0
`
`US006111831A
`
`United States Patent
`
`[19]
`
`[11] Patent Niunher:
`
`6,111,831
`
`Alon et al.
`
`[45] Date of Patent:
`
`*Aug. 29, 2000
`
`8/1988 Ando . . . .. .
`4,766,582
`3/1989 Webster
`4,815,067
`6/1989 Fennema
`4,839,876
`ll/1989 Takamura et al.
`4,882,546
`4,890,272 12/1989 Ando ....... ..
`
`,. 6
`,
`4 980 876
`4,982,395
`4,989,190
`
`4/1999 Mlyasaka
`11/1990 Rafner
`12/1990 Abate et al.
`1/1991 lVIacAnally
`..
`1/1991 Kuroe et al.
`.
`.
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`
`. . . ... 369/45
`359/97
`369/32
`329/310
`.. 369/45
`
`,369/.,3,2
`369/32
`. 369/44.11
`. 369/44.37
`.... .. 369/32
`
`
`
`089 264 A1
`273 384 A1
`506 447
`569 718
`598 611 A2
`643 388 A1
`712 119 A2
`
`714 096
`60’173724
`61-"8563
`2 188 474
`
`9/1983 European Pat. Off.
`7/1988 European Pat. Off.
`9/1992 European Pat. Off.
`11/1993 European Pat. Off.
`5/1994 European Pat. Off.
`3/1995 European Pat. Off.
`5/1996 European Pat. Off.
`
`.
`.
`.
`.
`.
`.
`.
`
`5/1996 E“‘°P”“ Pa" Off’ '
`9/1985
`Japan '
`5/1989
`-Tara" -
`9/1987 United Kingdom .
`
`OTHER PUBLICATIONS
`Ken C. Polilniann, The Compact Disc Handbook, A—R
`Editions, Inc,’ pp, 213-221, 1992,
`,
`,
`Primary Exammer—Paul W. Iluber
`Attorney, Agent, or Firm—Fish & Neave; James Trosino;
`Michael J. DeHaeiiier
`
`[57]
`
`ABSTRACT
`.
`Methods and apparatus are provided for synchronously
`reading data from multiple tracks of an optical disk using
`multiple illumination beams. Circuitry is provided for use
`with a photodetector array to read and buffer data in parallel
`from the multiple adjacent
`tracks, while asvnchronously
`rovidin '
`rocessed data to a host
`rocessor Circuitr' is
`P
`E1 h
`5 Pd d f
`.
`h
`'
`1 .
`f’
`rt er provi e
`or correcting p ase errors resu ting rom
`variations in the linear ‘velocity. of the tracks being read,
`depending upon the radial position of the tracks.
`
`20 Claims, 9 Drawing Sheets
`
`[54] METHODS AND APPARATUS FOR
`SIMUITANE()USI.Y READING MULTIPLE
`TRACKS OF AN OPTICAL STORAGE
`MEDIUM
`
`75]
`
`Inventors: Amir Alon, Sunnyvale, Calif; Jacob
`_
`,
`.
`,
`Flnkelstemr Kt“ 5343*‘, Israel
`_‘
`,
`73] Asslgneti Zen Research N- V-, Curacao,
`Netherlands Antilles
`
`*] Notice:
`
`This patent is subject to a terminal dis-
`elaiiiier.
`
`.
`7
`‘H APPL N0" 09/203394
`22]
`Filed;
`])e(;_ 1, 1998
`
`Related U_S_ Application Data
`
`60] Division of application No. ()8’8(J4,1U5, Feb. 20, 1997, Pat.
`No. 5,907,526, which is a contii1uation—in—part of application
`No. 08/559,429, Nov. 15, 1995, Pat. No. 5,627,805.
