WO2011077552A1

WO2011077552A1 - Two dimensional encoding method, holographic reproduction device, and holographic recording/reproduction device

Info

Publication number: WO2011077552A1
Application number: PCT/JP2009/071615
Authority: WO
Inventors: 昭人小川; 一右土井; 裕柏原
Original assignee: 株式会社東芝
Priority date: 2009-12-25
Filing date: 2009-12-25
Publication date: 2011-06-30

Abstract

A two dimensional encoding method wherein, with respect to a pixel set wherein pixels are arranged in square with n-pieces (n is an integer of at least 4) in the row direction and n-pieces in the column direction, two-dimensional code words are expressed by combinations of K-pieces (K is an integer of at least n) of ON-pixels and (n×n-K)-pieces of OFF-pixels; a code word group is selected from the two-dimensional code words, so that the group satisfies the conditions that (a) at least one ON-pixel is present within (2n-1)-pieces of pixels in the upper edge row and the right edge column, (b) at least one ON-pixel is present within (2n-1)-pieces of pixels in the right edge column and the lower edge row, (c) at least one ON-pixel is present within (2n-1)-pieces of pixels in the lower edge row and the left edge column, and (d) at least one ON-pixel is present within (2n-1)-pieces of pixels in the left edge column and the upper edge row (S401); and only the code words included in the code word group are allocated to user data (S402).

Description

Two-dimensional encoding method, holographic reproducing apparatus, and holographic recording / reproducing apparatus

The present invention relates to a two-dimensional encoding method suitable for holographic memory technology.

In holographic memory technology, user data is modulated (encoded) into a two-dimensional codeword and recorded on a recording medium. Usually, this two-dimensional code word is expressed by a combination of binary pixels (ON pixel and OFF pixel). More specifically, the user data is divided into predetermined unit data such as 10 bits and 8 bits. These predetermined units of data are converted into codewords uniquely assigned in advance.

In general, a code word in the holographic memory technology has a fixed size (the total number of pixels of the code word is constant), and the ratio between the ON pixel and the OFF pixel is also fixed. In Patent Document 1, a ² ON pixels, 3a ² OFF pixels in a pixel array arranged in a square shape with 2a (a is an integer of 1 or more) in the row direction and 2a in the column direction, The two-dimensional code word expressed by the combination of is illustrated. In Patent Document 2, b-1 ON pixels in a pixel set arranged in a square shape of b (b is an integer of 3 or more) in the row direction and b squares in the column direction, and b ² -b + 1 A two-dimensional code word expressed by a combination with an OFF pixel is illustrated.

From the principle of holographic memory technology, it is desirable that the encoding method and codeword satisfy the following three conditions (A) to (C).
(A) The ON pixel ratio is small.
(B) Encoding efficiency is high.
(C) The frequency band is limited.
Regarding the condition (A), the higher the ratio of ON pixels in the entire code word, the higher the diffraction efficiency of the hologram necessary for recording data of one page (the concept of the page will be described later). That is, the multiplicity of recording decreases. As an example, Non-Patent Document 1 discloses that an ON pixel ratio of about 20 to 30% is appropriate. That is, by using an encoding method and codeword that satisfy the condition (A), the multiplicity of recording increases.

Regarding condition (B), encoding efficiency is the ratio of the number of bits of user data to which a codeword is allocated to the total number of pixels (number of bits) of the codeword. For example, if a specific encoding method assigns a 16-pixel code word to 8-bit user data, the encoding efficiency of this encoding method is 8/16 (50%). On the other hand, if another encoding method assigns a 16-pixel code word to 12-bit user data, the encoding efficiency of this encoding method is 12/16 (75%). That is, if the conditions (the number of usable pixels, etc.) are the same, the latter encoding method can record 1.5 times as much user data as the former encoding method. That is, it is possible to substantially increase the recording capacity of the holographic memory by using an encoding method and a code word that satisfy the condition (B).

Regarding the condition (C), each code word has frequency characteristics corresponding to the arrangement of ON pixels and OFF pixels (for example, the number of continuous ON pixels or OFF pixels). When reproducing the holographic memory, signal processing is performed on the reproduction light of the recorded code word. However, the reproduced light includes noise components such as low-frequency noise (such as luminance unevenness) and high-frequency noise (such as scattered light and stray light) in addition to the signal component indicating the recorded code word. If the frequency band of the codeword is limited, the noise component can be removed by extracting the in-band component from the reproduction light (suppressing the out-of-band component). That is, by using an encoding method and a code word that satisfy the condition (C), high-precision recording / reproduction of the holographic memory can be performed. In addition, since the holographic memory is recorded on the Fourier transform plane, it is known that if the low frequency component of the code word is strong, the recording light distribution is concentrated on the optical axis, resulting in recording failure. Therefore, it is necessary to limit the frequency band so that the low frequency component is not particularly strong.

Japanese Patent Laid-Open No. 9-197947 JP 2001-75463 A

It can be evaluated that the two-dimensional encoding method described in Patent Document 1 has an ON pixel ratio of 25% and satisfies the condition (A). Also, in this encoding method, for example, when a = 1, the number of ON pixels in the code word is one (that is, the continuous number of ON pixels is limited to a maximum of one). It can be evaluated that C) is also satisfied. However, in this encoding method, for example, when a = 1, the encoding efficiency is 50%, and it cannot be evaluated that the condition (B) is satisfied. Encoding efficiency is improved by increasing to a = 2, for example. However, since the number of ON pixels in the code word increases in proportion to the square of a and the number of consecutive pixels is not limited, increasing a has an adverse effect on the condition (C).

The two-dimensional encoding method described in Patent Document 2 can be evaluated as satisfying the condition (A) because, for example, when b = 4, the ON pixel ratio is about 19% (3/16). Regarding the condition (C), this encoding method has a certain effect on the frequency band limitation because the number of ON pixels in the code word is limited to b-1. However, the two-dimensional encoding method described in Patent Document 2 has an encoding efficiency of 50% when b = 4, for example, and cannot be evaluated as satisfying the condition (B). Specifically, the number of patterns in which three of the 16 pixels are ON pixels is 560 ( ₁₆ C ₃ ). Therefore, in order to uniquely assign this code word, the number of patterns of user data in a predetermined unit must be 560 or less. Although the number of 9-bit patterns is 512 (2 ⁹ ), for convenience, multiple bits of 2 are generally used for dividing user data. That is, in the case of b = 4, the maximum number of user data that can be uniquely assigned a code word by this encoding method is 8 bits.

As described above, it is difficult to satisfy all the conditions (A) to (C) when the conventional two-dimensional encoding method is used. That is, the conventional two-dimensional encoding method has room for improvement, particularly with respect to the holographic memory technology.

Therefore, an object of the present invention is to provide a two-dimensional encoding method suitable for holographic memory technology.

