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`llllll1111111lllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`U8005537382A
`
`United States Patent
`
`McLaughlin et al.
`
`[19]
`
`[11] Patent Number:
`
`5,537,382
`
`[45] Date of Patent:
`
`Jul. 16, 1996
`
`[541
`
`[75]
`
`[731
`
`[211
`
`[22]
`
`[51]
`[52]
`
`[581
`
`[56]
`
`PARTIAL RESPONSE CODING FOR A
`MULTI-LEVEL OPTICAL RECORDING
`CHANNEL
`
`Inventors: Steven W. McLaughlin, Rochester;
`Arthur R. Calderbank, Princeton;
`Rajiv Lamia, Bridgewater, all of N.J.;
`John M. Gerpheide, Silver Spring, Md.
`
`Assignees: Optex Corporation, Del; AT&T
`Corp., N.Y.
`
`App]. No.2 340,353
`
`Filed:
`
`Nov. 22, 1994
`
`Int. Cl.6 ................................... G11B 7/00
`
`US. Cl.
`369/116; 369/59; 341/59;
`371/371; 360/40
`Field of Search ................................ 369/94, 116, 59,
`369/124; 341/59; 371/371, 37.8, 43; 360/40
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`
`
`............................. 340/347
`11/1983 Adler et al.
`4,413,251
`.. 340/347
`.
`7/1984 Adler et a1.
`4,463,344
`
`340/347
`4,488,142 12/1984 Franasuk
`.
`. 360/40
`4,566,044
`1/1986 Langdon, Jr.
`340/347
`4,691,193
`9/1987 Khu .................
`340/347
`4,760,378
`7/1988 l'ketani et a1.
`
`9/1989 Lindmayer ..
`.. 365/119
`4,864,536
`
`341/57
`9/1989 Karabed et al.
`4,870,414
`341/59
`..
`4,882,583
`11/1989 Dimitri et a].
`
`4/1990 Kameyama .
`341/59
`4,914,438
`
`360/40
`4,928,187
`5/1990 Rees
`360/40
`.
`4,949,196
`8/1990 Davie e
`
`9/1991 Weathers et al.
`341/59
`5,047,767
`
`.. 341/59
`5,099,237
`3/1992 Fitingof ......
`
`369/116
`..
`5,136,573
`8/1992 Kobayashi
`5,163,039 11/1992 Lindmayer .....
`369/100
`5,173,694 12/1992 Lynch, Jr. et a1.
`.. 341/59
`
`3/1993 Galbraith ........
`.. 341/59
`5,196,849
`371/371
`5,271,016 12/1993 Hilden et a1.
`
`2/1994 Sawaguchi et a1.
`...... 360/57
`5,287,228
`
`..
`5,329,512
`7/1994 Fukimoto et a1.
`369/116
`
`5,400,313
`3/1995 Belser et al.
`369/116
`
`OTHER PUBLICATIONS
`
`Siege}, Paul H., “Recording Codes For Digital Magnetic
`Storage,” IEEE Transactions 0n Magnetics, vol. 21, No. 5,
`pp. 1344—1349, Sep., 1985.
`Kobayashi, H. et 211., “Application of Partial—response Chan~
`nel Coding to Magnetic Recording Systems," IBM J. Res.
`Develop, pp. 368—375, Jul., 1970.
`Lindmayer, Dr. Joseph et al., “Electron Trapping Optical
`chhnology—Memory‘s Next Generation?,” Computer
`Technology Review, Summer, 1990.
`Batman, Allen, “Optical Data Storage With Electron Trap‘
`ping Materials Using M—ary Data Channel Coding," Pro-
`ceedings of the Optical Data Storage Conference, Feb.,
`1992, San Jose, California.
`Fomey, G. David at al., “Coset Codes For Partial Response
`Channels; or, Coset Codes With Spectral Nulls,” IEEE
`Transactions- on Information Theory, vol. 35, No. 5, Sep.,
`1989, pp. 925-943.
`
`(List continued on next page.)
