Part II........................
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In September 1995, Fujitsu Laboratories Ltd. announced "Super MO," which uses both the lands and grooves to store data, thus doubling the capacity, while also adding Magnetically-induced Super Resolution (MSR), which allows marks smaller than the laser beam spot to be read, and reduces crosstalk between the recorded land and groove tracks. In combination, the land and groove recording and MSR increased the theoretical capacity of the media by a factor of ten. No Fujitsu products using land/groove recording and MSR have been released to date.
Without a doubt, Fujitsu is a significant force behind ASMO, and its Super MO/MSR techniques will probably be used in ASMO. These techniques, however, were not designed for use on phase-change media, such as DVD-RAM or MMVF, since in phase-change, there is still the possibility of "thermal crosstalk" from marks in adjacent lands and grooves (thermal crosstalk is a phenomenon of the laser's recording heat affecting the areas surrounding recorded optical marks). But thanks to recent developments in noise-cancellation algorithms based on models that optimize the depth of grooves, the track pitch proposed for DVD-RAM, compared to the laser spot size, should not present a problem.
According to Werner Glinka, director of marketing, storage products, for Hitachi America, Ltd., "There is no crosstalk on DVD-RAM because the lands and grooves are at different physical levels, and the laser beam focuses on one or the other." Glinka calls this "crosstalk cancellation." For future, higher capacity versions of DVD-RAM, such as the 4.7GB "next generation" already being planned by the DVD Forum, it may be necessary to narrow the track pitch to increase capacity. This future version of DVD-RAM, therefore, might rely even more on land/groove geometry, such as steeper walls or increased depths of grooves, and, in turn, could require improved manufacturing techniques for media. MMVF, which is also a phase-change medium, proposes a narrower track pitch and smaller recorded mark size considered "aggressive" for existing technology, according to some optical storage engineers.
Another technique that has yet to appear in a real product to date is the use of land and groove tracks in a single spiral, rather than the parallel spiral design now in wide use. DVD-RAM, ASMO, and MMVF all specify media with a "switching point," where the spiral changes from a groove into a land, and would require the rewritable drives to force the focus system to adjust between land and groove, then groove and land, on every revolution. The actual physical distance between land or groove and optical pickup is microscopic, but it still requires an additional servo to change the focus. Of course, any DVD-ROM drive designed to read DVD-RAM media would need to provide for this kind of focus system change as well.
Which means that the tracking system for such drives must also negotiate a change. Today's single-spiral, groove-only tracking relies on "push-pull" signals to stay on track, which, stated very simply, means that the tracking signals received by the tracking servo are at optimum levels when the tracking system is centered over the groove, or the bottom center of the groove. If the tracking system moves slightly off-track, the servo detects a change in the signal, and corrects accordingly. With DVD-RAM, ASMO, and MMVF, when the tracking system reaches the switching point between a land and a groove, the groove is inverted, which also inverts the tracking signal. The servo, in turn, must adjust to this new inverted tracking signal, until the revolution is completed and the track switches back to a groove.
Another MO legacy adopted by DVD-RAM and MMVF for use in a phase- change medium are embossed sector headers for addressing. Unlike existing CD-Rewritable and CD-Recordable media, the DVD-RAM and MMVF discs would contain areas with pre-embossed pits, as well as rewritable areas. Because the embossed areas appear between every 2K sector, they represent a large and frequent amplitude change while reading data, and one that must be compensated for with Automatic Gain Control (AGC).
The rewritable region of a DVD-RAM disc contains 17 sectors per revolution at the inner diameter of the disc and 40 sectors per revolution at the outer diameter, for a total of 24 rewritable zones, with a change of one sector between adjacent zones. Within the zones, sectors are aligned by the embossed sector headers, which minimizes crosstalk, but also results in a lack of constant linear density over the entire disc (which means that a sector at the inner diameter will be denser than a sector at the outer diameter).
The sector and zone approach defined for DVD-RAM reduces capacity because of the ranges in linear density of sectors according to their placement on disc, and the use of embossed headers also makes DVD-RAM media more susceptible to damage from handling. The embossed sector headers are not covered by error correction, and if one is rendered unreadable, the entire 2K sector is lost. The DVD-RAM media itself physically resembles MO media, with "hash marks" delineating sectors and zones. At the switching point, the sectors are lined up across the radius of the disc.
Staying the Wobble and Groove
Wobble and groove is one of the basic technologies behind CD-R and CD-RW. Not surprisingly, the wobble and groove approach is central to DVD+RW as well. A big difference between the CD and DVD implementations of RW is a change from CD's Constant Linear Velocity (CLV) to DVD+RW's Constant Angular Velocity (CAV).