A two-dimensional encoding method according to an aspect of the present invention is a two-dimensional encoding method for holographic recording, in which n (n is an integer of 4 or more) in the row direction and n squares in the column direction. From the two-dimensional codeword expressed by a combination of K (K is an integer of n or more) ON pixels and (n × n−K) OFF pixels in the array of arranged pixels, (a) the top row And at least one ON pixel in (2n-1) pixels in the rightmost column, and (b) at least one ON pixel in (2n-1) pixels in the rightmost column and the bottom row. (C) there is at least one ON pixel among the (2n−1) pixels in the bottom row and left column, and (d) (2n−) in the left column and top row. 1) Satisfies the condition that at least one ON pixel exists in one pixel Comprising selecting a first code word group, and assigning only code word contained in the first code word group to the user data.

A holographic reproducing device according to another aspect of the present invention includes a light source that generates laser light, and a set of pixels arranged in a square shape with n (n is an integer of 4 or more) in the row direction and n squares in the column direction. From a two-dimensional codeword expressed by a combination of K (K is an integer of n or more) ON pixels and (n × n−K) OFF pixels in (a) (a) ( (2n-1) at least one ON pixel exists in the pixels, (b) at least one ON pixel exists in the (2n-1) pixels in the rightmost column and the bottom row, c) there is at least one ON pixel among the (2n-1) pixels in the lower end row and the left end column, and (d) (2n-1) pixels in the left end column and the upper end row. In the first codeword group satisfying the condition that at least one ON pixel exists in An image sensor that receives reproduction light generated by irradiating the laser beam onto a holographic recording medium on which user data encoded using only the code word is recorded, and the code word indicated by the reproduction light. And a playback unit for playing back user data.

A holographic recording / reproducing apparatus according to another aspect of the present invention includes a light source that generates laser light, n pixels (n is an integer of 4 or more) in a row direction, and n pixels that are arranged in a square shape in a column direction. From the two-dimensional codeword expressed by the combination of K (K is an integer equal to or greater than n) ON pixels and (n × n−K) OFF pixels in the set, (a) the top row and the right column (2n-1) pixels have at least one ON pixel, and (b) the rightmost column and the bottom row have (2n-1) pixels, at least one ON pixel exists, (C) there is at least one ON pixel among the (2n-1) pixels in the lower end row and the left end column, and (d) (2n-1) end pixels in the left end column and the upper end row A first codeword that satisfies the condition that at least one ON pixel exists in the pixel An encoding unit that performs encoding that uniquely assigns only the codeword included in the user data, and outputs recording light obtained by modulating the laser light in accordance with the codeword assigned to the user data to the holographic recording medium A modulation unit; an imaging element that receives reproduction light generated by irradiating the holographic recording medium with the laser beam; and a reproduction unit that reproduces the user data based on a codeword indicated by the reproduction light.

According to the present invention, a two-dimensional encoding method suitable for holographic memory technology can be provided.

The figure which shows a holographic recording medium. Explanatory drawing of a book and a page. The block diagram which shows a holographic recording / reproducing apparatus. Explanatory drawing of multiple recording. The block diagram which shows a recording signal processing part. The block diagram which shows a reproduction | regeneration signal processing part. The figure which shows a normal page data image. The figure which shows the image of page data in case the low frequency noise has arisen. The figure which shows a normal page data image. The figure which shows a page data image in case the high frequency noise has arisen. Explanatory drawing of page data. Explanatory drawing of the dotted-line area | region in FIG. Explanatory drawing of a code word. The graph which shows the relationship between ON pixel ratio and relative capacity | capacitance. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows the collection of codewords. The figure which shows the collection of codewords. The graph which shows the frequency component corresponding to FIG. 14A. The graph which shows the frequency component corresponding to FIG. 14B. The flowchart which shows the two-dimensional encoding method which concerns on 1st Embodiment. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a code word. The figure which shows a synchronization mark. Explanatory drawing of the relationship between a synchronous mark and the combination of a codeword. The figure which shows the partial area | region of the synchronous mark of FIG. The figure which shows the code word similar to the partial area | region of FIG. The figure which shows the code word similar to the partial area | region of the synchronous mark of FIG. The figure which shows the code word similar to the partial area | region of the synchronous mark of FIG. The figure which shows the code word similar to the partial area | region of the synchronous mark of FIG. Explanatory drawing of the relationship between a synchronous mark and the combination of a codeword. The figure which shows a synchronization mark. The figure which shows a synchronization mark. The figure which shows a synchronization mark. The figure which shows a synchronization mark.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 shows a recording medium 10 on which codewords are recorded based on the two-dimensional encoding method according to the first embodiment of the present invention. The outer shape of the recording medium 10 is a thick disk shape. The recording medium 10 has a plurality of layers in the thickness direction. Specifically, the recording medium 10 has a laminated structure in which the protective layer 50 and the protective layer 60 sandwich the recording layer 70 (including the servo information recording layer 80). The recording layer 70 has a sufficient thickness (for example, 1.0 mm) for multiple recording. The recording layer 70 is made of a holographic material whose optical characteristics (such as refractive index) change in proportion to the amplitude of the irradiated light. Therefore, the recording layer 70 is sensitive to the recording laser beam. A typical holographic material is a photopolymer. A photopolymer is a photosensitive material utilizing photopolymerization of a polymerizable compound (monomer). The photopolymer contains a monomer, a photopolymerization initiator, and a porous matrix as main components. This porous matrix plays a role of maintaining the volume of the recording layer 70 before and after recording. In addition to the photopolymer, the recording unit 70 can be made of any holographic material such as dichromated gelatin or a photorefractive crystal.

The protective layer 50 and the protective layer 60 serve to protect the recording layer 70 from scratches, dust, and the like and to maintain the shape of the recording layer 70. The protective layer 50 and the protective layer 60 are made of a material such as glass, polycarbonate, or acrylic resin. The protective layer 50 and the protective layer 60 are not limited to these materials, and are composed of other materials having various characteristics such as optical characteristics, mechanical strength characteristics, dimensional stability, and moldability with respect to the laser wavelength used. Is possible. In the servo information recording layer 80, information such as an encoder pattern for positioning and an address mark is recorded.

The recording medium 10 has a disk shape as described above, and has a clamp hole 20 near the center thereof. The clamp hole 20 is used to clamp the recording medium 10 in a device for holographic recording or holographic reproduction (not shown). The clamp hole 20 may be replaced with a hub. The recording medium 10 has a plurality of tracks 30 arranged concentrically from the center. A plurality of books 40 are recorded on each track 30. The book 40 is information of a predetermined unit. The track 30 disposed on the outer peripheral side can record more books 40 than the track 30 disposed on the inner peripheral side.

As described above, since the recording layer 70 is made of a holographic material, multiple recording is possible. Specifically, as shown in FIG. 2, each book 40 is configured by multiplexing predetermined units of information called pages 41. Therefore, in the recording medium 10, a plurality of pages 41 are multiplexed and recorded in the area for recording each book 40.

As shown in FIG. 3, the holographic recording / reproducing apparatus using the two-dimensional encoding method according to this embodiment includes a light source 100, a beam splitter 101, a beam expander 102, a spatial light modulator 103, and a spatial light modulator 104. , Lens group 105, spatial filter 106, beam splitter 107, objective lens 108, holding and driving mechanism 109, mirror 110, mirror 111, mirror 112, mirror 113, lens group 113, spatial filter 114, image sensor 115 and controller 116. Have. The holographic recording / reproducing apparatus in FIG. 3 uses a phase conjugate reproducing method.