`
`51]
`
`Int. Cl.7 .............................. .. C113 5/09; GIIB 3/74;
`G 1113 7/00
`369/49; 369/95; 369/124
`
`.................. .. 369/32, 33, 47,
`369/48, 49, 50, 54, 58, 60, 124, 95
`
`52] U-S- CL
`58] Field Of Search
`
`56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`.. 179/100.3 B
`2/1978 Russell
`.......... ..
`4,074,085
`369/32
`8/1981 Ciirryetal.
`4,283,777
`. 369/44
`5/1984 Reno ...........
`4,449,212
`. 369/44
`7/1984 C0rS0Ver 6! a1.
`4,459,690
`- 359/3/2
`'7/1984 G°I'f1°11 -----------
`4,460.~98{3
`‘ 369/32
`4>486?87” 12498‘! Pemg’e‘V et al‘
`369/112
`4’536’866
`8/1983 Jerome et a1’ ”
`369/59
`4,566,092
`1/1986 Gerard et al.
`369/59
`4,583,211
`41,1986 Nishikawa at al_ _
`_ 369/50
`4,64();2gD
`2/1987 Toyosawa
`369/112
`4,539,781
`8/1987 Ando
`369/46
`4,720,825
`1/1988 Kokado
`4,754,446
`6/1988 Reno ..................................... .. 369/112
`
`
`
`
`
`Apple 1037
`
`U.S. Pat. 9,189,437
`
`Apple 1037
`U.S. Pat. 9,189,437
`
`

`
`5,331,618
`
`6,111,831
`Page 2
`
`
`
`.
`.
`
`5,001,732
`5,081,617
`5,105,407
`
`5,111,445
`5,128,919
`5,140,577
`5,150,347
`5,199,017
`5310726
`
`U.S. PATENT DOCUMENTS
`3/1991 Nomura et al.
`............................. 377/3
`1/1992 Gelbart .................................. .. 369/112
`4/1992 Ishika ................................. .. 369/44.37
`N
`Maeda et al.
`
`......................... N 369/103
`5/1992 Psaltis et a1.
`7/1992 Narahara et al.
`....................... .. 369/97
`8/1992 Ohsato ..... ..
`369/44.37
`
`............................... .. 369/44.37
`9/1992 Yanagi
`3/1993 Kagami et al.
`.................... .. 369/44.28
`
`5/1993 Ja°kS°“ 6‘ a1~
`369/32
`2/1993 Ren9 .................................... 369/44.26
`............................ .. 369/48
`/1993 T0b1ta et al.
`,
`9
`8/1993 Seo etal.
`369/54
`5,239,530
`
`........................... .. 369/44.28
`9/1993 Lee et al.
`5,245,597
`9/1993 Yoshimaru et al.
`.................... N 369/48
`5,249,170
`5274507 12/1993 Lee .................. N
`. 360/39
`5,283,776
`2/1994 Takagi .
`369/58
`5,295,125
`3/1994 Oonishi et al.
`369/44.29
`5,301,174
`4/1994 Matoba et al.
`. 369/44.28
`5,313,448
`5/1994 Sukeda et al.
`. 369/121
`
`7/1994 Nagai
`...................................... .. 369/59
`§‘f0:‘f1‘f§:°;‘;1‘ fiiiiiiiiii:::f,639§ZQ§:
`5 377 178 12/1994 Saito et al
`369/124
`' """"""""""""""" "
`’
`’
`Park Ct 8.1.
`......................... ..
`3/1995 Maeda ................................... .. 369/124
`5,398,228
`3/1995 Oshiba et al.
`.......................... .. 369/32
`5,402,399
`6/1995 Alon et al.
`369/32
`5,426,623
`11/1995 Kobayashi et a1,
`369/50
`5,455,244
`1/1996 Jewell et a1.
`. 369/44.37
`5,483,511
`1/1996 Koyama .... ..
`.. 369/44.28
`5,485,438
`4/1996 Nagasaki et al.