The implementation of a wobble--a slight sinusoidal wave that meanders to the left and right of the center of a track--varies on CLV and CAV media, and affords different uses. In CLV-based CD-R media, the wobble provides Absolute Time In Pregroove (ATIP), which is used to guide the recording laser and time the speed of rotation, while for a CAV-based Minidisc, the wobble provides addressing information on non-prerecorded media. Because all CD formats use CLV, the amplitude and frequency of the wobble in CD-R and CD-RW media is constant throughout, relative to the linear speed of the spiral track. This ensures a constant linear density so that data stored near the hub of a disc has the same density as data stored near the rim.
One consequence of this data density uniformity is that, in order to achieve constant linear speed, the rotational spin of the disc must vary from the inner diameter to the outer diameter, and the drawback of wobbled CLV addressing is the variation in spin. It takes longer to slow down and speed up the disc than it does to actually locate the requested sector.
For CAV media, such as DVD+RW, the wobble frequency is still constant throughout the disc, but instead of being constant in relation to the linear speed of the track, it is constant relative to the speed at which the disc spins. This means that the rate of speed at which the wobbled groove travels past the read/write head varies, and thus, the waves of the wobble appear to be farther apart as the spiral moves towards the outer diameter of the disc. However, because the channel clock is adjusted according to the track number, the wobble appears at the same frequency to the read/write head, and the linear density remains constant throughout the disc. The CAV wobble improves performance by spinning the disc at a constant speed, for an estimated 1.5X performance advantage over the Zoned Constant Linear Velocity (Zoned CLV) used by DVD-RAM.
CD-R and CD-RW media use the wobble's ATIP sync mark, or pattern, for relative positioning, but DVD+RW media uses Address in Pregroove (ADIP) wobble and "fine clock marks" for improved clock timing and fine positioning. The fine clock marks, which are a single cycle at five times the normal wobble frequency, appear 96 times per revolution, and the ADIP wobble provides the track number, the segment number (eight segments per revolution), and a CRC (Cyclic Redundancy Check, a simple checksum derived from data, sync, and header). According to Chuck Weirauch, Hewlett-Packard's program manager of Advanced Technology and Standardization, "ADIP tells us track and segment number, but this does not give us fine enough positioning. The fine clock marks allow us to generate a write clock and calculate position more accurately. This addressing scheme allows us to map different layouts of data on the same media format, such as might be required for future higher densities, or writing with or without links."
DVD-RAM's lands and grooves are also wobbled, but because the discs use Zoned CLV, the frequency and amplitude of the wobble are constant within zones, relative to the rotational speed of the disc, but not constant over the entire disc. DVD-RAM uses the wobble in conjunction with the embossed sector headers for insurance, in case an embossed header is rendered unreadable by a scratch, fingerprint, dust, or normal media degradation. According to Hitachi's Glinka, "Using the embossed sector headers only, the error rate is 10 to negative eight (10-8), but using a recovery algorithm based on the wobble improves the address reading error rate to 10 to negative twenty-three (10-23)."
Scratch Areas for Errors, Anyone?
Rewritable optical media requires "scratch" areas between recorded data to provide an area in which the recorder can "sync" with the write clock. In existing CD-R and CD-RW media, this purpose is served by link blocks; for both DVD-RAM and DVD+RW, these areas are called "write splice areas" or link frames. In order to facilitate compatibility with existing DVD-ROM physical data organization, DVD+RW uses written splice areas between 32K ECC blocks. DVD-RAM uses embossed and written areas between 2K sectors.
DVD-ROM error correction is based on contiguous 32K ECC blocks, which allows the block to be read and corrected as a whole, while DVD-RAM requires the reading of 16 2K sectors, reassembling them in a buffer, and then applying error correction to the whole. This isn't particularly difficult, but it is very dissimilar to DVD-ROM error correction. According to Hewlett-Packard engineers, it would have been very easy to include link frames in the DVD-ROM specification, and only a small change would be required to make new DVD-ROM drives compatible with DVD+RW's write splices.
THE HIGH-DENSITY REWRITABLE OPTICAL TECHNOLOGIES, FORMATS, AND FUTURES
Obviously, the coming of the next generation in compact discs will have a wide-ranging effect on related technologies. There seems to be a general agreement that some of the factors that made the compact disc such a success--universality, compatibility, and versatility--are worthy of emulation, if only insofar as providing an additional playback platform for the read-only data distribution medium, in addition to large-capacity rewritable storage. It is no accident that these four emerging formats have one notable similarity--they all read pressed DVD. Each, however, brings its own unique set of advantages and disadvantages when it comes to providing rewritable storage.