The light source 100 generates laser light in accordance with control from the controller 116. The oscillation wavelength and intensity of the laser light are controlled by the controller 116. Generally, a laser with an external resonator (ELCD; External Cavity Laser Diode) configured by combining an external resonator with a semiconductor laser is used to control the oscillation wavelength of laser light. Although not shown, the light source 100 includes a spatial filter and a collimator for realizing a desired intensity distribution and beam diameter by making the spatial intensity of the output beam uniform, an isolator for avoiding the influence of return light, and the like. I have.

The beam splitter 101 splits the laser light (collimated light) from the light source 100 into two light beams. The division ratio is controlled by a wave plate (not shown). The holding and driving mechanism 109 physically holds the recording medium 10. The holding and driving mechanism 109 positions the recording medium 10 in accordance with an instruction from the controller 116.

The controller 116 controls each element of the holographic recording / reproducing apparatus shown in FIG. Specifically, the controller 116 notifies page data to the spatial light modulator 103, which will be described later, based on user data at the time of information recording, or user data based on a page data image from an image sensor 115, which will be described later, during information reproduction. Or play back. The holographic recording / reproducing apparatus shown in FIG. 3 is a so-called two-beam recording / reproducing method in which the reference light 120 and the recording light 130 have separate optical axes. However, the two-dimensional encoding method according to each embodiment is not limited to the two-beam recording / reproducing method, but can be applied to other recording / reproducing methods such as a coaxial method in which the reference light and the recording light pass through the same optical axis. is there.

Hereinafter, each part of the holographic recording / reproducing apparatus in FIG. 3 will be described focusing on the case of recording information on the recording medium 10. Light is output from the light source 100 when recording information.
The beam expander 102 shapes the light output from the light source 100 and passed through the beam splitter 101 into a desired beam diameter.

The spatial light modulator 103 and the spatial light modulator 104 superimpose recording information (such as user data) on the light from the beam expander 102. The recording information is converted into page data inside the controller 116. The spatial light modulator 103 and the spatial light modulator 104 are configured by a device such as a DMD (Digital Mirror Device) configured by a minute mirror, or an LCOS (Liquid Crystal Crystal on silicon) using a liquid crystal element. One of the spatial light modulator 103 and the spatial light modulator 104 modulates the intensity of light, and the other modulates the phase of light. In the following description, it is assumed that the spatial light modulator 103 modulates the light intensity and the spatial light modulator 104 modulates the light phase.

The spatial light modulator 103 sets ON / OFF of each pixel according to the page data notified from the controller 116. The page data pixels have a one-to-one correspondence with the pixels set by the spatial light modulator 103. For example, the spatial light modulator 103 sets a pixel corresponding to “1” of page data to ON (passes light) and a pixel corresponding to “0” of page data to OFF (blocks light). The spatial light modulator 104 modulates the phase of the modulated light from the spatial light modulator 103. For example, the spatial light modulator 104 randomly sets the phase of each pixel to either −π or π.

The modulated light from the spatial light modulator 104 passes through the lens group 105. The lens group 105 is
The modulated light is shaped into a desired beam diameter. A spatial filter 106 is provided between the lens groups 105. The spatial filter 106 realizes a desired filter process for adjusting the spatial frequency component of the light passing through the lens group 105. For example, a low-pass filter (LPF) can be realized by providing a pinhole. On the other hand, a high-pass filter (HPF) can be realized by providing a mark that blocks light at the center. Due to the action of the spatial filter 106, a specific frequency component of the light passing through the lens group 105 is suppressed or a specific frequency component is emphasized. The light passing through the lens group 105 further passes through the beam splitter 107 and the objective lens 108 and is irradiated to a desired position of the recording medium 10 (recording layer 70). In the following description, this irradiation light is referred to as recording light 130. On the other hand, the other light beam split by the above-described beam splitter 101 is also reflected by the mirror 110 and the mirror 113 and irradiated to a desired position of the recording medium 10 (recording layer 70). In the following description, this irradiation light is referred to as reference light 120.

The recording light 130 and the reference light 120 interfere with each other to form interference fringes in the recording layer 70. The recording medium 10 (recording layer 70) holds this interference fringe shape as recording information. As described above, the recording medium 10 can perform multiple recording. More specifically, as shown in FIG. 4, by changing the angle formed by the reference light 120 and the recording light 130 and the recording medium 10 and the incident angle of the reference light 120 with respect to the recording light 130, A plurality of information (page data) can be recorded at the same position. Such a multiplex recording system is called an angle multiplex system. In addition, the plurality of books 40 are recorded on the recording layer 70 by changing the irradiation positions of the reference light 120 and the recording light 130.

Hereinafter, each part of the holographic recording / reproducing apparatus in FIG. 3 will be described focusing on the case of reproducing information from the recording medium 10. Even when information is reproduced, light is output from the light source 100, but the path of the recording light 130 is blocked by a shutter (not shown).

The mirror 110 reflects the light output from the light source 100 and split by the beam splitter 101. The light reflected by the mirror 110 and the mirror 113 is irradiated to a desired position of the recording medium 10 (recording layer 70). Further, the light transmitted through the recording medium 10 is reflected by the mirror 111 and is irradiated again on the recording medium 10. When recording information exists at the irradiation position of these lights, the diffracted light of the irradiated light propagates to the objective lens 108. Recording information is superimposed on this diffracted light. In the following description, this diffracted light is referred to as reproduction light.

The reproduction light passes through the objective lens 108, is reflected by the beam splitter 107, is further reflected by the mirror 112, and is guided to the lens group 113. The lens group 113 shapes the reproduction light into a desired beam diameter. A spatial filter 114 is provided between the lens groups 113. The spatial filter 114 realizes a desired filter process for adjusting the spatial frequency component of the reproduction light. Due to the action of the spatial filter 114, a specific frequency component of the reproduction light is suppressed, or a specific frequency component of the reproduction light is emphasized. This reproduction light is guided from the lens group 113 to the image sensor 115.

The image sensor 115 receives the reproduction light from the lens group 113 and converts it into an electrical signal corresponding to the intensity. The image sensor 115 is generally constituted by a device such as a CCD (Charge-Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) which is a multi-pixel image sensor. The image sensor 115 converts this electrical signal into a digital signal by A / D conversion, and inputs it to the controller 116 as a page data image. For example, the value of each pixel of the page data image is represented by an 8-bit digital value (0 to 255) corresponding to the intensity of light received by each pixel of the image sensor 115. Note that the pixels of the page data image (pixels of the spatial light modulator 103) and the pixels of the image sensor 115 do not necessarily have a one-to-one correspondence. Specifically, more pixels (for example, four pixels) of the image sensor 115 may be assigned to one pixel of the page data image. Such a page data image capturing method is called an oversample method. According to the oversampling method, even when a positional deviation occurs between the pixel of the page data image and the pixel of the image sensor, the controller 116 can easily display the page data image in which the positional deviation is corrected by predetermined signal processing. Can be restored.

Hereinafter, a recording signal processing unit and a reproduction signal processing unit implemented as a part of the controller 116 will be described with reference to FIGS. These recording signal processing unit and reproduction signal processing unit are implemented by hardware, software, or a combination of hardware and software.