`
`Primary Examiner-Loha Ben
`Attomey, Agent, or Finn—Sterne, Kesslcr, Goldstein & Fox
`
`[57]
`
`ABSTRACT
`
`A system and method for recording multi~lcvel data to a
`multi—amplitudc recording channel encodes binary data to
`form multi—level data The multi-level data are recorded to
`the storage media for later recall. The system utilizes linear,
`multi-amplitude recording media which allows data to be
`stored as multi—level data—requiring fewer ‘bits’ to repre-
`sent the same number of symbols. To obtain greater data
`density in the storage media, a (infraction limited write laser
`is utilized, resulting in a smaller write-spot size. Because the
`read laser is of a longer wavelength, its difiraction limited
`spot size is larger. As a result, more than one mark is read
`at a given read time resulting in a inter-symbol interference.
`Trellis coded modultation techniques are adopted to convert
`the binary input data into M-ary data having M levels.
`Further coding is then performed to compensate for the
`effects of the inter-symbol
`interference. This is accom-
`plished by preceding the data using a Tomlinson-Harashima
`precoder. The preceding results in multi-level data (of m
`levels, where méoo).
`
`18 Claims, 5 Drawing Sheets
`
`
`
`CHANNEL
`
`lk-llT
`
`kT
`
`LSI corps await 1033
`
`Page 1
`
`LSI Corp. Exhibit 1033
`Page 1
`
`

`

`5,537,382
`Page 2
`
`OTHER PUBLICATIONS
`
`Laroia, Rajiv et al., “A Simple and Effective Preceding
`Scheme for Noise Whitening on Intersymbol Interference
`Channels,” IEEE Transactions on Communications, vol. 41,
`No. 10, Oct. 1993, pp. 1460—1463.
`McLaughlin, Steven et al., “M—ary Runlength Limited
`Codes for High Density Optical Recording,” I994 Int’l
`Symposium on Information Theory, Trondheim, Norway,
`Jul, 1994.
`McLaughlin, Steven, “Improved Distance M—ary (d,k)
`Codes for High Density Recording,” Rochester Institute of
`Technology, Rochester, New York, 1994.
`Ungerhoeck, Gottfried, ‘Trellis-Coded Modulation with
`Redundant Signal Sets, Part I: Introduction," IEEE Commu-
`nications Magazine, vol. 25, No. 2, pp. 12—21.
`Ungerboeck, Gottfried, “Trellis—Coded Modulation with
`Redundant Signal Sets, Part Ii: State of the Art,” IEEE
`Communications Magazine, vol. 25, No. 2, pp. 12—21.
`Marcus, Brian et 31., “Finite—State Modulation Codes for
`Data Storage,” IEEE Joumal 0n Selected Areas In Com-
`munications, vol. 10, No. l, 1992.
`Adler, Roy et al., “Algorithms for Sliding Block Codes,”
`IEEE Transactions in Information Theory, vol. IT~29, No. I,
`pp. 5—22, Jan, 1983.
`
`Forney, G. David et 211., “Combined Equalization and Cod-
`ing Using Precoding,” IEEE Communications Magazine, pp.
`25—34, Dec. 1991.
`
`McLaughlin, Steven et al., “Modulation Codes for Multi-
`—arnplitude Optical Recording Channels,” Rochester Insti-
`tute of Technology, Rochester, New York, paper presented
`Nov. 1994.
`
`M. Tomlinson, “New Automatic Equalizer Employing Mod-
`ule Arithmetic,“ Electronic Letters, vol. 7, pp. 138—139,
`Mar., 1971.
`
`G. Ungerboeck, “Channel Coding With Multi—Level/Phase
`Signals,” IEEE Trans. on Information Theory, vol. ITOCZS,
`pp. 56—67, Jan, l982.
`
`S. W. McLaughlin, “Improved Distance M—ary (d,k) Codes
`for High Density Recording,” Rochester Institute of Tech-
`nology, Rochester, NY, Jun, 1994.
`
`Brita H. Olson et al., “Multidimensional Partial Response
`For Parallel Readout Optical Memories,” SPIE, vol. 2297,
`pp. 331-337, May, 1984.
