JP2007066464A - Correction processing of reproducing signal of multivalued information - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To increase the separation degree of the distribution of reproducing signals and to reduce the effect of inter-code interference of multivalued information. <P>SOLUTION: The ratio of the change of reproducing signal amplitude that a center cell can take for the total value of the multiple values of front and rear cells in three consecutive cells is acquired beforehand. Then, when performing reproduction, the total value of the multiple values of the adjacent front and rear cells is acquired for each cell, and the reproducing signal amplitude acquired in the center cell is corrected on the basis of the total value and the ratio of the change. As a result, by reproducing the multivalued information on the basis of the corrected reproducing signal amplitude, the effect of the inter-code interference of the multivalued information is reduced. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is an optical information recording / reproducing apparatus and method for recording or / and reproducing multi-value information, in which information pit levels are recorded or reproduced using values of three or more values. In particular, it is for correcting the influence of intersymbol interference and / or the influence of non-linearity from a reproduction signal obtained from an optical information medium.

  In recent years, the optical memory industry has been expanding, and it has been developed from read-only CDs and DVDs to write-once using metal thin films and dye-based recording materials, as well as rewritable using magneto-optical materials and phase change materials. Its application is expanding from consumer to external memory of computers.

  Research and development for further increasing the recording capacity has been promoted, and the wavelength of the light source has changed from red (650 nm) to blue-violet (405 nm) as a technique for miniaturizing the light spot related to information recording / reproduction. It is going The numerical aperture of the objective lens is also being increased from 0.6 or 0.65 to 0.85. On the other hand, more efficient multilevel recording / reproducing techniques using the same light spot size have also been proposed.

  For example, the inventor of the present application has proposed Japanese Patent Application Laid-Open No. 5-128530 as a method of multilevel recording / reproducing technology. The recording method records multi-value information on an information track of an optical information recording medium by combining the width of the information pit in the track direction and the amount of shift in the track direction with respect to the reproduction light spot of the information pit. When reproducing the multi-value recorded information pit, a reproduction method for reproducing multi-value information from the correlation between a detection signal learned in advance and a detection signal obtained from a light spot is proposed (Patent Document). 1).

  Also, ISOM 2003, which is an international conference in research on the field of optical discs, has announced that multilevel recording / reproduction of 8 levels has been performed (Non-patent Document 1). The optical system is a blue-violet light source (405 nm) and NA 0.65. For an optical disc having a track pitch of 0.46 μm, the width in the track direction of a region (hereinafter referred to as a cell) in which one information pit virtually provided is recorded is 0.26 μm.

In addition, the applicant further uses a blue-violet light source (405 nm) and an NA 0.85 optical system to reduce the light spot and adapt to the multi-value scheme announced at ISOM 2003, about 30 Gbit / inch ² There has been a proposal to increase the density.

  According to this, the selection of 8-level information pits, for example, divides the width of the cell in the track direction (A direction in the figure) into 16 equal parts (16 channel bits) as shown in FIG. No information pit is recorded at level 0. Level 1 is 2 channel bits wide. Level 2 is 4 channel bits wide. Level 3 is 6 channel bits wide. Level 4 is 8 channel bits wide. Level 5 is 10 channel bits wide. Level 6 is 12 channel bits wide. Level 7 is 14 channel bits wide.

  FIG. 14 shows a schematic diagram when a random information pit is recorded on a track on the optical disc and the relationship between the light spots.

  In order to increase the storage capacity, it is necessary to reduce the size of the cell, and if it is reduced, information pits of 2 to 3 cells are included in the light spot as shown in FIG. In the figure, the direction of arrow A similarly indicates the track direction, each cell is virtually provided with a region delimited by a broken line, 11 is a track on the optical disc, 12 is a random information pit, Indicates an optical disc to be recorded.

Here, the cell width is set to 0.2 μm with respect to the size of the light spot of about 0.405 μm. With this scale, the surface density is improved by about 1.5 times compared to the surface density of about 19.5 Gbit / inch ² when the conventional method of binary level (for example, 1-7PP modulation, 2T = 139 nm) is used. Is possible.

  Next, in order to know the state of the reproduction signal by this method, the result of optical simulation will be described.

