1. Technical Field
The present invention relates to an optical disc medium, a method of optically reproducing information recorded on an optical disc, and an optical disc apparatus.
2. Background Art
There are various formats and reproduction technologies developed for increasing the recording density of optical discs by employing different readout methods from those for the optical discs that have so far been put to practical use. These newer technologies for recording density improvement according to the prior art include (1) a multi-value recording technique whereby a plurality of bit information items are recorded at a single mark edge; (2) narrowing of the intervals of tracks (track pitch) in which information is written; and (3) narrowing of the intervals (mark pitch) of the minute marks in which information is recorded.
As an example of the multi-value recording technique, JP Patent Publication (Unexamined Application) No. 6-76303 discloses a method of reading multi-value information recorded on a disc. According to this method, the disc is scanned with an optical spot over a mark along the track to produce an optical readout signal (RF signal) corresponding to the position of a recorded mark edge, and multi-value information recorded at the edge is read from the waveform of the RF signal. The publication also describes a method of compensating a one-dimensional non-linear inter-symbol interference by employing a learning pattern, so that the coded information can be decoded accurately. Another method of compensating the one-dimensional inter-symbol interference on a medium pattern is disclosed in JP Patent Publication (Nonexamined Application) No. 11-353652.
JP Patent Publication (Unexamined Application) No. 10-74322 discloses an example of the technique of narrowing the track pitch. In this example, the track pitch is set at a value smaller than the size (diameter) of the optical spot used for readout. The optical spot scans not immediately above a track but an intermediate position between two adjacent tracks. Pit patterns for the two tracks are simultaneously read by an optical readout signal, and the information recorded in the individual tracks is decoded separately by signal processing. This separate decoding method is called a one-dimensional PRML (Partial Response Maximum Likelihood).
JP Patent Publication (Nonexamined Application) No. 2000-207747 discloses an example of the technique of narrowing the mark pitch in each track. In this example, the recording density of an optical disc is increased by making both the mark pitch and the track pitch smaller than the optical spot diameter. When playing back the disc, a single optical spot simultaneously captures four pit edges in a checkered, alternating bit pattern. The multi-value information recorded at the four pit edges is read by an optical readout signal and processed to decode the information recorded at each pit edge separately. This separation and decoding method is called a two-dimensional PRML.
The above method disclosed in JP Patent Publication (Nonexamined Application) No. 2000-207747 will be hereafter described by referring to FIGS. 2 to 4.
The pits are drawn on a disc original by an electron-beam drawing apparatus such that they are arranged on the optical disc as shown in FIG. 2. The regions having no pits are called mirror regions or areas. The pits include a synchronization pit 1 for signal synchronization, a reference pit 2 for level compensation, a servo pit 3 for detecting misalignment from the right track position, and a data pit 4 for recording data. The reference pit may also serve the function of a servo pit, and similarly the synchronization pit may serve the function of a reference pit. Accordingly, the presence or absence of the synchronization pit, reference pit and servo pit may be appropriately determined as desired.
In the present example, the data pit 4 for recording data is a modulation pit including a modulation area. The start and end positions of the pit are modulated in accordance with the data recorded, so that the pit length varies. As shown, the basic structure according to this technique comprises a plurality of data pits 4 arranged in each track at predetermined periods. It should be noted, however, that since the length of each end of the data pit is modulated, the position of the center of mass, so to speak, of each data pit changes depending on the degree of modulation at either end. Thus the data pits are not in reality arranged at completely uniform periods. Therefore, the periods in the present example will be referred to as substantially uniform intervals. By arranging the individual tracks one by one in the radius direction 7 of the optical disc, the pits are two-dimensionally arranged on the disc. In this example, the modulation pits are arranged in a checkered pattern, as shown in FIG. 2, so that they have mutually opposite phases between adjacent tracks.
The length of each end of the data pit 4 (modulation pit edge 6) is varied depending on the data recorded. Code data is recorded by elongating or shortening the pit length with reference to a non-modulated pit length. The point at which an independent item of information is placed, that is the point which is modulated by the recording data, is called a modulation point. Thus, in the present example where the two pit edges on either side of each pit are independently (irrelevantly) modulated, each pit has two modulation points. Although the pits are arranged in a checkered pattern, the pit edges, that is the modulation points 50, are arranged in a regular grid pattern, as indicated by the hatched areas in FIG. 3. The following description will refer to both way so of looking at the pattern.