`
`LSIVUMN 00167939
`
`LSI Corp. Exhibit 1033
`
`Page 2
`
`LSI Corp. Exhibit 1033
`Page 2
`
`

`

`US. Patent
`
`Jul. 16, 1996
`
`Sheet 1 of 5
`
`5,537,382
`
`ENCODER
`
`3023
`
`104
`
`302A
`
`302C “6.33
`
`303
`
`104
`
`‘02:
`
`FIG 3c
`
`LSI ©0va Emmit 1033
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`Page 3
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`LSI Corp. Exhibit 1033
`Page 3
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`

`

`US. Patent
`
`Jul. 16, 1996
`
`Sheet 2 of 5
`
`5,537,382
`
`40
`
`8
`
`2:2
`
`' 208
`
`bk
`
`
`
`
`PRECODER
`
`CHANNEL
`
`(k-nT H
`
`b:
`
`”0
`
`504 /
`
`Fl G. 5A PRIOR ART
`
`
`
`LSI—UMN 00167941
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`LSI Corp. Exhibit 1033
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`Page 4
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`LSI Corp. Exhibit 1033
`Page 4
`
`

`

`US. Patent
`
`Jul. 16,1996
`
`Sheet 3 of 5
`
`5,537,382
`
`
`
`LSI 60wa Efififiit 1033
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`Page 5
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`LSI Corp. Exhibit 1033
`Page 5
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`

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`
`
`Oxno
`
`exam
`
`Ox8exam
`
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`ZSS‘LES‘S
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`S J0 P 193‘18
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`9661 ‘91 ‘Inf
`
`mama 'STI
`
`LSI Corp. Exhibit 1033
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`Page 6
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`LSI Corp. Exhibit 1033
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`
`

`

`
`
`zss‘Lss‘s
`
`1119ch '31]
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`LSI Corp. Exhibit 1033
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`LSI Corp. Exhibit 1033
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`

`

`5,537,382
`
`1
`PARTIAL RESPONSE CODING FOR A
`MULTI-LEVEL OPTICAL RECORDING
`CHANNEL
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates generally to data recording,
`and more specifically to a system and method for multi-level
`recording in a partial response channel.
`2. Related Art
`
`Conventional recording techniques use saturation record-
`ing to store information on the recording media Saturation
`recording techniques typically store the information in a
`two-level (i.e., binary) form, using digital data encoding
`methods to mark the recording medium. The process used to
`encode the data is limited to codes requiring no more than
`two, or possibly three, symbol amplitudes. Thus, such
`techniques provide limits to data storage capacity and trans-
`fer rates of the medium.
`
`The most common signal processing technique employed
`in saturation recording is run-length limited coding and peak
`detection. The application of more advanced methods of
`coding,
`such as partial
`response maximum likelihood
`(PRML) sequence detection has recently been considered
`for saturation recording applications. PRML techniques pro-
`vide an increase in recording density and reliability over
`runlength limited codes and peak detection.
`Significant advances in electron trapping materials have
`lead to the development of a storage medium that provides
`a linear response characteristic. Such a linear response
`characteristic provides an advantage over saturation-type
`media in that it adds an analog dimension to the storage
`capacity of the medium. Because the response is linear, the
`electron trapping material presents the ability to encode
`information in two dimensions—amplitude and phase. As a
`result, the storage medium is no longer confined to storing
`binary or tri-level data. Instead, the concept of M-ary, or
`non-binary, data coding and storage is provided. The
`increased symbol alphabet allowed by such encoding pro-
`vides the opportunity to increase dramatically the data
`recording density and transfer rate of the storage device. For
`example, the potential storage capacity of a single 5% inch
`disk can be extended to several gigabytes if that disk were
`to be implemented using the electronic trapping materials.
`Examples of materials that can be used as the storage
`media for M-ary storage are described in U.S. Pat. Nos.
`4,915,982, 4,834,536, and 4,830,875. Other materials useful
`as the storage media are disclosed in U.S. Pat. Nos. 4,839,
`092 and 4,806,772, and 4,842,960. Examples of an optical
`disk and an optical disk drive incorporating electron trapa
`ping materials for data storage are disclosed in U.S. Pat.
`Nos. 5,142,493 and 5,007,037, respectively. The full disclo-
`sure of each of these Patents is incorporated by reference
`herein.