  FIG. 15 shows parameters used for the optical simulation. The track pitch is 0.32 μm. The size of the light spot is 0.405 μm (wavelength 405 nm, numerical aperture of objective lens: NA 0.85). The cell size is 0.2 μm. The shape of the given information pit was given as shown in FIG. 16 for each level shown in FIG.

  In FIG. 17, eight types of levels are sequentially combined to three consecutive cells (all combinations are 8 × 8 × 8 = 512), and the light spot is the third cell from the center of the first cell (previous cell) ( The reproduction signal (the amount of reflected light) when moving to the center of the back cell was calculated.

In the lower diagram of FIG. 17, eight combinations of levels (0, 1, 6) to (7, 1, 6) of three cells are shown as examples. (All three cells are set to 0 level)
The positions of the three solid lines in the figure indicate the reproduction signal (cell median value) when the light spot is in the center of each cell, and the positions of the two broken lines are the light spot at the boundary between the cell and the following cell. The reproduction signal (inter-cell value) in this case is shown.

  It can be seen from this that, in this parameter, the cell center value of the middle cell corresponds to the level “1”, but the same value is obtained by changing the level of the left cell from “0” to “7”. It can be seen that it has a width. This is the effect of intersymbol interference. However, it can be seen that if the level of the middle cell is the same as “1”, the cell median value of the right cell is almost the same regardless of the level of the left cell. That is, it can be seen that the range of influence of intersymbol interference on the cell median is only from the left and right neighboring cells, and the influence outside it can be ignored. In FIG. 14, it can be understood intuitively that the hem of the light spot on the middle cell is on the left and right cells.

  FIG. 18 shows the distribution of the amplitude of each reproduction signal, with the horizontal axis representing the level of the middle cell in all combinations of levels recorded in three consecutive cells. The distribution from A to H in the figure corresponds to levels 0 to 7, respectively (normalized by the reflectance of the mark part and the non-mark part here).

As can be seen from the figure, the overlap of the distributions of the reproduction signals at adjacent levels increases, and it is difficult to identify the level using a fixed threshold value as it is. Therefore, in general, the reproduction signal is subjected to signal processing such as waveform equalization to increase the degree of separation of the reproduction signal distribution. For example, a 3-tap waveform equalization as shown in FIG. 18 is calculated. Here, T is the time during which the light spot moves from the center of one cell to the center of the adjacent cell, and a is an equalization count, where a = −V1 / (1 + V1), V1 = 0.237. As calculated. (V1: Amplitude value in adjacent cell for isolated waveform with amplitude 1)
FIG. 19 shows the result. A 'to H' correspond to the distribution of levels 0 to 7, respectively. As can be seen from the figure, each distribution can be separated with a fixed threshold.

Japanese Patent Laid-Open No. 5-128530 ISO2003 (Write-once Disks for Multi-level Optical Recording: Proceedings Fr-Po-04)

  In the result shown in FIG. 20, the separation can be performed with a fixed threshold value. However, this is an ideal simulation result, and medium noise, recording noise, system noise, and the like are added to this, and the respective distributions are expanded. This increases the possibility of a reproduction error. Also, the reproduction margin is reduced. Therefore, it is necessary to correct the reproduced signal with a higher degree of separation.

  Accordingly, an object of the present invention is to further increase the degree of separation of the reproduction signal distribution and reduce the influence of intersymbol interference of multilevel information.

  In order to achieve the above object, the present invention provides the following.

  For an optical information recording medium provided with a track, the width of the information pit or the area of the information pit is changed in the track direction in a virtual fixed interval cell provided on the track, and In an optical information recording / reproducing apparatus for recording and / or reproducing n-level multi-level information by making the amplitude multi-stage, possible reproduction of the central cell with respect to the sum of the multi-level values of the preceding and following cells in three consecutive cells A means for acquiring in advance the rate of change in signal amplitude; a means for acquiring a total value of multi-values of adjacent cells adjacent to each cell during playback; and a center based on the total value and the rate of change. An optical information recording / reproducing apparatus comprising: reproducing means for correcting reproduction signal amplitude acquired in a cell and reproducing multi-value information based on the corrected reproduction signal amplitude.

  According to the present invention, the influence of intersymbol interference of multilevel information can be reduced, the degree of separation of the reproduction signal distribution is increased, and the reproduction accuracy is improved.

  Next, the best mode for carrying out the invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of an embodiment of an optical information recording / reproducing apparatus according to the present invention.