As shown in FIG. 5, the recording signal processing unit includes an error correction code calculation unit 210, a modulation unit 220, and a page data generation unit 230. The recording signal processing unit notifies the spatial light modulator 103 of the page data 240 based on the user data 200 to be recorded.

The error correction code calculation unit 210 divides the user data 200 into data of a predetermined unit. The error correction code calculation unit 210 calculates an error correction code for each predetermined unit of data according to a predetermined algorithm. The error correction code calculation unit 210 adds an error correction code to data of a predetermined unit and inputs the data to the modulation unit 220.

The modulation unit 220 converts a predetermined unit of data (including the error correction code) from the error correction code calculation unit 210 into a code word according to a predetermined modulation table and modulation rule. The modulation table and the modulation rule are based on the two-dimensional encoding method according to the present embodiment. Details of the two-dimensional encoding method according to the present embodiment will be described later. The modulation unit 220 inputs the code word to the page data generation unit 230.

The page data generation unit 230 generates page data 240 in which a predetermined number of code words are arranged. The page data 240 includes additional information such as address information. In the page data 240, a predetermined bit pattern called a synchronization mark is arranged at predetermined intervals. As described above, the spatial light modulator 103 sets the pixel ON / OFF based on the bit pattern of the page data 240.

The page data 240 includes, for example, a plurality of synchronization marks 43 and a plurality of subpages 42 as shown in FIG. The plurality of subpages 42 have the same or similar shape. Synchronization marks 43 are arranged at the four corners of the subpage 42, respectively. In the subpage 42, additional information such as address information is arranged in an area adjacent to the synchronization mark 43. The address information indicates a book number, a page number in the book, a subpage number in the page, a modulation type, and the like. In the subpage 42, a codeword in which user data is modulated (encoded) is arranged in an area other than the additional information.

FIG. 10 shows an enlarged view of the dotted area in FIG. As shown in FIG. 10, a plurality of code words 44 are arranged in each subpage. In each code word 44, as shown in FIG. 11, a plurality of pixels are arranged. In the following description, unless otherwise specified, the hatched pixel in the drawing corresponding to the code word represents “0” and the white pixel represents “1”. In the example of FIG. 10, the code word 44 is composed of 4 × 4 pixels, and the synchronization mark 43 is composed of 8 × 8 pixels. In general, the synchronization mark 43 has a pattern having higher symmetry than the code word 44. For example, referring to FIG. 10, the synchronization mark 43 has a pattern in which “0” and “1” are alternately arranged concentrically. Such a highly symmetrical pattern is suitable for correlation peak detection in template matching processing described later.

As shown in FIG. 6, the reproduction signal processing unit includes a filter unit 310, a coordinate detection unit 320, a coordinate conversion unit 330, a filter unit 340, a binarization unit 350, a demodulation unit 360, and an error correction unit 370. The reproduction signal processing unit reproduces user data 380 based on the page data image 300 from the image sensor 115. Basically, the reproduction signal processing unit performs the reverse process of the recording signal processing unit. However, there is a possibility that the page data image 300 includes noise or has a spatial shift. Therefore, the reproduction signal processing unit also performs processing for suppressing noise and processing for correcting spatial deviation.

FIG. 7A shows an example of a normal page data image 300. On the other hand, FIG. 7B shows an example of the page data image 300 when low-frequency noise is generated. Such low frequency noise is also called luminance unevenness. In the example of FIG. 7B, the four corners of the page data image 300 are darker than the center. In such a state, since the light intensity of the ON pixel and the light intensity of the OFF pixel are close to each other, a bit error is likely to occur. Low-frequency noise such as uneven brightness is caused by non-uniformity in the intensity distribution of recording light or reference light, aberrations in the optical system, non-uniformity in sensitivity of the spatial light modulator or image sensor, misalignment, changes in environmental temperature, etc. Due to various factors.

FIG. 8A shows an example of a normal page data image 300 in an enlarged state. On the other hand, FIG. 8B shows an enlarged view of an example of the page data image 300 when high-frequency noise occurs. In the example of FIG. 8A, the ON pixel and the OFF pixel can be clearly distinguished. In the example of FIG. 8B, fine bright spots (high-frequency noise) different from the ON pixels are generated on the entire screen. When such high-frequency noise is generated, it is difficult to distinguish between a bright point due to the noise and the ON pixel, and it becomes difficult to distinguish the ON pixel. In addition, high-frequency noise may appear as distortion of the shape of the ON pixel or OFF pixel, but in this case as well, it is difficult to discriminate both. That is, even when high frequency noise is generated in the page data image 300, a bit error is likely to occur. High frequency noise is generated by various factors such as scattered light or stray light generated by an optical system, and electrical noise.

The filter unit 310 performs a filtering process on the page data image 300 from the image sensor 115. For example, the filter unit 310 performs HPF processing using a two-dimensional FIR (Finite Impulse Response) filter or the like in order to suppress low frequency noise, or brightness of the entire page data image 300 by luminance difference averaging processing. May be averaged. In this brightness difference averaging process, for example, the page data image 300 is divided into a plurality of sections, the brightness amplitudes of the pixels in each section are measured, and weighting for increase / decrease is performed so that these brightness amplitudes are equal. This is the process to be performed. The filter unit 310 may perform LPF processing to suppress high frequency noise. Further, the filter unit 310 may perform an equalizing process for amplifying a specific frequency band component in order to emphasize the recording information component. Note that the processing of the filter unit 310 is preferably optimized in consideration of the processing of the coordinate detection unit 320 and the coordinate conversion unit 330 in the subsequent stage. The filter unit 310 inputs the filtered page data image to the coordinate detection unit 320.

The coordinate detection unit 320 corrects the spatial shift of the page data image from the filter unit 310 and detects the coordinates of the page data image corresponding to the center coordinates of each pixel of the page data. There is no one-to-one correspondence between the pixel of the image sensor 115 and the pixel of the page data between the pixel of the page data set at the time of recording and the pixel of the page data image captured at the time of reproduction. A positional shift between the pixel 103 and the pixel of the image sensor 115, and a spatial shift due to factors such as image enlargement, reduction, and distortion due to optical aberration occur. Specifically, the coordinates of the page data image corresponding to the center coordinates of each pixel of the page data may change every time the page data image is processed. First, the coordinate detection unit 320 detects the coordinates corresponding to the synchronization mark of the page data from the page data image. As described above, the synchronization mark is arranged at a predetermined position such as the center or edge of the page data. Since these synchronization mark patterns are known, for example, the coordinate detection unit 320 performs a template matching process using the known pattern as a template for the page data image. The coordinate detection unit 320 detects the coordinate having the highest correlation in the page data image as the coordinate corresponding to the synchronization mark. Then, the coordinate detection unit 320 detects the coordinates of the page data image corresponding to the center coordinates of the other pixels of the page data by complementing processing based on the detected coordinates.

The coordinate conversion unit 330 resamples the page data image based on the coordinates detected by the coordinate detection unit 320. This resampling process is realized by, for example, a bilinear method, a bicubic method, or the like. By this re-sampling process, a page data image having a one-to-one correspondence with the page data pixels is generated. The coordinate conversion unit 330 inputs the page data image generated by the resampling process to the filter unit 340.