`
`SUMMARY OF THE INVENTION
`
`The present invention is directed toward a system and
`method for recording multilevel data to a multi—amplitude
`recording channel. According to the invention, binary data
`are encoded to form multi—level data. The multilevel data are
`recorded to the storage media for later recall.
`One key difference between data recording as disclosed
`herein and conventional recording techniques is that of the
`storage media. Conventional storage media are saturation-
`
`IO
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`type recording media, capable of storing only two—level
`(binary) data To record a large number of information bits
`to the media requires a large number of channel symbols.
`In contrast, the present invention utilizes linear, multi-
`arnplitude recording media which allows dam to be stored as
`multi-level data—requiring fewer symbols to represent the
`same number of information bits. Preferably, this media is an
`optical storage media that is written to and read from using
`a write and a read laser, respectively.
`To obtain greater data density in the storage media, a
`diffraction limited write laser is utilized, resulting in a
`smaller write—spot size. This smaller spot size yields a higher
`recorded data density. Because the read laser is of a longer
`wavelength, its diffraction limited spot size is larger. As a
`result, more than one mark is read at a given read time
`resulting in inter-symbol interference.
`To take full advantage of this multi-amplitude storage
`media trellis coded modultation techniques are adopted.
`Such techniques convert the binary input data into M-ary
`data having M levels. Further coding is then performed to
`compensate for the effects of the inter-symbol interference.
`This is accomplished by precoding the data using a Tom-
`linson-Harashima precoder. The preceding results in multi-
`level data (of in levels, where méw). The data are precoded
`prior to recording so that the decoding process can be
`implemented in a somewhat straightforward manner.
`Further features and advantages of the present invention,
`as well as the structure and operation of various embodi-
`ments of the present invention, are described in detail below
`with reference to the—accompanying drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention is described with reference to the
`accompanying drawings. In the drawings,
`like reference
`numbers indicate identical or functionally similar elements.
`Additionally, the left-most digit(s) of a reference number
`identifies the drawing in which the reference number first
`appears.
`FIG. 1 is a diagram illustrating a portion of a track written
`with “marks” having different intensity and length.
`FIG. 2 is a block diagram illustrating a systems-level
`channel model for the M-ary recording channel.
`FIG. 3A through 3C illustrate the efiects of intersymbol
`interference that occurs during the read process.
`FIG. 4 is a block diagram illustrating the system model of
`FIG. 2, with encoder 204 divided into the two phases to
`implement the preferred embodiment.
`FIG. 5A is a block diagram illustrating a finite state
`machine used to implement a simple rate one—half binary
`convolutional code.
`
`’ FIG. 5B is a trellis illustrating the output bits that result
`for a stream of input bits for the specific coder illustrated in
`FIG. 5A.
`
`FIG. 6A depicts the trellis of FIG. 5B when the four
`output sequences 00, 10, 01, and 11 are replaced with four
`signal sets C0 through C3.
`FIG. 6B depicts an example of partitioning of signal sets
`C0 through 0,.
`FIG. 7 illustrates an example of a time varying trellis.
`FIG. 8 is a block diagram illustrating one embodiment of
`a Tomlinson-Harashima precoder.
`FIG. 9 depicts a time varying one dimensional 8 state
`trellis.
`
`LSI—UMN 00167945
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`5,537,382
`
`3
`DETAILED DESCRIPTION OF THE
`EMBODIMENTS
`
`1. Overview and Discussion of the Invention
`
`The present invention is directed to a system and method
`for multi-level encoding of data for a multiple amplitude
`optical recording channel.
`Traditional optical recording products for data storage
`utilize binary (digital) encoding methods to mark the optical
`media. The intrinsic nature of optical marking requires the
`use of such encoding methods. For example, write-once
`recording, commonly found in compact disks, is accom-
`plished by marking the optical material with pits or other
`similar features to indicate the data recorded. For example,
`a 1 may correspond to the presence of a pit, and a O to the
`absence of a pit. Similarly, phase~change recording is
`accomplished by manipulating crystalline or amorphous
`features of the recording material to indicate the presence of
`a 1 or a 0. Finally, magneto-optic recording is accomplished
`by altering the polarities of the magnetic material. Due to the
`nature of the materials,
`these marking techniques have
`limited channel encoding to binary digital code sets. Con-
`ventional magnetic recording is likewise limited to two-state
`encoding at very high dam densities where the feature
`dimensions approach the minimum domain dimensions.