As the pit pattern is scanned by a beam spot 5 focused by a lens for signal reproduction, the amount of reflected light varies depending on the area of pit portions that are located within the beam spot. The depth of each pit is adjusted during the formation of the optical disc such the phase of reflected light is inverted where pits exist. As the beam spot 5 passes over the pit pattern as shown in FIG. 2A, the reflected light from the pit portions where the phase is inverted and the reflected light from other portions with no phase inversion cancel each other, thus creating a reflected-light intensity profile shown in FIG. 2B. This reflected-light intensity profile constitutes the RF signal, which is herein referred to as a readout signal as is conventionally done. The readout signal shown in FIG. 2B is also called an eye pattern in which readout signals of the data pits 4, when their start and end positions are variously modulated, are superposed.
In the present technique, two or three pits are located within a single beam spot, so that a total of four modulation pit edges 6 are included in the spot. Thus, the sum of the length of each of these modulation pit edges appear in the readout signal when the beam spot is located at SP1 (modulation data sampling point) as shown in FIG. 4. By sampling the intensity of the readout signal at the position of FIG. 4, the value of the sum of the data recorded in each pit can be obtained. When the lengths of three modulation pit edges are known (known-level edges 52), the length of the remaining pit A (unknown-level edge 51) can be determined on the basis of the value of the sum of the data at the four modulation pit edges. Thus, the unknown-level edge 51 can be turned into a known-level edge 52. Further, as the beam spot moves into a new position SP2 (modulation data sampling point), as shown in FIG. 5, since the length of the modulation pit edge A is already known, the length of the remaining pit edge B can be determined. By repeating this process, the length of all of the modulation pit edges in each track can be known. By repeating the same process from one track to another, the digital data recorded in all of the data pits two-dimensionally can be restored. This is how the signal reproduction method in the two-dimensional PRML system is carried out.
As described above, there is an ongoing effort to increase the recording density of optical discs by narrowing the mark pitch and/or the track pitch. However, as the mark pitch is narrowed, another problem has been identified, which relates to interference between symbols. This problem, called inter-symbol interference, will be hereafter described by referring to FIGS. 6 and 7.
As the mark pitch and/or track pitch is narrowed and consequently the pattern is made denser for the purpose of increasing the recording density, edges of adjoining marks come into the field of view of the single beam spot, as shown in FIG. 6. As a result, leakage light from the surrounding mark edges are mixed in, creating an error in the readout signal. This mixing of leakage light is the inter-symbol interference. When the leakage light is produced not only within the track that is scanned but also from the adjoining tracks on both sides, this is called a two-dimensional inter-symbol interference, because the interference is taking place both radially and circumferentially on the disc.
FIG. 7 shows calculated magnitudes of the two-dimensional inter-symbol interference caused by the mixing of leakage light in an optical disc adapted for the two-dimensional PRML technology. In the calculation, the limit value of an error in the readout signal caused by inter-symbol interference which exceeds one level of the multi-value recording is determined, so that the number of recording bits that can be recorded at one modulation point and the recording density per square inch can be determined. The calculation method for FIG. 7 will be described below.
FIG. 7 plots the magnitude (=y) of inter-symbol interference mixing from the surrounding mark edges (modulation points) in logarithm (with a multiplier of 2 to the power of minus x) on the right vertical axis and the recording density on the left vertical axis, against different recording intervals (horizontal axis) in the case of the example of FIG. 6 where one half of mark pitch is set to be equal to track pitch (recording interval) and equal recording intervals are provided. The beam spot has a Gaussian profile with a diameter of 390 nm, on the assumption that blue light (wavelength λ=405 nm) is focused with a lens having a numerical aperture (NA) of 0.85. The beam spot size (diameter) is herein defined as φ=0.82λ/NA. The values on the right vertical axis are logarithms (LOG) to the base 2 of the ratio of the total amount of light mixing from 12 surrounding modulation points, to the four modulation points (mark edge modulation areas). At point zero of the right vertical axis, the total amount of the mixed light is so great as to equal the modulated amount of light at a single modulation point (mark edge). At point 3 on the right vertical axis, the total amount of mixed light from the surrounding modulation points (mark edges) is one eighth (two to the power of minus three) the modulated amount of light at one modulation point (mark edge). When the total amount of mixed light due to inter-symbol interference is one-eighth the modulated amount, multi-value recording of maximum eight levels can be performed at a single modulation point. Namely, in this case, three bits (log 8=3) of information can be recorded per modulation point.
