data from a modulator 24 via a phase adjusting circuit 18. The modulator 24 converts input recording data 101 to a prescribed code.
At the time of playback, the head driver 14 receives a clock signal, via a playback adjusting circuit 20, from the clock generator 26. The playback adjusting circuit 20 generates a synchronization signal for adjusting the timing and amplitudes of pulses applied to the magnetic head 12. A recording/playback switch 16 is provided for switching or selecting the respective signal to be applied to the head driver 14 at the time of recording and at the time of playback.
The optical pick-up unit 30 also comprises a detector for detecting the laser light reflected from the disk 10 and for generating a corresponding reading signal. This reading signal is applied to a decoder 28 which is arranged to decode the reading signal to generate output data 102. The reading signal generated by the optical pick-up unit 30 is also applied to a clock generator 26 in which a clock signal obtained from reading embossed clock marks on the disk 10 is extracted. This clock signal is applied, for synchronization purposes, to the recording pulse adjusting circuit 32, the playback adjusting circuit 20, and the modulator 24. In particular, a data channel clock may be generated in the PLL circuit of the clock generator 26.
During data recording, the laser source in the optical pick-up unit 30 is modulated with a fixed frequency corresponding to the period of the data channel clock. In this way, the data recording area (that is, the optical spot) on the rotating disk 10 is locally heated at equal distances. Furthermore, the data channel clock output by the clock generator 26 controls the modulator 24 to generate a data signal with a standard clock period. The recording data 101 are modulated and code-converted by the modulator 24 to obtain a binary runlength information corresponding to the information of the recording data.
The structure of the magneto-optical recording medium 10 may, for example, correspond to the structure described in JP-A-2000-260079.
The occurrence of false signals due to a large overlap (caused, for example, by a radiation power which is too high) should normally be avoided. However, when the correct data in the storage layer is known or can be deduced from coding constrains on that data, the occurrence and the number of false peaks gives direct information on the spatial width of the copy window which again is related to the thermal laser profile. This information can be used not only to correct the previous and/or subsequent data on the disc, but also provides a direct way to correct the radiation power and/or the field strength of the external magnetic field.
In the embodiment shown in
The control unit 25 receives a comparison result of a comparing unit 22 which compares the result of an analysis of the read-out data obtained from the decoder 28 with reference data stored in a non-volatile memory, for example, a look-up table 23. The analyzing is performed by an analysis unit 21 which receives the read-out from the decoder 28.
The resultant time dependency of the overlaps 61, 62, 63 upon scanning with different copy window sizes w1, w2 and w3 are indicated in the second graph from above. Overlap 61 corresponds to a copy window size w1, while overlap 62 corresponds to a copy window size w2 and overlap 63 corresponds to a copy window size w3.
The lower graph in
In conventional systems, the copy window size should for correct read-out be smaller than half the channel bit length b, as applies to the copy window size w1 in
For larger copy window sizes, such as, for example, copy window size w2, additional MAMMOS peaks 76 will be generated for space regions near a mark region because of the larger overlap. This situation is indicated by the dashed lines 62, 76 in
Even larger copy window sizes, having a size w between 2.5b and 4.5b (2.5b<w<4.5b; b being the channel bit length) such as, for example, copy window size w3, cause a difference of four peaks in space and mark run length detection, while a −I5 space is the smallest space run length that can be detected (by one missing peak).
The actual values in a table as shown in
Using a table as shown in
The run length violations are determined by the analyzing unit 21. This determination is, for example, based on a determination of the number of peaks in the read-out signal by a pulse counting function. Alternatively, this determination is based on a measurement of the space periods in the read-out signal by a timer function.