  The optical information recording / reproducing apparatus 1 includes a control circuit 2, a spindle motor 3, an optical disk 4, an optical head 5, an optical head control circuit 6, an information recording circuit 7, an information reproducing circuit 8, a spindle motor controller 9, and an interface controller 10. .

  The control circuit 2 controls transmission / reception of information to / from an information processing apparatus such as an external computer, controls information recording / reproduction with respect to the optical disc 4 using the information recording circuit 7 and the information reproduction circuit 8, and other Control the working part. The information recording circuit 7 records multi-value information as will be described later, and the information reproduction circuit 8 reproduces multi-value information.

  The spindle motor 3 is controlled by the spindle motor controller 9 and drives the optical disk 4 to rotate. The optical disk 4 is an optical information recording medium that is inserted into or ejected from the optical information recording / reproducing apparatus 1 by a mechanism (not shown).

  The optical head 5 optically records information on the optical disk 4 and reproduces it. As for the optical head 5, as in the conventional example, when the wavelength of the light source is 405 nm and the numerical aperture of the objective lens is 0.85, the size of the light spot is approximately 0.405 μm. The track pitch of the optical disk 4 is 0.32 μm. The optical head control circuit 6 controls the position of the light spot by the optical head 5, and performs auto tracking control, seek operation control, and auto focusing control.

  The width in the track direction (direction A in the figure) due to the difference in the level of the multilevel information pit used in the optical information recording / reproducing apparatus of the present invention is the same as that shown in FIG.

  This method is 8-level recording, and 3 bits can be recorded in one cell. For example, (0, 0, 0) is 0 level for 3-bit information. (0, 0, 1) is 1 level. (0,1,0) is 2 levels. (0, 1, 1) is 3 levels. (1, 0, 0) is 4 levels. (1, 0, 1) is 5 levels. (1,1,0) is 6 levels. (1, 1, 1) is assumed to correspond to 7 levels. Of course, other corresponding methods may be used.

  2 to 11 show the results of optical simulation, and the present invention will be described using these results. The parameters used for the calculation are the same as those described with reference to FIGS.

  As in FIG. 18, FIG. 2 shows the distribution of the amplitude of each reproduction signal before waveform equalization by taking the level of the middle cell on the horizontal axis in all combinations of levels recorded in three consecutive cells. It is shown. The distribution from A to H in the figure corresponds to levels 0 to 7, respectively (normalized by the reflectance of the mark part and the non-mark part here). Due to intersymbol interference, adjacent level distributions overlap and cannot be detected with a fixed threshold.

  The points indicated by X in the figure are (3, 0, 2), (3, 1, 2), (3, 2, 2), (3, 3, 2), (3, respectively. 4, 2), (3, 5, 2), (3, 6, 2), (3, 7, 2). As can be seen from this, it can be seen that if the combination of the preceding and following cells is the same, there is an X mark at the relatively same position in the distribution of each level.

  FIG. 3 further plots the results of FIG. 2 with the horizontal axis as the sum of the levels of the preceding and following cells. The vertical axis represents the amplitude of the reproduction signal, which is normalized by the reflectance of the mark portion and the non-mark portion. Each distribution from A to H is developed into a distribution of the mark ● on a straight line from I to P. From the figure, it can be seen that if the sum of the preceding and succeeding cells is the same, the amplitude of the reproduction signal takes substantially the same value. For example, when viewed in a straight line corresponding to the 0 level of I, a combination of (0, 0, 5), (1, 0, 4), (2, 0, 3, ), (3, 0, 2), (4, 0, 1), (5, 0, 1) all overlap the Q point.

  Considering the waveform equalization processing performed in the conventional example, it can be understood that the straight line distribution obtained in FIG. 3 may be corrected to a straight line having a slope of 0 passing through a certain reference value.

  The method will be specifically described. In FIG. 4, for the straight line distribution obtained in FIG. 3, the reference value is a value when the sum of the levels of the preceding and subsequent cells is 7. The reference value of each level is indicated by a circle, and is located at the approximate center of each of the distributions A to H corresponding to each level shown in FIG.

  The reference value and slope of each straight line I to P (straight line corresponding to each level) are shown together in FIG. In order to correct the reproduction value to the reference value, the difference between the reproduction value and the reference value may be subtracted.