`As described above, the implementation of storage media
`using electron trapping materials allows data to be stored at
`multiple levels. Thus, the use of a M-ary, or non—binary code
`set would enable the recording system to take full advantage
`of the properties of the electron trapping material. This
`results in significant increases in both data density and
`transfer rate. The full potential of optical recording media
`based on materials which exhibit the electromtrapping phe-
`nomenon and provide a very broad linear amplitude
`response can be realized by implementing M—ary data code
`sets.
`
`2. The Multi-Amplitudc Optical Channel
`
`2.l Multi-chcl Recording Media
`An example of a multi—amplitude optical storage media is
`now described. Electron trapping is an opto-electronic
`approach to optical recording. To prepare the electron—
`trapping media, a disk, or other storage substrate, is coded
`with a II—Vl phosphor material that is doped with two rare
`earth metals. The fundamental process responsible for the
`storage of information in the electron-trapping material is
`the transfer of an electron charge from one dopant atom to
`a neighboring different dopant atom under the stimulus of
`incident light radiation. Thus, to facilitate the operation of
`writing to the material, the material is illuminated with light
`at a first wavelength so that electrons from one dopant atom
`are accelerated to a higher energy state in a second dopant
`atom, where they remain trapped at energy levels deter-
`mined by the dopant materials.
`To read the information, the material is illuminated with
`light of a second wavelength. The absorption of a photon at
`this second wavelength provides the trapped electron with
`enough energy to elevate it out of the trap, and return it to
`the ground state of the first dopant atom, thereby releasing
`the stored energy in the form of visible light.
`the elfective
`Although this
`is a two‘state process,
`“domain” is delimited by the adjacent dopant pair within the
`crystal lattice. Because the effective domain is very small as
`compared to the marking resolution (defined by the spot size
`of the light radiation), a marking region contains many
`
`10
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`effective domains. The high ratio of mark—tddomain dimen—
`sion provides a linear amplitude response in a similar
`manner to analog audio-video magnetic recording. That is,
`the number of emitted photons is proportional to excitation
`energy, and hence, is linear in this sense.
`The above description provides one example of how an
`electron~trapping material can be implemented to provide
`multivlevel recording media. This description is provided by
`way of example only, and it is not intended to limit the scope
`of the invention in any way. It will become apparent to a
`person skilled in the relevant art how to implement the
`invention using other types of multi—amplitude recording
`media.
`2.2 A Multi-Level Recording Channel
`A model of an M—ary channel according to the invention
`is now described. FIG. 1 is a diagram illustrating a portion
`of a track written with “marks” having different intensity and
`length. The channel is written and read optically by separate
`lasers focused to ‘spots’ on the media. A write spot 102 of
`diameter LW and a read spot 104 of diameter LR result from
`the focusing of a write laser and a read laser onto the media.
`Because of the gaussian prOperties of the coherent laser
`beams, the spots are generally circular. However, because
`the disk is spinning during read and write operations, the
`remnant spots 108 take the oblong shape that is illustrated in
`FIG. 1.
`
`the write laser is
`For writing multiple levels of data,
`positioned above a track on a rotating disk and its intensity
`is modulated The strength of the remnant mark on the disk
`is proportional to the write laser intensity. Specifically, the
`trapping level, or number of electrons trapped, is propor-
`tional to the intensity of light impinging on the media. Thus,
`multiple levels of encoding are provided by modulating the
`intensity of the write laser.
`For reading, the read laser is positioned above the track to
`be read and a constant intensity illumination is provided to
`the writtendo media. As a result of the excitation provided
`by the read laser, trapped electrons are released from the
`trapped state resulting in the emission of photons. The
`intensity of the emission is proportional to the number of
`electrons that were trapped during the write process. This
`allows the multiple levels to be detected at the read stage.