Assuming a (d,k)=(0,6) RLL modulation, which means that the lengths of the smallest marks and of the smallest spaces are I1 and −I1 while the lengths of the largest marks and of the largest spaces are I7 and −I7, respectively. When the smallest mark run length in a data sequence observed by the analyzing unit 21 is larger than 1, the comparing unit 22 determines a correction. When, for example, the smallest mark run length in a data sequence observed is 3 successive peaks PM, the comparing unit 22 determines a correction of 2 peaks and thus a copy window size w between b/2 and 2.5b. When the largest allowed space run length (−I7) shows, for example, only 3 missing peaks PS instead of 7, the comparison unit 22 determines a correction of 4 peaks and a copy window size w between 2.5b and 4.5b. It is noted that for such a relatively large copy window, only −I5 and larger space run lengths can be detected and corrected. This demonstrates the need for rather tight radiation power control. In both above examples the radiation power and/or the field strength should be reduced by a corresponding amount which is determined in the control unit 25 on the basis of the comparison results. The run length characteristics of the modulation may be preset in the analyzing unit 21 or in the look-up table 23. The run length characteristics of the modulation may be based on information provided on and read from the disk 10 or, alternatively, on input by an input function in the disk player.
For even better control of the radiation power, a look-up table and/or a number of algorithm parameters are prerecorded on the disc 10 at a predetermined area. The look-up table stores a predetermined copy window vs laser power characteristic of the disk 10. This look-up table, or the algorithm parameters, is read from the disk and used by the control unit 25 to generate the control signals 38, 39. For relatively high linear velocities (for example, in Constant-Angular-Velocity operation or at different read-out speeds), the laser power should be increased with the linear velocity to reach appropriate temperatures on the surface of the disk 10. Therefore, the table could also include the disk radius as a variable or, alternatively, a radiation power interpolation scheme between inner and outer radius of the disk 10.
The present invention can be applied to any reading system for domain expansion magneto-optical disk storage systems. The functions of the analyzing unit 21, the comparing unit 22, the look-up table 23 and the control unit 25 may be combined into a single unit which may be a hardware unit or a processor unit controlled by a corresponding control program. The control signals 38,39 may be applied only to head driver 14, only to the optical pickup unit 30, or to both. The read-out data may alternatively be applied directly from the optical pickup-unit 30 to the analyzing unit 21. The preferred embodiments may thus vary within the scope of the attached claims.
Claims
1. (Cancel)
2. A method of controlling an external magnetic field applied during a reading operation from a magneto-optical recording medium (10) comprising a storage layer and a read-out layer, wherein an expanded domain leading to a pulse in a reading signal is generated in said read-out layer by copying a mark region from said storage layer to said read-out layer upon heating by a radiation power when applying said external magnetic field, said method comprising
- an analyzing step for analyzing a pulse pattern in said reading signal;
- a step for comparing the result of said analyzing step with a run length characteristic of the data stored in said storage layer; and
- a controlling step for controlling the strength of said external magnetic field on the basis of the comparison result.
3-13. (Cancel)
14. A reading apparatus for controlling an external magnetic field applied during a reading operation from a magneto-optical recording medium (10) comprising a storage layer and a read-out layer, wherein an expanded domain leading to a pulse in a reading signal is generated in said read-out layer by copying a mark region from said storage layer to said read-out layer upon heating by a radiation power and when applying said external magnetic field, said apparatus comprising:
- analyzing means (21) for analyzing a pulse pattern in said reading signal;
- comparing means (22) for comparing the result of said analyzing by said analyzing means (21) with a run length characteristic of the data stored in said storage layer; and
- field control means (25) for controlling the strength of said external magnetic field on the basis of the comparison result.
15-16. (Cancel)
17. A magneto-optical record carrier comprising a storage layer and a read-out layer, wherein an expanded domain is generated in said read-out layer by copying a mark region from said storage layer to said read-out layer upon heating by a radiation power and when applying an external magnetic field, said record carrier (10) comprising control information for use in the control of the magnetic field and/or the radiation power during a reading operation.
18. A record carrier as claimed in claim 17, wherein said control information defines a medium-related characteristic between a copy window size and the radiation power.
19. A magneto-optical record carrier comprising a storage layer and a read-out layer, wherein an expanded domain is generated in said read-out layer by copying a mark region from said storage layer to said read-out layer upon heating by a radiation power and when applying an external magnetic field, said record carrier (10) comprising a test pattern with a predetermined run-length characteristic for use in the control of the magnetic field and/or the radiation power during a reading operation.
20. A record carrier as claimed in claim 17, wherein said record carrier is a MAMMOS disk (10).
Type: Application
Filed: Aug 12, 2004
Publication Date: Jan 20, 2005
Inventor: Coen Verschuren (Eindhoven)
Application Number: 10/917,214