  That is, when a straight line is obtained as Y = −bX + c (X is a horizontal axis, Y is a vertical axis, b is a slope, and c is an intercept value), a Y value when X = 7: (−7b + c) Is the reference value for each.

When the reproduction signal is S, and the total value Xs of the levels of the cells before and after that is obtained: Xs,
[S + {(− 7b + c) − (− Xs * b + c)}] = [S + (Xs−7) b]] is corrected.

  As can be seen from FIG. 3 or FIG. 5, the slopes of the eight straight lines from I to P are not the same. This is because the shape of the light spot is not uniform, and the amount of light decreases with increasing distance from the center of the spot. Therefore, as the mark with a large width (or area) is involved, the change rate decreases from linear. This is because of nonlinearity. Moreover, it turns out that not only the inclination of 8 straight lines but the space | interval of each straight line becomes so narrow that it goes down. This is also a non-linearity that occurs because the light spot is not uniform. These two nonlinearities cannot be corrected by conventional waveform equalization. Therefore, even if an ideal calculation is performed, the distribution of FIG. 2 does not converge to one point, and the spread remains as shown in FIG.

  Now, when performing correction, for example, if the slope of the straight line at level 7 is used as a representative value for correction value counting, the correction result is as shown in FIG. Seven levels adopting the slope converge to almost one point. Further, it can be seen that even if the counter electrode is at the 0 level, it is converged compared to the result of FIG. Here, the 7-level gradient with many non-linearities has been described as the representative value, but the central level, for example, the gradient of the 3rd or 4th level, may be used as the representative value for the correction value counting. The inclination correction value count may be an average value of inclinations of eight straight lines from I to P.

  In the result of FIG. 6, the reference value (center value) of the distribution of each level becomes smaller as the level increases. Next, a method for correcting this will be described.

The left side of FIG. 7 shows the reproduction values of the isolated marks at each level (the preceding and following cells are at the 0th level and the central cell is at each level). Considering correcting this to an equally spaced value as shown on the right side, the correction curve is obtained as {y = −0.7031x ² + 2.0556x−0.3894}. (X is a value before correction, y is a value after correction) Using this correction curve, if the nonlinearity of the distribution of FIG. 6 is corrected, FIG. 8 is obtained.

  As can be seen from the figure, the reference value (median value) of each distribution is corrected at equal intervals. After the above correction is performed on the sample value, the absolute value of the difference from the reference value of each level is calculated, and the level having the smallest value is adopted as the reproduction value. Although the non-linearity is corrected using the value of the isolated mark, any mark may be used as long as the position in the distribution is the same as other reference values.

  Up to this point, the description has been made using ideal values, but in reality, system noise, recording noise, and the like are added to the sample values, and the distributions shown in FIGS. Become.

  A series of corrections is an operation of bringing each sample value close to the original reproduction level reference value. In reproduction, the difference between the reference value of each level after correction and the sample value after correction is obtained, and the level having the smallest difference is set as the reproduction level. Thereby, multi-level reproduction with high accuracy can be performed.

  In FIG. 6, since the slope of the level 7 is adopted as the representative value of the correction value count, the distribution spreads toward the level 0. Therefore, a method of correcting the inclination of each level as a correction value count will be described next.

  For the sample value: S, the correction formula is [S + (Xs−7) b] as described above. Here, however, b employs the inclination shown in FIG. 5 according to each level.

FIG. 9 shows the result. It can be seen that the distribution converges to one point at all levels. Further, considering the correction of non-linearity in the same manner as in FIG. 8, if the correction value on the right side is adopted for the isolated mark of each level in FIG.
The correction curve is {y = −0.75551x2 + 2.1435x−0.4295}.

  FIG. 11 shows the distribution after final correction. It can be seen that the levels converge to almost one point, and the respective reference values are equally spaced. After the above correction for the sample value, the absolute value of the difference from the reference value of each level is calculated, and the smallest level is adopted as the reproduction value.

  The correction for removing the influence of the intersymbol interference and the correction for removing the non-linearity have been described above with respect to the sample value. A method for reproducing the information by performing the correction will be described again with reference to the flowchart of FIG. Here, a method of correcting using the slope of each level will be described.