`369/60
`5,508,990
`9/1996 Kamisada et al
`369/219
`5 555 539
`,
`9
`9
`55619654 10/1996 Ham{1‘°“ °“‘1~
`369/97
`5,566,159
`10/1996 Shz1p1ra ................................... .. 369/99
`5,581,715
`12/1996 Ver1nsky et al.
`...................... .. 395/309
`5,600,626
`2/1997 Yokogawa et al.
`. 369/275.3
`5,608,716
`3/1997 Koyama et al.
`. 369/275.1
`5,627,805
`5/1997 Finkelstein et al.
`369/32
`5,828,643
`10/1998 Takeda et al.
`........................ .. 369/103
`
`
`
`'
`
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`
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`
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`Aug. 29,2000
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`U.S. Patent
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`6,111,831
`
`1
`METHODS AND APPARATUS FOR
`SIMULTANEOUSLY READING MULTIPLE
`TRACKS OF AN OPTICAL STORAGE
`MEDIUM
`
`RELATED APPLICATION
`
`This application is a division of commonly assigned
`application Ser. No. 08/804,105, filed Feb. 20, 1997, now
`U.S. Pat. No. 5,907,526, issued May 25, 1999, which is a
`continuation-in-part of application Ser. No. 08/559,429,
`filed Nov. 15, 1995, now U.S. Pat. No. 5,627,805, issued
`May 6, 1997.
`
`FIELD OF THE INVENTION
`
`This invention relates to methods and apparatus for
`retrieving information from an optical disk at high data rates
`by simultaneously and synchronously reading multiple adja-
`cent tracks.
`
`BACKGROUND OF THE INVENTION
`
`Due to their high storage density, long data retention life,
`and relatively low cost, optical disks are becoming increas-
`ingly popular as a means to distribute information. Large
`format disks have been developed for storing full length
`motion pictures. The compact disk (CD), and more recent
`mini disk (MD) formats were developed and marketed for
`the distribution of musical recordings and have essentially
`replaced vinyl records. High-capacity, read-only data stor-
`age media, such as CD-ROM, have become prevalent in the
`personal computer field, while the new Digital Video Disk
`(DVD) format may soon replace videotape as the distribu-
`tion medium for video information.
`
`An optical disk is made of a transparent disk or substrate
`in which data, in the form of a serial bit-stream, is encoded
`as a series of pits in a reflective surface within the disk. The
`pits are arranged along a spiral or circular track. Data is read
`from the optical disk by focusing a low power laser beam
`onto a track on the disk and detecting the light reflected from
`the surface of the disk. By rotating the optical disk, the light
`reflected from the surface of the disk is modulated by the
`pattern of the pits rotating into and out of the laser’s field of
`illumination. Optical and imaging systems detect
`the
`modulated, reflected, laser light and produce an electrical
`signal which may be decoded to recover the digital data
`stored on the optical disk. The recovered digital data, which
`may include error correcting codes and additional subcoded
`information, is further processed to recover the stored data
`which may then be converted to audio signals, or used as
`executable programs and data depending on the type of
`optical disk being read.
`To be able to retrieve data from anywhere on a optical
`disk, the optical systems include a pickup assembly which
`may be positioned to read data from any disk track. Servo
`mechanisms are provided for focusing the optical system
`and for keeping the pickup assembly positioned over the
`track, despite disk warpage or eccentricity.
`Because in most previously known systems the data is
`retrieved from the disk serially, i.e. one bit at a time, the
`maximum data transfer rate for an optical disk reader is
`determined by the rate at which the pits pass by the pickup
`assembly. The linear density of the bits and the track pitch
`is fixed by the specification of the particular optical disk
`format. For example, CD disks employ a track pitch of 1.6
`gm, while the DVD employs a track pitch only about
`one-half as wide.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`Previously known methods of increasing the data transfer
`rate of optical disk readers have focused on increasing the
`rate at which the pits pass by the pickup assembly by
`increasing the rotational speed of the disk itself. Currently,
`drives with rotational speeds of 2x to 10x standard speed are
`commercially available, and 12x designs are on the horizon.