The filter unit 340 performs a filtering process or an equalizing process on the page data image from the coordinate conversion unit 330. The processing of the filter unit 340 may be the same as or similar to the processing of the filter unit 310, or may be completely different processing, but is preferably optimized in consideration of the processing of the subsequent binarization unit 350. . Note that the pixels of the page data image to be processed by the filter unit 310 do not have a one-to-one correspondence with the pixels of the page data, so the sampling frequency is unknown, but the page data image to be processed by the filter unit 340 is Since the corresponding pixels have a one-to-one correspondence with the page data pixels, the sampling frequency can be fixed. The filter unit 340 inputs the filtered page data image to the binarization unit 350.

The binarization unit 350 binarizes the value of each pixel of the page data image from the filter unit 340 to generate a bit string. For example, regarding an 8-bit value pixel, the binarization unit 350 determines “1” (ON pixel) if the pixel value is 127 or more, and determines “0” (OFF pixel) otherwise. The binarization unit 350 inputs the bit string to the demodulation unit 360.

The demodulation unit 360 converts the bit string from the binarization unit 350 into data of a predetermined unit (including an error correction code) in accordance with a predetermined demodulation table and demodulation rule in units of codewords. The demodulation table and the demodulation rule correspond to the modulation table and the modulation rule used by the modulation unit 220 described above. Demodulation section 360 inputs demodulated data to error correction section 370.

The error correction unit 370 performs error correction on the demodulated data and reproduces data of a predetermined unit. The error correction process is realized using the error correction code given by the error correction code calculation unit 210. The reproduced data of a predetermined unit is integrated as user data 380 and output to the outside.

Hereinafter, the two-dimensional encoding method according to the present embodiment will be described. The two-dimensional encoding method according to the present embodiment realizes the following modulation table, for example.

In this modulation table, a 16-bit (pixel) code word is assigned to 10-bit user data. That is, since the encoding efficiency of this encoding method is 62.5% (10/16), the above-described condition (B) is satisfied. According to this modulation table, for example, code words “0100; 0000; 0011; 0100” are assigned to user data “526”. The notation of the code word in this modulation table indicates the bit pattern in the raster order of the code word. For example,

bits

0, 1, 0, and 0 are arranged in order from the left in the upper row of the code word. In the second row from the top of this code word,

bits

0, 0, 0, 0 are arranged in order from the left. That is, this code word has the bit pattern shown in FIG.

In the two-dimensional encoding method according to the present embodiment, it is assumed that the total number of pixels of the code word is the same, and the ratio of “1” (ON pixel) and “0” (OFF pixel) is fixed. For example, in the modulation table, the code word is a bit pattern of 4 rows and 4 columns, and the number of ON pixels is fixed to 4. In this way, fixing the total number of pixels of the code word and the ON pixel ratio simplifies the cut-out process of the code word in the reproduction signal processing, improves its reliability, and improves the determination accuracy of the binarization process. Useful. For example, if the number of ON pixels determined by the binarization processing is different from the specified number, the parameter is changed and the binarization processing is executed again, or the ON number is set so as to match the specified number in order of increasing brightness. It is possible to determine a pixel.

Also, it is desirable that the ON pixel ratio is suppressed to about 20 to 30% as described regarding the condition (A). This basis is disclosed in Non-Patent Document 1. Specifically, as shown in FIG. 12, when the ON pixel ratio is about 20% to 30%, the holographic recording capacity (relative capacity based on the case where the ON pixel ratio is 50%) is maximum. It becomes. In the modulation table, since the ON pixel ratio is 25%, the above condition (A) is satisfied.

In the following description, the notation E (J, K, L) is used to simply describe the parameters of the two-dimensional encoding method. J represents the total number of pixels of the code word. K represents the number of ON pixels included in the code word. L represents the number of bits of user data to which a code word is assigned. That is, the modulation table is based on the two-dimensional encoding method E (16, 4, 10). By the way, the number of patterns of this code word is ₁₆ C ₄ = 1820. On the other hand, the number of patterns of user data is 2 ¹⁰ = 1024. That is, in this two-dimensional encoding method E (16, 4, 10), the number of codeword patterns is surplus compared to the number of user data patterns. Therefore, in the two-dimensional encoding method according to the present embodiment, a codeword suitable for the holographic memory technology is selected from all codewords so as not to be less than the number of user data patterns, and then assigned to user data. In other words, in the two-dimensional encoding method according to the present embodiment, codewords that are not suitable for the holographic memory technique are excluded from all codewords within a surplus range and then allocated to user data. Hereinafter, a method for selecting a codeword suitable for the holographic memory technology or a method for eliminating a codeword not suitable for the holographic memory technology will be described.

The above-described two-dimensional encoding method E (16, 4, 10) selects a code word suitable for the holographic memory technology based on the following selection conditions.

Selection condition (1-1) The four ON pixels are not concentrated in any of the 3 × 3 pixel partial areas in the code word.
Selection condition (1-2) The maximum number of ON pixels included in a pixel region in which ON pixels are adjacent to each other in the row direction or the column direction is not three or more (two or less).
Selection condition (1-3) The maximum number of ON pixels included in the same row or column is not three or more (two or less).

Of course, the selection condition (1-1) may be read as the following selection condition (1-1 ′) or (1-1 ″).
Selection condition (1-1 ′) At least one ON pixel exists in the seven pixels in the uppermost row and the rightmost column, and at least one ON pixel in the seven pixels in the rightmost column and the lowermost row And at least one ON pixel among the seven pixels in the bottom row and the left column, and at least one ON pixel among the seven pixels in the left column and the top row To do.
Selection condition (1-1 ″) The maximum number of ON pixels included in the 3 × 3 pixel partial area in the code word is not four (three or less).

For example, the code words in FIG. 13A are concentrated in a partial area of 3 × 3 pixels excluding the rightmost column and the lowermost row. Further, in this code word, the maximum number of ON pixels included in the pixel region in which the ON pixels are adjacent to each other in the row direction or the column direction is three. Therefore, this code word does not satisfy the selection conditions (1-1) and (1-2). In the code word of FIG. 13B, the maximum number of ON pixels included in the pixel region in which the ON pixels are adjacent to each other in the row direction or the column direction is three. Therefore, this code word does not satisfy the selection condition (1-2). In the code word of FIG. 13C, the maximum number of ON pixels included in the pixel region in which the ON pixels are adjacent to each other in the row direction or the column direction is three. Further, this code word has the maximum number of ON pixels 3 included in the same row or the same column. Therefore, this code word does not satisfy the selection conditions (1-2) and (1-3). In the code word of FIG. 13D, the maximum number of ON pixels included in the pixel region in which the ON pixels are adjacent to each other in the row direction or the column direction is four. This code word has a maximum number of ON pixels of 4 in the same row or column. Therefore, this code word does not satisfy the selection conditions (1-2) and (1-3). In the code word of FIG. 13E, the maximum number of ON pixels included in the same row or the same column is three. Therefore, this code word does not satisfy the selection condition (1-3).