`According to a preferred embodiment of the invention,
`the wavelength of the read laser, AR, is different from the
`wavelength, kw, of the write laser. Specifically,
`in one
`embodiment, the data are written with a blue light laser
`having a kw of 488 nanometers (run) and they are read with
`a red laser having a AR of 647 nm.
`2.3 A Multi—Level Channel Model
`
`FIG. 2 is a block diagram illustrating a systems-level
`channel model for the M—ary recording channel. This model
`includes an encoder 204, an optical channel 208 and a
`multiplier 212. In this model, bk denotes a user bit sequence
`(i.e., the data to be recorded). Typically, the user data to be
`recorded as provided by the user are in digital (binary) form.
`To take full advantage of the multi~level properties of the
`storage medium, these digital data are encoded by encoder
`204 to provide multi—level data. To this end, encoder 204
`receives the user bit sequence bk and codes this sequence to
`produce a coded channel symbol sequence ak. The coded
`channel symbol sequence is the actual multi—level coded
`data that are to be recorded onto the multi—lcvcl recording
`channel.
`
`the sequence is used to
`To record the sequence ak,
`modulate the write laser as represented by multiplier 212.
`Variations in the amplitude of ak result in a variation in the
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`5,537,382
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`5
`intensity of the modulated write-laser signal w(t). Because
`sequence a,c is a sequence having In levels (theoretically
`méoo), the write laser is modulated to in levels. Therefore,
`in levels of data are written to the medium.
`
`illustrated in FIG.2, w(t) represents the
`In the model
`modulated laser light, or, in reality, the mark pattern written
`on the disk where
`
`w(t)= ii agb(l—iT,)
`M
`
`(1)
`
`IO
`
`In equation (1), b(t—kT5) is a “box” function representing
`one pulse of the unmodulated write laser and having a
`nominal height of one (I), a duration time of TS and n<u°.
`Although there is flexibility in choosing a value for T3, in
`one embodiment, TS is chosen to be substantially close to or
`equal to TW=LWJV, where Lw is the spotsize of the write laser
`and v is the velocity of the medium. In an alternative
`embodiment, at least one class of modulation codes, how-
`ever, uses T5<TW (M-ary runlength limited codes are con-
`sidered later).
`During the read process, information is read from the
`media using a read laser with spotsize LR. The read laser
`illuminates the recorded marks and the media releases a
`number of photons. The number released is proportional to
`the mark intensity as originally determined by the intensity
`of the write laser (i.e., as modulated by ak). The photons are
`counted using a photodetector whose output is an analog
`voltage r(t). An approximation to r(t) is a sliding, ideal
`integrator, namely
`1
`
`(2)
`
`r(t)=K J'
`
`1 ~ T];
`
`.
`w(r)dr+ n(t)
`
`where the duration of the impulse is TR=LR/v. K is a
`proportionality constant and n(t) is a white gaussian noise.
`Without loss of generality it can be assumed that K=l. A
`decoder (not illustrated in FIG. 2) uses r(t) to estimate the
`code sequence ak.
`The purpose of encoder 204 is to convert a sequence of
`user data bits to a channel waveform. Specifically, encoder
`204 converts the user binary data to multi-level data. The
`performance of the code is measured by its storage density
`D (in bits/unit area) and error probability Pc (bit error rate,
`or BER).
`The bit density D in user—bits/arca is computed as D=bl
`vTW, where b is the number of bits stored during period T
`on a disk rotating at speed v with a track width W. Assuming
`a normalized product vW=1, and for convenience let T=TR,
`the linear density D (often called density ratio) becomes
`(in bits/sec). D=b/RR, (3)
`Given this model, the ideal coded modulation scheme is
`one that densely stores user data with high reliability.
`‘
`2.4 Induced Intersymbol Interference
`According to one embodiment of the invention, the read
`and write lasers are diffraction limited. As a result, the read
`and write spot diameters, LR and Lw as focused on the
`medium are made as small as possible. This has the effect of
`maximizing storage density on the disk. The diiiraction
`limits lawn], and Lani] are lower bounds on the spot sizes
`LW and LR, respectively. Because the diffraction limit is
`proportional to the laser wavelength, 1, and because the
`lasers are ditiraetion limited, the read spot size LR is larger
`than the write spot size LW. In fact, in the specific embodi-
`ment described above using a blue write laser and a red read
`laser, LR is approximately 11/3 times the size of LW. The
`manner in which this spot size differential is used to induce
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`a controlled intersymbol interference (i.e. partial response)
`channel is now described.