`However higher disk rotational speeds place increasing
`demands on the optical and mechanical subsystems within
`the optical disk player, making such players more difficult
`and expensive to design and manufacture.
`Other previously known techniques for increasing aver-
`age data transfer rates involve methods to intelligently
`anticipate future read requests by a host processor. It has
`been observed that data access by computers frequently
`exhibit “locality of reference,” which means that a future
`data access will be local,
`in either space or time,
`to a
`previous data access. Thus a CD-ROM drive or controller
`can “read ahead” and buffer the data that the host processor
`is likely to request next. When the host processor next
`requests data from the optical disk drive,
`the drive first
`checks if the requested data has already been read and
`buffered. If the data has already been buffered, the drive
`simply sends the buffered data to the host, avoiding the
`delays associated with repositioning the pickup assembly
`and reading data from the optical disk itself. While such
`caching techniques may speed up average data transfer rates,
`the maximum data transfer rate is still
`limited by the
`rotational velocity of the optical disk within the optical disk
`reader.
`
`U.S. patent application Ser. No. 08/559,429, filed Nov. 15,
`1995, now, U.S. Pat. No. 5,627,805, incorporated herein by
`reference, describes a system to increase disk reading speeds
`by reading multiple tracks simultaneously. The data is read
`using a matrix detector that provides track signal data from
`each of a plurality of adjacent tracks. The system described
`therein employs a source of wide-area illumination to illu-
`minate multiple tracks, which are then imaged onto the
`single matrix detector.
`The present application is directed to an improvement in
`the system described in the above-incorporated patent,
`wherein the matrix detector and source of wide-area illumi-
`
`nation are replaced by a multi-beam, multi-detector pickup
`assembly. Apparatus in accordance with the present inven-
`tion obviates the Virtual Tracking System described in the
`foregoing application,
`instead employing conventional
`servo methods for tracking.
`It would therefore be desirable to provide optical disk
`reading apparatus and methods that provide high speed
`retrieval of information from an optical disk while avoiding
`the limitations imposed on optical disk rotation speeds
`encountered by previously known devices.
`It would also be desirable to provide an optical disk
`reading apparatus and methods that provide high speed
`retrieval of information from an optical disk using a multi-
`beam, multi-detector pickup assembly.
`SUMMARY OF THE INVENTION
`
`In view of the foregoing, it is an object of the present
`invention to provide an optical disk reading apparatus and
`methods that provide high speed retrieval of information
`from an optical disk while avoiding the limitations imposed
`on optical disk rotation speeds encountered by previously
`known devices.
`
`It is a further object of this invention to provide an optical
`disk reading apparatus and methods that provide high speed
`retrieval of information from an optical disk using a multi-
`beam, multi-detector pickup assembly.
`
`

`
`6,111,831
`
`3
`These and other objectives of the invention are accom-
`plished by providing methods and apparatus for processing,
`tracking, and reading data from multiple adjacent tracks
`simultaneously.
`In particular, apparatus constructed in
`accordance with the present invention employs a pickup
`assembly including a diffraction grating that splits a source
`of laser light into a plurality of beams for illuminating
`multiple tracks of an optical disk. A plurality of photode-
`tectors simultaneously generate electrical data signals rep-
`resentative of the information-bearing pits on respective
`ones of the multiple adjacent data tracks of the optical disk.
`Methods and apparatus are provided for synchronizing the
`readout of the data from the multiple adjacent tracks to
`account for radial variations in linear velocity, and its effect
`on signal phase and frequency. Electrical data signals are
`then processed in accordance with previously known
`demodulation, decoding and error correction schemes and
`the resulting bit stream is buffered. The buffered data is
`subsequently asynchronously read out of the buffer for
`further processing per se known in the fields of digital audio,
`video, and computer processing.