The values on the left vertical axis are obtained by multiplying the values on the right vertical axis with the modulation point density (the modulation point interval raised to the power of minus two). Thus, the left vertical axis indicates the limit of recording density at which reproduction is possible without compensating inter-symbol interference.
In the current optical disc technology, multi-value recording of up to 64 levels (two raised to the 6th power) per modulation point is the limit. This is due to limitations in controllability of the film thickness of the recording film or in the precision with which the mark is formed, which creates errors. Accordingly, it is calculated that values in the neighborhood of 32 gigabits per square inch are the limit of recording density attainable by the current two-dimensional PRML system when the two-dimensional inter-symbol interference is not compensated, unless some measures are taken, such as shortening the wavelength of the light source used for reading the disc, or in the method of focusing light or compensation. Thus, the light mixing in from the surrounding marks due to the effect of inter-symbol interference creates errors in the readout signal, thereby limiting the recording density during reproduction with a certain beam spot.
That was the problem of recording density caused by inter-symbol interference. In the following, the problem of how to deal with the noise during reproduction created by inter-symbol interference will be described.
There are many factors during the manufacture and reproduction of the optical disc medium adapted for the two-dimensional PRML system which causes errors in the readout signal. These factors include errors in the pattern recorded on the medium, deformation of the pattern during transcription by stamping (mass reproduction), variation in the film thickness of the plastic cover layer formed on the recording layer, and distortion in refractive index. Pattern errors include an overall displacement of the positions of the pits formed as marks between adjoining tracks, and errors in width or height of the pits. These are production errors in the media. In a reproducing apparatus, when the medium is scanned by a beam spot for reproduction, there are errors such as tracking error, focusing error and tilting of the optical axis in the optical system.
These multiple factors produce mainly two types of fluctuations in the readout signal, namely deviations in medium frequency range which undulate slowly at periods of more than several tens of times of the mark pitch, and deviations in high frequency range which are produced at periods identical to the mark pitch.
The deviations in high frequency range are observed in the form of an AC component in which the signal moves up and down alternately at sampling points. The major causes of this include overall discrepancies of the marks between adjoining tracks that are introduced during the manufacture of the medium, and tracking error in the optical system during reproduction. For example, FIG. 25 illustrates the vertical vibration of the readout signal caused by an overall displacement of the marks between adjoining tracks on the medium. When the positions of the marks between the adjacent tracks are aligned as shown in FIG. 25A, the readout signal produces a pattern in which the eyes are opened at the same height at each sampling timing. But when the positions of the marks between the adjacent tracks are misaligned, as shown in FIG. 25B, the readout signal swings in the form of sine wave in which the eyes move up and down alternately at each sampling timing. This is due to the fact that the marks are concentrated on one side at mark-pitch periods. Similar effects are observed when the beam spot is displaced away from the center of the two tracks toward one of the tracks, creating regular deviations in high frequency range.
On the other hand, the deviations in medium frequency range are observed in the form of a DC component of the readout signal slowly changing. Most of the causes other than the causes for the deviations in high frequency range can be causes for this deviations in medium frequency range. Further, deviation factors such as deviations in refractive index, which used to be typically found in a frequency range an order of magnitude lower, have come to be translated into the mid-frequency range as a result of the reduction in the cover-layer thickness from 0.6 mm to 0.1 mm. Thus, the problem of deviation noise is compounded by additional factors that come into play as a result of increasing the recording density.