Hereinafter, the technical significance of the selection conditions (1-1), (1-2), and (1-3) will be described.
The selection condition (1-1) is useful for eliminating codewords in which ON pixels are concentrated in a local region. When the ON pixels are concentrated in the local area, the low frequency component of the codeword image tends to be strong. That is, the signal component of the code word tends to concentrate on the low frequency band. When signal components are concentrated in the low frequency band, more signal components are blocked (lost) when the reproduction signal processing unit applies HPF processing for low frequency noise suppression. Further, since the holographic memory is recorded on the Fourier transform plane, if the low frequency component of the code word is strong, the distribution of the recording light is concentrated on the optical axis, resulting in a recording failure. The selection conditions (1-2) and (1-3) are also useful for eliminating codewords in which ON pixels are concentrated in a local region. That is, the selection conditions (1-1), (1-2), and (1-3) are also effective for satisfying the above-described condition (C). In the selection conditions (1-1), (1-2), and (1-3), the maximum number of ON pixels included in the local area is limited to a predetermined value or less. May be. In addition, although the selection condition (1-1) defines 3 × 3 pixels as a local region, the local region may be changed as appropriate.

FIG. 14A shows a set of four code words that do not satisfy the selection condition (1-1). On the other hand, FIG. 14B shows a set of four code words that satisfy the selection condition (1-1) (and the selection conditions (1-2), (1-3)). 15A and 15B represent the frequency components of the images corresponding to FIGS. 14A and 14B, respectively. In FIGS. 15A and 15B, the signal intensity, the column direction frequency, and the row direction frequency are all normalized. 15A and 15B, the signal intensity near the center indicates the low frequency component of the image corresponding to FIGS. 14A and 14B. As is apparent from FIGS. 15A and 15B, the signal component in the low frequency band is suppressed in the image corresponding to FIG. 14B compared to the image corresponding to FIG. 14A. Therefore, low-frequency noise can be effectively suppressed by performing HPF processing on the image corresponding to FIG. 14B. That is, according to the code word satisfying the selection condition (1-1), an improvement in the signal band noise ratio (SNR) is expected.

Regarding the two-dimensional encoding method E (16, 4, 10), the number of codeword patterns satisfying the above selection condition (1-1) is 1587. The number of codeword patterns that satisfy the selection condition (1-2) in addition to the selection condition (1-1) is 1267. The number of codeword patterns satisfying all the selection conditions (1-1), (1-2), and (1-3) is 1123. Since the number of 10-bit patterns is 1024, the two-dimensional encoding method E (16, 4, 10) according to this embodiment satisfies all selection conditions (1-1), (1-2), and (1-3). A satisfying codeword can be uniquely assigned to user data. Therefore, the two-dimensional encoding method E according to the present embodiment satisfies the conditions (A) and (B) by setting J, K, and L, and the selection conditions (1-1), (1-2), It can be evaluated that the condition (C) is satisfied by applying (1-3).

Hereinafter, the two-dimensional encoding method E (J, K, L) according to the present embodiment will be generalized and described.
In the two-dimensional encoding method according to the present embodiment, the total number of pixels J of the code word is determined to be the square of n (n is an integer of 4 or more). In the two-dimensional encoding method according to the present embodiment, the number of ON pixels K of the code word is determined so that the ON pixel ratio (K / J) falls within about 20 to 30%. Furthermore, in the two-dimensional encoding method according to the present embodiment, the number K of ON pixels in the code word is determined so as to ensure the number of code words sufficient to satisfy the desired encoding efficiency. As described above, the encoding efficiency is the ratio (L / J) of the number of bits L of user data to which a code word is allocated to the total number of pixels J of the code word. The number of codeword patterns is _J C _K , and the number of user data patterns is ^2L . In the two-dimensional encoding method according to the present embodiment, the number of ON pixels K of the code word is determined to be a value of n or more, for example, in order to realize high encoding efficiency exceeding 50%. By determining J, K, and L in this way, the above conditions (A) and (B) can be satisfied.

After determining J, K, and L as described above, a code word for satisfying the above condition (C) is selected or excluded. In the above example (E (16, 4, 10)), the number of codeword patterns satisfying all the selection conditions (1-1), (1-2), (1-3) is the user data pattern. Although it is more than the number, such a relationship is not necessarily established depending on the values of J, K, and L. Therefore, for example, it is effective to perform selection / exclusion of codewords step by step in order from the selection condition with the highest priority.

The selection condition (1-1) can be read as follows regarding the two-dimensional encoding method E (J, K, L).
Selection condition (2-1) The K ON pixels are not concentrated in any of the partial areas of (n−1) × (n−1) pixels in the code word.
Selection condition (2-1 ′) At least one ON pixel exists in (2n−1) pixels in the uppermost row and rightmost column, and (2n−1) pixels in the rightmost column and lowermost row At least one ON pixel, and at least one ON pixel among (2n−1) pixels in the lower row and the left column, and (2n -1) There is at least one ON pixel among the pixels.
Selection condition (2-1 ″) The maximum number of ON pixels included in the partial area of (n−1) × (n−1) pixels in the code word is not K ((K−1) or less). ).

If the number of codeword patterns satisfying the selection condition (2-1) is equal to or greater than the number of user data patterns (2 ^L ), the following selection condition (2-2) is added to the codeword group satisfying the selection condition (2-1). ) Is further applied. The selection condition (2-2) is obtained by replacing the selection condition (1-2) with respect to the two-dimensional encoding method E (J, K, L).

Selection condition (2-2) The maximum number of ON pixels included in a pixel region in which ON pixels are adjacent to each other in the row direction or the column direction is not (K-1) or more ((K-2) or less).
If the number of codeword patterns satisfying these selection conditions (2-1) and (2-2) is greater than or equal to the number of user data patterns, the codeword group satisfying the selection conditions (2-1) and (2-2) The following selection condition (2-3) is further applied. The selection condition (2-3) is obtained by replacing the selection condition (1-3) with respect to the two-dimensional encoding method E (J, K, L). On the other hand, if the number of codeword patterns satisfying these selection conditions (2-1) and (2-2) is less than the number of user data patterns, for example, application of this selection condition (2-2) is canceled, A code word satisfying the selection condition (2-1) is assigned to the user data.

Condition (2-3) The maximum number of ON pixels contained in the same row or column is not (K-1) or more ((K-2) or less).
If the number of codeword patterns satisfying these selection conditions (2-1), (2-2), and (2-3) is greater than or equal to the number of user data patterns, selection conditions (2-1) and (2-2) , (2-3) is assigned to the user data. On the other hand, if the number of codeword patterns satisfying these selection conditions (2-1), (2-2), and (2-3) is less than the number of user data patterns, for example, the selection condition (2-3) The application is canceled and a code word satisfying the selection conditions (2-1) and (2-2) is assigned to the user data.