`Further features of the invention, its nature and various
`advantages will be more apparent from the accompanying
`drawings and the following detailed description of the
`preferred embodiments.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is an illustrative block diagram of a previously
`known optical disk reader;
`FIG. 2 is an illustrative block diagram of an optical disk
`reader constructed in accordance with the principles of the
`present invention;
`FIG. 3 is a block diagram of an illustrative embodiment
`of a multi-beam, multi-detector pickup assembly suitable for
`use in the present invention;
`FIG. 4 is a detailed view of the arrangement of the
`detector elements in the pickup assembly of FIG. 3;
`FIG. 5 is a block diagram of the front end circuitry for
`extracting data from the signals output by the pickup assem-
`bly of FIG. 3;
`FIGS. 6A and 6B are more detailed block diagrams of the
`clock generation circuitry and exemplary frequency detector
`circuitry, respectively, of FIG. 5;
`FIG. 7 is a block diagram of the data aligner and data
`sampler circuitry of FIG. 5;
`FIG. 8 is a more detailed block diagram of exemplary data
`synchronization circuitry of FIG. 7;
`FIGS. 9A and 9B are, respectively, an alternative embodi-
`ment of the data aligner of FIG. 7 and a corresponding
`timing diagram;
`FIG. 10 is yet another illustrative embodiment of the data
`aligner of FIG. 7; and
`FIG. 11 is a flow chart outlining illustrative processes for
`reading a requested block of data from an optical disk and
`providing the requested block of data to a host processor.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`By way of overview, a brief description of the compo-
`nents and operation of a previously known optical disk
`reader 10 is provided with respect to FIG. 1. The detailed
`description of the design and operation of such previously
`known optical disk readers may be found, for example, in
`Compact Disk Technology, H. Nakajima and H. Ogawa,
`
`4
`
`translated by
`published by Ohmsha, Ltd., Japan (1992),
`Aschmann, C., IOS Press, Inc., Burke, Va., and The Com-
`pactDiskHana'book, Ken C. Pohlmann (2nd Ed. 1992), A-R
`Editions, Inc., Madison, Wis., both of which are incorpo-
`rated herein in their entirety by this reference. The present
`invention is then described where it differs in major respects
`from the previously known system of FIG. 1.
`It will of course be understood that the prior art system of
`FIG. 1 is merely illustrative of the various types of optical
`disk apparatus in which the methods and apparatus of the
`present invention may be employed. Thus, for example,
`applicants expect that the invention described herein may be
`advantageously employed in any optical disk system,
`including DVD systems.
`
`Overview of a Prior Art Optical Disk System
`
`Illustrative previously known optical disk reader 10 com-
`prises a spindle motor 11 that rotates optical disk 100 at high
`speed and pickup 12 including an illumination source and a
`photodetector for generating electrical signals representative
`of information-bearing pits formed in a reflective surface
`within optical disk 100. The electrical signals from the
`photodetector of pickup 12 are then passed to front end
`circuitry 13 for extracting a digital data signal. Under the
`control of controller 24, the data signal is further processed
`by eight-to-fourteen (EFM) demodulation circuitry 17,
`Cross Interleaved Reed-Solomon Code (CIRC) circuitry 18,
`error correction code (ECC) circuitry 19, and subcode
`circuitry 16. Controller 24 also controls focus and tracking
`circuitry 14, as well as buffer 20 and interface 22.
`For a digital audio system,
`the data signals may be
`processed into suitable analog signals (using circuitry not
`shown) connected to audio means 21. Similarly,
`if the
`optical disk contains video images, the data signals may be
`processed for direct display on a TV or monitor. In computer
`applications the data signals are typically transferred from
`buffer 20 to host processor 23 via interface 22.