  First, playback operation is started at Step 1. In the Set 2, the distribution (learning table) shown in FIG. 2 or 3 is obtained. In Step 3, the slope of each level: b and the reference values: t0, t1,... T7 are obtained from the learning table. (As shown in FIG. 5).

  The method for obtaining the learning table described so far is to record all the patterns of three consecutive cells together with information on the optical disc as learning data, read it out during reproduction, and create a learning table. Anyway. Furthermore, a learning table pattern obtained by simulation may be stored, and a learning table may be obtained by fitting by reproducing a part of data recorded on the optical disk.

  In Step 4, the reproduction of information is started and the sample value Sk is sequentially obtained. The process proceeds to Step 5 to determine what the level of the sample value Sk obtained sequentially, and it is provisionally determined and stored in a memory or the like. At this time, as a provisional determination method, the absolute value of the difference between the reference value of each level obtained in Step 3 and the sample value Sk obtained in Step 4 is obtained. The lowest level may be set as a provisional level, or may be determined by obtaining the difference between the reference value and the sample value obtained after performing the waveform equalization shown in the conventional example.

  Proceeding to Step 6, the influence of intersymbol interference is corrected with the correction formula [Sk + (Xs−7) b]. In this case, Xs, which is the total value of the preceding and succeeding levels, uses the provisionally determined preceding and following cell level values obtained in Step 5. Of course, the value after reproduction may be used as the value of the previous cell. Further, a series of reproduction may be repeatedly processed. Here, since individual inclinations are used for eight levels, eight correction values are obtained for one sample value Sk. (Sk0, Sk1,... Sk7) When the gradient of one level is used as a representative value, or when the gradients of eight levels are averaged, the correction value is one.

Proceeding to Step 7, the nonlinearity is corrected. Here, the correction curve obtained from the sample value of the isolated mark is used. (As shown in FIG. 10) The eight correction values obtained in Step 6 are further corrected for nonlinearity. (Sk′0, Sk′1,..., Sk′7), the reference value is also corrected at equal intervals. (T′0, t′1,..., T′7)
Proceed to Step 8 and correspond to each level. The difference | Sk′n−t′n | between the correction value and the reference value (n = 0 to 7) is calculated. Then, in Step 9, the level having the smallest value is set as the reproduction value.

  When the process proceeds to Step 10 and information reproduction is continued, the process returns to Step 4 to continue reproduction sequentially, and when it is finished, the process proceeds to Step 11 and the reproduction operation is completed.

  Here, in Step 7, since the correction of the reference value becomes the same value after the second time, it can be omitted.

  The eight levels have been described above. In general, in the case of the n level, the learning table is obtained with the reproduction signals of all combinations of the n levels of three consecutive cells. Accordingly, a correction value is obtained, and the reproduction signal of the central cell is corrected so as to approach the reference value according to the level of the central cell with a correction amount according to the total number of levels of the preceding and succeeding cells with respect to the central cell. This correction can reduce the influence of intersymbol interference. At this time, the nonlinearity can also be corrected by using a correction value according to each level.

  Furthermore, the non-linearity can be further corrected by correcting so that the reference values of the respective levels are equally spaced.

  In FIG. 12, the method has been described in which correction is performed by tentatively determining the levels of the preceding and subsequent cells. In this case, n levels of the preceding and following cells are set as integers, but may be expressed as analog values according to the sample values. For example, if the sample value is between the reference value of 3 and the reference value of 4 and the difference between the two reference values is normalized to 1, and the sample value is at the position of 0.35, the sample value is 3.35 and analog. It will be expressed as a value. In this way, the levels of the preceding and succeeding cells may be determined as analog values to be Xs, and similarly corrected according to the formula [Sk + (Xs−7) b].

  In the case of using a phase change material as a recording material that can be recorded and erased as the optical disk 4, the optical spot 4 is irradiated with a light spot 13 to adjust the light quantity and timing of each of the recording pulse, the erasing pulse, and the cooling pulse. Thereby, the shape of the information pit is changed, and information pits having a plurality of reproduction levels are formed. In the above description, for convenience, the information pits are shown as rectangular information pits, and the state in which the width in the track direction is changed is shown. However, as long as it corresponds to the area of each level, the information pits may not be rectangular information pits. The essence of this proposal does not change even if it is a round, oval, or arrowhead type mark.