Thus, by selecting the codeword group satisfying the selection conditions in stages, suitable codewords are narrowed down with the number of user data patterns as a lower limit. For example, when the selection conditions (2-1), (2-2), and (2-3) are sequentially applied to the two-dimensional encoding method E (16, 4, L), as shown in FIG. 13A first. Code words that do not satisfy the selection condition (2-1) are excluded. Next, codewords that do not satisfy the selection condition (2-2) as shown in FIGS. 13B, 13C, and 13D are eliminated, and finally the selection condition (2-3) as shown in FIG. 13E is satisfied. No codewords are eliminated. Such stepwise narrowing down makes it possible to assign a suitable codeword to user data. In the present embodiment, the priority difference between the selection conditions (2-1), (2-2), and (2-3) is determined for the following reason.

In a code word that does not satisfy the selection condition (2-1), all ON pixels (K ON pixels) are concentrated in a local area of (n-1) × (n-1) pixels. Therefore, for a codeword that does not satisfy the selection condition (2-1), for example, as shown in FIG. 13A, there is a high possibility that signal components are concentrated in the low frequency bands in both the row direction and the column direction. For codewords that do not satisfy the selection condition (2-2), for example, as shown in FIGS. 13A and 13B, signal components may be concentrated in the low frequency band in both the row direction and the column direction. However, in this code word, for example, as shown in FIGS. 13C and 13D, signal components may be concentrated in a low frequency band only in one of the row direction and the column direction. For codewords that do not satisfy the selection condition (2-3), as shown in FIGS. 13C, 13D, and 13E, there is a possibility that signal components are concentrated in only one of the low frequency bands in the row direction and the column direction. high. Therefore, in the present embodiment, the order of the selection conditions (2-1), (2-2), and (2-3) is considered by comprehensively considering the influence on the low frequency components in the row direction and the column direction of the code word. Is given priority.

As shown in FIG. 16, the two-dimensional encoding method according to the present embodiment selects a codeword group that satisfies the selection condition (excludes codewords that do not satisfy the selection condition) (step S401), and is selected. And assigning only code words included in the code word group (only code words not excluded) to user data (step S402). As a result, codewords assigned to user data by the two-dimensional encoding method according to the present embodiment are narrowed down to those suitable for the holographic memory technology.

Hereinafter, as one of the two-dimensional encoding methods according to the present embodiment, a two-dimensional encoding method E (16, 5, 12) will be exemplified. Since the ON pixel ratio of the two-dimensional encoding method E (16, 5, 12) is about 31% (5/16), the condition (A) is satisfied. The encoding efficiency of the two-dimensional encoding method E (16, 5, 12) is 75% (12/16), which satisfies the condition (B). The number of codeword patterns is 4368 ( ₁₆ C ₅ ), and the number of user data patterns is 4096 (2 ¹² ). Therefore, the code word has a surplus of 372 patterns compared to the user data.

This two-dimensional encoding method E (16, 5, 12) applies the selection condition (2-1) in order to narrow down the codewords assigned to the user data. As a result, codewords as shown in FIGS. 17A, 17B and 17C are selected, while codewords as shown in FIGS. 18A, 18B, 18C and 18D are eliminated. The number of codeword patterns satisfying this selection condition (2-1) is 4096, which matches the number of user data patterns. Therefore, in the two-dimensional encoding method E (16, 5, 12), the selection conditions (2-2) and (2-3) are not applied, and only code words that satisfy the selection condition (2-1) are user data. Assigned uniquely.

As described above, the two-dimensional encoding method according to this embodiment satisfies the conditions (A) and (B) by setting J, K, and L. Furthermore, the encoding method according to the present embodiment satisfies the condition (C) by applying the selection conditions (2-1), (2-2), and (2-3) stepwise to the codeword. Therefore, according to the two-dimensional encoding method according to the present embodiment, a code word suitable for the holographic memory technology can be assigned to user data.

(Second Embodiment)
In the two-dimensional encoding method according to the first embodiment described above, the selection conditions (2-1), (2-2), and (2-3) are applied step by step to select / exclude codewords. Therefore, the condition (C) is satisfied. The two-dimensional encoding method according to the second embodiment of the present invention assigns codewords more suitable for the holographic memory technology to the user by defining further selection conditions.

As shown in FIG. 10, the page data includes a synchronization mark (43) in addition to the code word (44). The synchronization mark is used for detecting the coordinates of the codeword pixel in the reproduction signal processing. For example, the position, inclination, size, and the like of the page data image are estimated based on the detection result of the position and shape of the synchronization mark.

The synchronization mark has more pixels than the code word. Further, as shown in FIG. 19, the synchronization mark has a pixel pattern in which ON pixels are relatively long and continuous. A synchronization mark having such a characteristic pattern is easier to detect than a normal codeword. However, when specific codewords are combined, a pixel pattern similar to (highly correlated with) such a characteristic pattern may be generated. The two-dimensional encoding method E (16, 4, 10) exemplified in the first embodiment can use 1123 patterns of code words. When a part of the codewords of the 1123 pattern is combined, a pixel pattern similar to the synchronization pattern is generated as shown in FIG. In FIG. 20, the synchronization mark of FIG. 19 and the combination of codewords are displayed superimposed. Also, in FIG. 20, details of pixels at positions that are out of the synchronization mark (do not overlap with the synchronization mark) among the pixels included in the codeword combination are omitted. In FIG. 20, each code word is divided by a dotted line region, and has ON pixels at positions where “x” marks are given, and has OFF pixels at other positions. That is, in FIG. 20, the Hamming distance between the codeword combination and the synchronization mark matches the total number of white pixels (ON pixels with respect to the synchronization mark but OFF pixels with respect to the codeword combination). To do. Specifically, the Hamming distance is “8”. The Hamming distance is an example of a distance (index) indicating the similarity between bit patterns. In the example of FIG. 20, when a total of eight white pixels (OFF pixels) in a combination of codewords are determined as ON pixels in reproduction signal processing due to some factor (such as noise), the combination of these codewords is determined. Misdetected as a synchronization mark.

19 includes a partial area shown in FIG. The size of this partial area is the same as the code word. A code pattern similar to the synchronization mark is generated by combining a code word similar to each partial region of the synchronization mark. Therefore, the two-dimensional encoding method according to the present embodiment further defines the following selection condition (3-1).

Selection condition (3-1) The minimum hamming distance from each partial region of the synchronization mark is equal to or greater than a predetermined value (“3” in the following description).
This selection condition is further applied after, for example, the above-described selection conditions (2-1), (2-2), and (2-3) are applied. That is, if the number of codeword patterns satisfying the selection conditions (2-1), (2-2), and (2-3) is greater than or equal to the number of user data patterns, the selection condition (3-1) is applied. . In addition, this application order is an example and may be changed to a different application order.

FIG. 22A shows a code word having a Hamming distance “2” from the pixel pattern shown in FIG. 22B, 22C, and 22D also show codewords having a Hamming distance “2” from the partial region of the synchronization mark in FIG. Regarding the two-dimensional encoding method E (16, 4, 10), codes that satisfy the selection conditions (2-1), (2-2), and (2-3) and do not satisfy the selection condition (3-1) The words are the above four patterns (FIGS. 22A, 22B, 22C, and 22D). The two-dimensional encoding method E (16, 4, 10) according to the present embodiment uses a selection condition from 1123 patterns of codewords that satisfy the selection conditions (2-1), (2-2), and (2-3). Four patterns of code words that do not satisfy (3-1) are eliminated. As a result, 1119 patterns of codewords remain, which is more than 1024 user data patterns. Therefore, the two-dimensional encoding method according to the present embodiment can use codewords that satisfy all the selection conditions (2-1), (2-2), (2-3), and (3-1). In the two-dimensional encoding method E (16, 4, 10) according to the present embodiment, it is guaranteed that all codewords have a Hamming distance of “3” or more with each partial region of the synchronization mark. For example, as shown in FIG. 23, the Hamming distance is “12” even if each partial region of the synchronization mark in FIG. 19 is combined with a code word having a close Hamming distance. That is, the possibility that a combination of code words is erroneously detected as a synchronization mark is lower than in the example of FIG.