`Spindle motor 11 spins optical disk 100 at a speed that
`depends upon the radial location of pickup assembly 12 (for
`example, for a 1x CD-ROM spindle speed, approximately
`200—500RPM), to maintain a constant linear velocity of an
`optical disk track relative to pickup assembly 12. For a
`CD-ROM format, this linear velocity is generally 1.4 m/s,
`while for the DVD format
`it approaches 4 m/s. Pickup
`assembly 12 typically includes a laser diode that illuminates
`only a single data track on optical disk 100 and an optical
`sensor onto which an image reflected from the optical disk
`is projected. The intensity, or other property, of the light
`beam reflected from the surface of optical disk 100 is
`modulated by inhomogeneities in the reflective surface of
`the optical disk (i.e., bumps or pits, referred to hereinafter as
`“data spots”) arranged in spiral or circular tracks on optical
`disk 100.
`
`Pickup assembly 12 includes circuitry to generate an
`electronic signal representative of the modulation in the
`illumination impinging upon its optical sensor due to the
`presence of the data spots. To ensure that the laser illumi-
`nation remains focused on the reflective surface of optical
`disk 100, pickup assembly 12 also provides signals to focus
`and tracking subsystem 14.
`The data spots are recorded on optical disk 100 using a
`modulation code that permits a data clock to be recovered
`from the data as it is read off of the optical disk. Clock
`circuitry 15 includes phase-locked-loop (PLL) circuitry for
`recovering the data clock from, and maintaining the data
`clock in synchrony with, the modulated electronic signal
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`from pickup assembly 12. In addition to being used for
`extracting the data from the modulated signal, the data clock
`is representative of the linear velocity of the data track
`relative to pickup assembly 12 and may be used as a
`feedback signal to control the speed of spindle motor 11 to
`maintain a constant linear velocity.
`Front end circuitry 13 uses the data clock from PLL 15 to
`recover a serial stream of bits from the electronic signal.
`Front end circuitry 13 contains additional circuitry to iden-
`tify synchronization codes in the bit stream so that the serial
`bit stream may be correctly assembled into multi-bit data
`words which are transferred to demodulation circuitry 17.
`Demodulation circuitry 17 may be programmed for eight-
`to-fourteen demodulation, eight-to-fifteen demodulation (as
`in the SD systems), eight-to sixteen demodulation (EFM
`Plus), or may use another suitable demodulation scheme.
`The demodulated data words, or symbols, are then
`assembled into blocks and decoded by CIRC decoder 18
`using a form of Cross Interleaved Read-Solomon code, for
`example, CIRC for CD-formats and CIRC Plus for DVD.
`Demodulated data words are also provided to subcode
`processor 16 which extracts data, such as block numbers, or
`song titles, that may be recorded in the subcode channels
`embedded in each block of data words.
`
`For video and audio optical disk players, the data from
`CIRC decoder 18 represents, in digital form, the video or
`audio signal that was originally recorded and stored on the
`disk. These signals may then be converted to analog signals
`and the original recorded signal reproduced using conven-
`tional audio or video devices 21. Errors in the recovered
`
`audio or video signals are handled by interpolation and
`filtering circuitry (not shown) to calculate a value to use in
`place of the erroneous data. Because of the interpolation
`process,
`isolated errors in an audio or video signal are
`unlikely to be noticed when listening to the audio or viewing
`the video signals.
`However, since a single bit error in data representing a
`computer program may render the program inoperable or the
`data unusable, optical disks used for the storage and distri-
`bution of data and programs must have very low data error
`rates. To reduce the data error rates to acceptably low levels,
`error correction codes (ECC) are added to the data when it
`is recorded to the disk. ECC circuitry 19 uses error correct-
`ing codes to detect and possibly correct errors in the data.
`Finally, the data is buffered in memory buffer for transfer to
`host processor 23 via interface circuitry 22. Controller 24
`coordinates operation of each of the optical disk reader
`subsystems and to control the operation of the optical disk
`reader as a whole.
`
`In the previously known optical disk reader of FIG. 1, the
`rate of data transfer between the optical disk itself and the
`host processor is limited by the rate at which the data can be
`processed by the circuitry shown in FIG. 1. For example, for
`a 1x CD-ROM reader, the data rate of the signal being read
`from the optical disk is about 4.32 MHZ, well within the
`processing capabilities of the electronic circuits involved.