  In addition to the phase change material, a magneto-optical material can be used as the recording material that can be erased. In this case, in the optical information recording / reproducing apparatus described above, the information pit shape is changed by cooperative work with a magnetic field from a magnetic head (not shown) other than the light spot, and information pits having a plurality of reproduction levels are formed.

  Furthermore, it is also possible to apply a recording material that can only be additionally recorded. An organic dye or a metal film can be used as a recording material, and the optical spot is irradiated with a light spot. Then, by adjusting the recording light quantity and their timing, the shape of the information pit is changed, and information pits having a plurality of reproduction levels are formed. Similarly, even in a read-only recording medium, information pits can be formed on a substrate as uneven shapes called phase pits, and multi-level recording is performed by modulating the area of the phase pits or the optical depth of the phase pits. Is possible.

  In order to obtain the slope of the straight line obtained in FIG. Specifically, the slope of the straight line can be obtained by obtaining two or more reproduction signal values that can be taken by the central cell with respect to the total value of the multi-values of the preceding and succeeding cells.

  Thereby, learning can be performed in a small area, and the efficiency of the medium can be improved.

1 is a block diagram showing an embodiment of an optical information recording / reproducing apparatus according to the present invention. The figure which shows the characteristic of the learning table which concerns on this invention. Similarly, the figure which shows the characteristic of the learning table which concerns on this invention. The figure explaining the reference value used for correction | amendment and reproduction | regeneration of this invention. The figure which shows the example of the reference value and inclination used for correction | amendment and reproduction | regeneration of this invention. The figure which took the inclination of level 7 as an example in the Example which correct | amends the influence of the intersymbol interference of this invention. The figure explaining the correction value at the time of correct | amending the nonlinearity of this invention. The figure which shows the result which correct | amended the nonlinearity of this invention. The figure which shows the result using the inclination of each level in the other Example which correct | amends the influence of the intersymbol interference of this invention. The figure explaining the correction value at the time of correct | amending the other nonlinearity of this invention. The figure which shows the result of having corrected other nonlinearity of the present invention. The flowchart explaining the correction | amendment and reproduction | regeneration method of this invention. The figure explaining a multi-value mark. The figure which shows the relationship between a schematic diagram when recording a random information pit with respect to a track | truck, and a light spot. The figure explaining the parameter of optical simulation. The figure explaining the shape of the information pit given by the optical simulation. The figure explaining the reproduction signal with respect to the combination of the information pit written in three continuous cells by the calculation result of optical simulation. A graph showing the amplitude distribution of the cell median. The horizontal axis is the level of the center cell. The figure for demonstrating the waveform equalization of 3 taps. The figure explaining the result after waveform equalization.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical information recording / reproducing apparatus 2 Control circuit 3 Spindle motor 4 Optical disk 5 Optical head 6 Optical head control circuit 7 Information recording circuit 8 Information reproducing circuit 9 Spindle motor controller 10 Interface controller 11 Track,
12 Information pit 13 Light spot

Claims (5)

For an optical information recording medium having a track, the width of the information pit or the area of the information pit is changed in the track direction to a virtual fixed interval cell provided on the track, and In an optical information recording / reproducing apparatus for recording and / or reproducing n-level multilevel information by making the amplitude multistage,
Means for acquiring in advance a ratio of a change in reproduction signal amplitude that can be taken by the central cell with respect to a total value of multi-values of preceding and succeeding cells in three consecutive cells;
Means for obtaining the total value of the multi-values of adjacent cells before and after each cell when playing,
Reproducing means for correcting reproduction signal amplitude acquired in a central cell based on the total value and the rate of change, and reproducing multi-value information based on the corrected reproduction signal amplitude. An optical information recording / reproducing apparatus.   2. The optical information recording / reproducing apparatus according to claim 1, wherein the ratio of the change in the reproduction signal amplitude is acquired for each level of the central cell.   3. The optical information recording / reproducing apparatus according to claim 2, wherein the reproduction signal amplitude change ratio is one of the change ratios acquired for each level of the central cell.   2. The optical information recording / reproducing apparatus according to claim 1, wherein the change rate of the reproduction signal amplitude is an average value obtained for each level of the central cell. 2. The optical information recording / reproducing apparatus according to claim 1, wherein the correction is performed by setting the corrected reproduction signal amplitude at equal intervals for each level.

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