`As described above, both the read and the write laser are
`diffraction limited. The minimum mark Tmin is constrained
`by the spot size of the write laser LW and speed of the disk,
`namely T,,.,»,,=Tw. The maximum mark Tmx is constrained
`by system timing requirements. Because many recording
`systems derive timing from transitions in the recorded signal
`level, periodic transitions
`in recorded amplitude are
`required, thus constraining the maximum width Tm“ of a
`mark.
`Due to the diffraction limiting, in the above—described
`embodiment having a blue write and a red read laser, read
`spot 104 is actually approximately 1% times the size of write
`spot 102. One reason for diffraction limiting the write laser
`is to increase the density at which marks, and thus dam, are
`written to the medium. Because spot size Lw is constrained
`to the minimum beam waist size, the data density is maxi—
`mized for a given write laser wavelength, 1“,.
`This advantage of a minimum LW, however, is somewhat
`ofiset by a resultant intersymbol interference that occurs
`during the read process. To elaborate, because LR is larger
`than Lw, when data are read, more than one symbol are read
`at a time. For example, assume that LR is twice the diameter
`of Lw. In this example, the read laser actually stimulates
`emission from the spatial equivalent of two marks. This is
`illustrated in FIGS. 3A and 3B. In FIG. 3A, read laser
`actually reads the levels stored in marks 302A and 302B. In
`FIG. SB, read laser reads the levels stored in marks 302A,
`3023, and 302C. Because the detector cannot distinguish
`which of the received photons are from which mark, the
`detector provides a signal level that is the total of all photons
`detected. Thus, this intersymbol interference can result in an
`erroneous reading.
`As another example, consider the embodiment described
`above where LR is l’/3 times the size of Lw. In this example,
`illustrated in FIG. 3C, the read laser results in the detection
`of the current mark 304 (the mark we want to detect) plus
`one third of the previous mark 303. This is illusuated in FIG.
`3C.
`To minimize the effects of the resultant intersymbol
`interference,
`two solutions can be implemented. A first
`solution attempts to decode the results of the read operation
`to remove the efiects of the intersymbol interference. Such
`a solution requires a decoder that estimates the effect of the
`extra V3 of the mark that is read and subtracts this amount
`from the detected value to produce a decoded signal.
`A second solution is to provide an additional encoding
`phase in encoder 204 to subtract the effect of the additional
`V: of the mark before the data are written to the medium. It
`is this second solution that is the preferred embodiment of
`the invention.
`Note that for the example embodiment described herein,
`there is intersymbol interference because the read laser spot
`size is approximately 1% that of the write laser. The present
`invention is described in terms of this example embodiment.
`Description in these terms is provided for convenience only.
`It is not intended that the invention be limited to application
`in this example embodiment, where LR=1‘/3LW. In fact, after
`reading the following description, it will become apparent to
`a person skilled in the relevant at how to implement the
`invention with alternative partial responses.
`
`3. Encoder
`
`As stated above, encoder 204 is implemented to encode
`the binary input data into multi-level data that makes opti-
`
`LSI—UMN 00167947
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`LSI Corp. Exhibit 1033
`
`Page 10
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`LSI Corp. Exhibit 1033
`Page 10
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`TABLE I
`
`signal set C0
`00
`signal set Cl
`01
`signal set C2
`10
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`11 signal set C3
`
`The combined total of signal sets C0 through C3 contains
`eight (8) discrete values between zero (0) and the saturation
`level. Thus, the trellis—coded modulator in this example
`produces an M-ary data stream where M=8.
`From a given node N in signal set equivalent trellis 600,
`the next input bit of the input stream bK determines the path
`taken and the specific value chosen from the signal set of
`that path. This determination is made using the output bits
`that result from the convolutional coder finite state machine.