`Even in optical disk readers having a spindle speed 8x the
`standard speed, the data transfer rate is limited by the speed
`at which the data can be read off the disk.
`
`Overview of the Present Invention
`
`Referring now to FIG. 2, optical disk reader is described
`that provides a high data transfer rate, in accordance with the
`principles of the present
`invention, by reading multiple
`tracks of data from an optical disk simultaneously. Much of
`the circuitry of FIG. 2 may be common to or readily adapted
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`from the circuitry of the system of FIG. 1. Accordingly, the
`following description describes in detail
`the differences
`between a previously known optical disk reader and appa-
`ratus 25 constructed in accordance with the principles of the
`present invention.
`In particular, apparatus 25 includes a multi-beam, multi-
`detector pickup assembly for illuminating and reading mul-
`tiple adjacent data tracks; phase-lock loop circuitry that
`permits a clock associated with a reference track to be used
`for synchronizing the recovery of data from neighboring
`tracks; and a parallel write/asynchronous read architecture
`that enables blocks of data to be read from the optical disk,
`processed and written to a buffer in parallel while being
`asynchronously retrieved from the buffer by a host com-
`puter.
`Apparatus 25 of FIG. 2 includes pickup assembly 27
`including a source of laser illumination, a diffraction grating
`for splitting the laser illumination into three or more illu-
`mination beams and a corresponding number of photode-
`tectors onto which multiple illumination beams, reflected
`from the optical disk, are focused by an optical system.
`Pickup assembly 27 is described in greater detail hereinbe-
`low. Each of the multiple photodetectors in pickup assembly
`27 generates an electrical signal representing data read from
`a corresponding data track on optical disk 100, and provides
`that electrical signal to front end circuitry 30.
`Front end circuitry 30 performs a function similar to that
`of front end circuitry 13 of FIG. 1, except that multiple bit
`streams are processed concurrently, so additional circuitry is
`provided for buffering and synchronizing data transfers to
`subsequent processing circuitry. Front end circuitry also
`includes a multiplexer for routing multiple data streams to
`demodulation circuitry 32.
`Memory 33 is provided to buffer the data read from the
`multiple data tracks, and to decouple the process of reading
`data from optical disk 100 from the process of transferring
`the data to host processor 37. Memory 33 therefore is large
`enough to hold about as many data blocks from multiple data
`tracks of optical disk 100 as can be read in one revolution of
`optical disk 100. Controller 38 maps data from the multiple
`data tracks to memory 33 so that individual data blocks will
`be correctly assembled without overwriting one another. As
`will be appreciated by those of skill in the art of buffer
`design, this mapping may be either dynamic or static.
`With respect to FIG. 3, pickup assembly 40 suitable for
`use in an optical disk reader constructed in accordance with
`the principles of the present invention is described. Pickup
`assembly 40 includes source of laser illumination 41, i.e., a
`laser diode, diffraction grating 42, beam splitter 43, objec-
`tive lens 44, and photodetector array 46. Diffraction grating
`42 splits the laser light emitted by laser diode 41 into three
`(or more) illumination beams, which are bent by beam
`splitter 43 and focused by objective lens 44 onto three (or
`more) adjacent tracks of information-bearing pits on optical
`disk 100. As will of course be understood, the illumination
`beams are spaced apart by the track pitch, for example, for
`the CD-ROM format, 1.6 pm apart. The illumination beams,
`once reflected from the information bearing surface within
`optical disk 100, pass through beam splitter 43 and are
`focused on corresponding photodetectors in photodetector
`array 46. Alternatively, multiple beams may be formed from
`the light emitted by the laser diode using the beam splitter
`apparatus described, for example, in Corsover et al. U.S. Pat.
`No. 4,459,690.