Of these problems, the deviations in medium frequency range can be compensated by using a filter for tracking and compensating the DC component fluctuations. However, the method based on the detection of the passing of a compensation pattern, which is often used in the prior art, requires many reference patterns to be placed within the pattern on the medium. In this conventional example, in order not to adversely affect the recording density, each frame (sector) typically contains one or two such compensation patterns, and compensation is made at long periods (low frequency) with intervals of more than 100 marks. Thus, this conventional technique is hardly capable of carrying out effective compensation with regard to error (noise) components having periods shorter than a single frame.
Hereafter the deviations in high frequency range will be described. As mentioned above, there are two major factors contributing to this type of deviations.
The deviations in high frequency range caused by an overall misalignment of the marks between adjoining tracks appears in the readout signal as if substantially only a sinusoidal wave component has been added, without changes in the signal intensity ratio between the two tracks that are simultaneously scanned. This phenomenon is reported in Technical Report MR99-76, IEICE (Institute of Electronics, Information and Communication Engineers), as a mixing of the carrier signal caused by misalignment of the pit edge position. The report states that the mixing can be removed by using a filter (carrier canceller) for removing a signal appearing at carrier periods (carrier signal) that is constantly included in the readout signal. This conventional compensation method employing a carrier canceller requires averaging over a relatively long time, because the method carries out tracking compensation by detecting the carrier signal constantly included in the readout signal separately from the original symbol modulation signal. Consequently, there was the problem that the compensation cannot track the overall positional misalignment of the pits, which fluctuates at relatively short periods of eight frames or less, thus failing to provide sufficient compensative effects.
At the same time, the deviations in high frequency range caused by tracking error during reproduction appears in the readout signal, with the signal from only one of the simultaneously scanned tracks emphasized. In this case, not only is the sinusoidal component of the same frequency as in the case of the misalignment between adjoining tracks added, but also different modulation ratios of the symbols in the individual tracks appear in the readout signal. As a result, the readout signal cannot be decoded accurately by the conventional method of using the carrier canceller, which subtracts the sinusoidal component.
To these error factors is further added the signal error caused by the two-dimensional inter-symbol interference, so that the filter response time necessary for eliminating the carrier signal becomes even longer, thus limiting the noise frequency range that can be compensated.
Another method of compensating the two-dimensional inter-symbol interference runs an equalizer filter on the readout signal after reproduction. This method, however, has the disadvantage that the noises due to the various factors mentioned above are also enhanced when the signal at a high region is enhanced.
Thus, in a high-recording density optical disc in which pits are arranged at smaller intervals than the beam spot, in order to further increase the recording density and allow the information to be reproduced at high reliability, the problems of limited recording density due to the two-dimensional inter-symbol interference have to be solved and means must be provided for responding to the manufacturing error of those optical disc medium and the control error during reproduction as fast as possible and compensating them accurately. It is, therefore, desired to provide an optical disc capable of playing back information highly reliably and an inexpensive optical disc apparatus capable of recording a great amount of information at high density.
Hereafter, the term “marks” will be used instead of “pits” that are used on the ROM (read-only memory) optical disc so far described in the present section. This is in order to include the optical discs in which information is recorded based on changes in the characteristics (such as the crystalline structure and refraction index) of the substance formed by writing, such as the RAM (random access memory) media. Correspondingly, the length between the center of the mark to its mark edge will be referred to as “mark edge length” instead of “the length of the modulation pit edge.” Modulating the length of the pit edge in accordance with the recorded information, which corresponds to modulating the area of the mark edge of a modulation point by shifting the mark edge position, will be referred to as “modulating the mark edge length.”
In the checkered mark arrangement, since the lengths of the left-half and right-half of the mark are independently modulated, the mark edge lengths are different on the left-side and right-side of the mark. This is defined in FIG. 12. The center (of gravity) of a non-modulated mark is designated as a mark center 90. The interval between the centers of any two adjacent non-modulated marks is designated as a mark pitch 91 along the tracks (circumferential direction) and a track pitch 92 radially.
The length between the mark center 90 and the edges of a mark is designated as a left-side mark edge length 93 on the left-hand side and as a right-side mark edge length 94 on the right-hand side. When the left- and right-side mark edge lengths are mentioned as a whole, they will be designated simply as the mark edge length.
The four edges including any two mark edges adjoining each other along the radius and any two mark edges adjoining each other along the tracks (including the edges on opposite sides of the same mark) are designated as adjacent mark edges.