`For example, consider what happens when an input bit is
`received and the current state of the coder is at node N2. As
`with the binary trellis, an input bit of ‘0’ results in output bits
`b0, b1 having the values ‘1' and '0‘, respectively. However,
`unlike the binary trellis, the input bit does not select the path
`directly. Instead, one of the output bits (e.g., be) directly
`selects the path to follow (i.e., the signal set to choose), and
`the second of the two bits (cg, bl) selects which level in the
`signal set is to be chosen as the output level. Following the
`same example, for output bits b0, b1 of ‘1’, ‘0’, b0=‘ 1 ’ is used
`to select signal set C,, and b1=‘0’ is used to select the value
`5Al7 as the output level of the coder. Note that 5N7 is the
`sixth of eight levels.
`Techniques for trellis—coded modulation similar to that
`described here are Well—known in the modem communica-
`tions industry. These Ungerboeck-style codes can produce a
`coded sequence having a constant amplitude over a consid-
`erable number of symbols. However, in one embodiment of
`the optical recording channel, timing is derived from symbol
`amplitude transitions. As a result, a string of constant
`amplitude symbols can result in a loss of timing synchro-
`nization. To overcome this difiiculty, a time-varying trellis is
`implemented.
`An example of a time varying trellis is illustrated in FIG.
`7. As with the trellis presented in FIG. 6, C0, C1, C2 and C3
`are the signal sets obtained by standard Ungerboeck parti-
`tioning. However in the time varying trellis 700, the even
`stages of the four-state trellis are reassigned. For example, in
`every other stage, C0 is replaced with C2. As a result of this
`reassignment, the maximum runlength of any signal subset
`(and hence code symbol) is limited to three. A similar type
`of reassignment can be made for an eight—state trellis to
`obtain a code that has a maximum symbol runlength of four.
`A further advantage of a time-varying trellis is that it suffers
`no loss in rate and no loss in minimum distance.
`3.2 Tomlinson—Harashima Precoder
`As stated above, the differential between read and write
`spot sizes L, and LW is used to force inter-symbol interfer—
`ence to increase recording density. The Tornliuson—Ha—
`rashima precoder 408 is implemented to precede the modu~
`lated data to compensate for this inter-symbol interference
`prior to the data even being written to the optical recording
`channel 208. Although Tomlinson—Harashima preceding is
`generally known in the communications industry, it is a new
`and novel feature of the invention to provide a partial
`response optical recording channel 208 induced by inter-
`symbol interference and to precompensate for this interfer-
`ence prior to recording using a Tomliuson—l-larashima pre-
`coder 408. In summary, by anticipating the inter—symbol
`interference in the channel 208, the data are precoded so that
`
`7
`mum usage of the multi—level recording channel 208. Addi-
`tionally,
`in the preferred embodiment where intersymbol
`interference results from diffraction limited read and write
`lasers at two difierent wavelengths, encoder 204 provides an
`additional encoding stage to precode the data. This precod-
`ing is implemented to counteract the effects of the intersym—
`bol interference before the data are written to the recording
`channel. Because the data are preceded before being
`recorded, the decoding process is simplified. Note that in
`this embodiment, as will become apparent to a person skilled
`in the relevant an after reading the below disclosure of the
`precoder 408, the actual data recorded onto the recording
`channel can theoretically have an infinite number of levels.
`FIG. 4 is a block diagram illustrating the system model of
`FIG. 2, with encoder 204 divided into the two phases to
`implement the preferred embodiment. In the embodiment
`illustrated in FIG. 4, these phases are a trellis coder 404 and
`a Tomlinson-Harashima precoder 408.
`Trellis coder 404 receives the binary input dam, bk, and
`encodes this input data to generate an M-ary output signal Ck
`(having M levels). TornlinsomHarashima precoder 408
`accepts the Mary data ck and codes this data to anticipate
`the effects of the inter-symbol interference. This results in
`preceded multi~level data ak. It is this precoded multi-level
`data at that modulates the write laser to record the preceded
`multi—level data onto optical channel 208. Trellis coder 404
`and Tomlinson—Harashima precoder 408 are now described.
`3.] Trellis Modulation Cod