`As shown in FIG. 4, photodetector array 46 includes a
`central four quadrant detector comprising elements
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`46a—46d, and outboard detectors 46e and 46f for reading
`adjacent data tracks. Photodetector array 46 illustratively
`includes three detectors for reading three adjacent tracks of
`optical disk 100, although additional detectors may be
`disposed on either or both sides of the four quadrant detector
`(indicated by dotted line detector elements g and h).
`Detectors 46a to 46d are summed to provide an electrical
`signal representative of the information contained in the data
`track imaged onto the four quadrant detector by the optical
`system, while detectors 46e and 46f provide electrical sig-
`nals representative of the information contained in the
`adjacent data track on either side of the track imaged onto
`the four quadrant detector. As is conventional, the difference
`between the sums of the diagonal quadrants, i.e., efoCu5=
`(46a+46d)—(46b+46c), may be computed to generate a focus
`signal using the well known astigmatism method, while a
`tracking signal may be generated as the difference of the
`sums of the quadrants on the same side of the track, i.e.,
`etmCk=(46a +46c)—(46b+46a) The focus error signal efocm
`and tracking error signal etmck are input
`to focus and
`tracking circuitry 26.
`
`Multi-track Phase Lock Loop Circuitry
`
`Referring now to FIG. 5, pickup assembly 27 outputs
`track data signal, T1 .
`.
`. Tm, corresponding to m tracks being
`read (illustratively, three for the pickup assembly of FIGS.
`3 and 4). The track data signals, T1 .
`.
`. Tm, output by
`photodetector array 46 are then processed by front end
`circuitry 30,
`including clock generating circuitry 50 and
`track processing circuitry 51, to extract data from each track
`signal. Multiplexer 54 selects extracted data words from
`each of track processing circuitries 51 for decoding by
`eight-to-fourteen decoder 32.
`An accurate data clock is needed to reliably extract the
`data from the track data signals. By design, a track data
`signal is self-clocking, that is, the data stored in a data track
`is formatted so that a data clock can be recovered from the
`
`track signal. Typically, a Phase-Locked Loop (PLL) is used
`to recover the clock signal from the track data signal. In
`optical disk reader 25 of the present invention, clock gen-
`eration circuitry 50 recovers a reference clock signal from a
`selected one of the multiple data tracks being read. The
`reference track may be, for example, the middle, innermost
`or outermost track of the multiple tracks being read.
`The reference clock, CIDREF, generated by clock generation
`circuitry 50 has a frequency and phase which are correct for
`the reference track data signal. However, because the track
`data signals are read from tracks having different radii, and
`therefore slightly different linear velocities, the correspond-
`ing track data signals differ slightly in frequency and may
`differ substantially in phase. For example, for a CD-ROM
`optical disk, applicants have determined that the change in
`linear velocity, and therefore the difference in track data
`signal
`frequency, between any two adjacent
`tracks is
`approximately 0.01% anywhere on the optical disk.
`Furthermore, since the track data signal frequencies differ,
`the phase difference between any pair of tracks varies
`continuously. Consequently, a single data clock cannot be
`used directly to extract data from each track data signal.
`Track processing circuitries 51 therefore include data aligner
`circuitry 52, for synchronizing reference data clock CIDREF to
`the individual track data signals, and data sampler circuitry
`53, for sampling the track data signal.
`Track processing circuitry 51 also includes first-in/
`first-out buffer (FIFO) 49 for assembling the serial data into
`parallel data words and for synchronizing transfer of the
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`assembled data words from track processing circuitry 51 to
`EFM circuitry 38 via multiplexer 54. Advantageously,
`assembling the data words in front end circuitry 51 reduces
`the frequency at which subsequent circuitry operates. For
`example, in a standard speed CD-ROM drive, each track has
`a data rate of approximately four million bits per second
`(Mbps). Thus, multiplexer 54 would have to operate at a
`frequency of approximately 40 MHz (4 Mbps><10 tracks).
`However, by convertin