In addition, although the above description exemplifies the synchronization mark shown in FIG. 19, we have investigated various patterns that can be used as the synchronization mark. 24A, 24B, 24C, and 24D show examples of synchronization marks that have been investigated. As a result of this investigation, if the predetermined value is set to “3” in the selection condition (3-1), the number of codewords is sufficiently secured, and the Hamming distance between the entire synchronization mark and the codeword combination Has been found to be able to be expanded beyond “8”.

As described above, the two-dimensional encoding method according to the present embodiment defines that the minimum hamming distance from each partial region of the synchronization mark is a predetermined value as an additional selection condition. Therefore, according to the two-dimensional encoding method according to the present embodiment, the possibility that a combination of code words is erroneously detected as a synchronization mark is reduced.

Note that the present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiments. Further, for example, a configuration in which some components are deleted from all the components shown in each embodiment is also conceivable. Furthermore, you may combine suitably the component described in different embodiment.

For example, a holographic recording / reproducing apparatus (holographic recording apparatus) that records recording information encoded using the two-dimensional encoding method according to each embodiment on a holographic recording medium is an aspect of the invention disclosed by the present application. include. A holographic recording medium that records recording information encoded using the two-dimensional encoding method according to each embodiment is also included in the aspects of the invention disclosed in the present application. Furthermore, a holographic reproducing device (holographic recording / reproducing device) that reproduces the recorded information from a holographic recording medium that records the recorded information encoded by using the two-dimensional encoding method according to each embodiment. It is contained in the aspect of the invention which this application discloses. Note that the holographic recording apparatus can be configured by removing elements necessary only for holographic reproduction from the holographic recording / reproducing apparatus of FIG. 3, for example. Further, the holographic reproducing apparatus can be configured by removing elements necessary only for holographic recording from the holographic recording / reproducing apparatus of FIG. 3, for example.

DESCRIPTION OF SYMBOLS 10 ... Holographic recording medium 20 ... Clamp hole 30 ... Track 40 ... Book 41 ... Page 42 ... Subpage 43 ... Synchronization mark 44 ... Code word 50, 60 ... Protective layer 70 ... Recording layer 80 ... Servo information recording layer 100 ... Light source 101 ... Beam splitter 102 ...

Beam expander

103, 104 ... Spatial light modulator 105 ... Lens group 106: Spatial filter 107 ... Beam splitter 108 ... Objective lens 109 ... Holding and

driving mechanism

110, 111, 112, 113 ... Mirror 113 ... Lens group 114 ... Spatial filter 115 ... image sensor 116 ... controller 120 ... reference light 130 ... recording light 140 ... reproduction light 200 ... User data 210 ... Error correction code calculation unit 220 ... Modulation unit 230 ... Page data generation unit 240 ... Page data 300 ... Page data image 310 ... Filter unit 320 ... Coordinate detection Unit 330 ... coordinate conversion unit 340 ... filter unit 350 ... binarization unit 360 ... demodulation unit 370 ... error correction unit 380 ... user data

Claims

In a two-dimensional encoding method for holographic recording,
K (K is an integer of n or more) ON pixels in a pixel set arranged in a square shape with n (n is an integer of 4 or more) in the row direction and n in the column direction, and (n × n−K) (A) From a two-dimensional codeword represented by a combination with OFF pixels, (a) there is at least one ON pixel in (2n-1) pixels in the top row and right column, and (b) There is at least one ON pixel in (2n-1) pixels in the rightmost column and bottom row, and (c) at least one in (2n-1) pixels in the bottom row and left column. A first codeword group that satisfies the condition that there are two ON pixels and (d) at least one ON pixel is present in (2n-1) pixels in the leftmost column and the uppermost row To do
A two-dimensional encoding method comprising: allocating only code words included in the first code word group to user data.
From the first code word group, a second code word group satisfying a condition that the maximum number of ON pixels included in a pixel region in which ON pixels are adjacent to each other in the row direction or the column direction is (K−2) or less. Further comprising selecting
The user data is assigned only to codewords included in the second codeword group.
The two-dimensional encoding method according to claim 1.
Further comprising selecting from the second codeword group a third codeword group satisfying a condition that the maximum number of ON pixels existing in the same column or row is (K-2) or less,
The user data is assigned only to codewords included in the third codeword group.
The two-dimensional encoding method according to claim 1.
Further comprising selecting, from the third codeword group, a fourth codeword group that satisfies a condition that a minimum Hamming distance with each partial region of the synchronization mark is a predetermined value or more,
The user data is assigned only to codewords included in the fourth codeword group.
The two-dimensional encoding method according to claim 1.
A light source for generating laser light;
K (K is an integer of n or more) ON pixels in a pixel set arranged in a square shape with n (n is an integer of 4 or more) in the row direction and n in the column direction, and (n × n−K) (A) From a two-dimensional codeword represented by a combination with OFF pixels, (a) there is at least one ON pixel in (2n-1) pixels in the top row and right column, and (b) There is at least one ON pixel in (2n-1) pixels in the rightmost column and bottom row, and (c) at least one in (2n-1) pixels in the bottom row and left column. Included in the first codeword group that satisfies the condition that (d) at least one ON pixel exists in the (2n-1) pixels in the leftmost column and the uppermost row Holographic recording of user data encoded using only encoded codewords An imaging element for receiving the reproduction light generated said is laser light irradiated on the recording medium,
A holographic reproduction device comprising: a reproduction unit that reproduces the user data based on a codeword indicated by the reproduction light.
A light source for generating laser light;
K (K is an integer of n or more) ON pixels in a pixel set arranged in a square shape with n (n is an integer of 4 or more) in the row direction and n in the column direction, and (n × n−K) (A) From a two-dimensional codeword represented by a combination with OFF pixels, (a) there is at least one ON pixel in (2n-1) pixels in the top row and right column, and (b) There is at least one ON pixel in (2n-1) pixels in the rightmost column and bottom row, and (c) at least one in (2n-1) pixels in the bottom row and left column. Included in the first codeword group that satisfies the condition that (d) at least one ON pixel exists in the (2n-1) pixels in the leftmost column and the uppermost row An encoding unit that performs encoding to uniquely assign only codewords to be assigned to user data;
A modulator that outputs recording light obtained by modulating the laser light in accordance with a code word assigned to the user data to a holographic recording medium;
An imaging device for receiving reproduction light generated by irradiating the holographic recording medium with the laser beam;
A holographic recording / reproducing apparatus comprising: a reproducing unit that reproduces the user data based on a codeword indicated by the reproduction light.