DVD-RAM has the distinction of being the only "official" DVD Forum-approved format, with all the psychological and economic importance that bears. With influential companies, such as Matsushita, Toshiba, and Hitachi, bringing products to market earliest, plus the imprimatur of being deemed the "standard," DVD-RAM's gate position is enviable. However, in a comparison of technical merits, DVD-RAM offers smaller capacity, presents a collection of new, relatively untried techniques, and insists on the requirement of a cartridge. Even though these traits could handicap its performance in the marketplace, some of DVD-RAM's specific characteristics may also reflect a desire to break away from older CD technologies--and patents--and the desire to establish a new technological dynasty based on heretofore unrealized patent revenues.
DVD+RW, despite its reputation in some general trade press reports as a "rogue" format, nonetheless bears the DVD name and the support of two powerful DVD Forum companies, as well as four others with excellent records in the CD-R and CD-RW market. Not coincidentally, DVD+RW claims to facilitate backwards compatibility, compliance with much of the DVD-ROM format, and closer compliance with the recommendations of the DVD-RAM TWG (Technical Working Group). DVD+RW may be a favorite for those who would like to get the most out of their investment in existing technology. Those investors, of course, are not limited to the owners of CD-R and CD-RW products; they also include the makers and sellers of those products and the owners of the licensed technology that makes them possible. And when it comes to user acceptance, DVD+RW arguably has a subtle, simple, but still compelling advantage: it is the only one of the proposed high-density rewritable formats that has the look and feel of the familiar CD and DVD, and does not require a cartridge.
ASMO is an interesting anomaly in this group of contenders, as the only magneto-optical-based format in the group. ASMO is a loner in the traditional MO family as well; a "Magneto-Optical Technology Roadmap" published by OSTA shows clear lines of descent in past and future 90mm and 130mm form-factor MO technologies, but ASMO stands alone with no lines to it or from it. However, as the first MO format to target time-shift consumer recording, as well as computer storage, and with the 15-member ASTC behind it, ASMO's potential should not be overlooked. Technically, ASMO could be very appealing to those who require the highest-capacity, fastest, and most permanent medium for development and storage.
MMVF is no less of an anomaly, as the only proprietary format in the group. Little is known as yet about MMVF's particular technological approach to high-density rewritable storage other than the bare specifications, but the MMVF objective is no mystery: MMVF was designed to record digital video. Interestingly enough, MMVF's non-adherence to DVD formats--despite the common misconception that DVD denotes a format optimized for video--is what may actually give this technology an edge. MMVF is also a product of the DVD Forum's internal wrangling over the specifications for DVD-RAM--NEC originally proposed MMVF to the Forum, but was rebuffed, presumably in part because the company is not a Forum member. The DVD Forum, subsequent to the announcement of MMVF, invited NEC to join in hopes of avoiding a conflict.
With the stakes for rewritable high-density optical storage great not only in the world of computers, but carrying the potential to exceed the huge success of VCRs, camcorders, and other such devices in the consumer electronics domain, it is no wonder then that there's a vital sense of competition for these formats.
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Companies Mentioned in This Article
Advanced Storage Technology Conference (ASTC) Secretariat
+81 3 5448 3608; Fax +81 3 5448 9430;
DVD Forum
The Secretary of the DVD Forum, 15th Floor, Toshiba Building, 1-1, Shibaura 1-Chome, Minato-ku, Tokyo 105-01, JAPAN; +81 3 5444 9580; Fax +81 3 5448 3458
Fujitsu Computer Products of America, Inc.
2904 Orchard Parkway, San Jose, CA 95134-2009; 800/626-4686; 408/894-3565; Fax 408/894-1709; fcpa.com
Hewlett-Packard Company
Information Storage Americas, 800 South Taft Avenue, Loveland, CO 80537; 800/826-4111; 970/635-1500; Fax 970/679-5933; hp.com
Hitachi America Ltd.
Computer Division, 2000 Sierra Point Parkway, MS: 500, Brisbane, CA 94005-1835; 650/589-8300; Fax 650/244-7647; hitachi.com
Japanese Technology Evaluation Center (JTEC)
Loyola College, 4501 North Charles Street, Baltimore, MD 21210; mcc.com projects/ilo/JTEC
Matsushita Electrical Corporation of America
One Panasonic Way, Secaucus, NJ 07094; 201/348-7000, 201/348-7064; Fax 201/348-7016
Mitsubishi Chemical America, Inc.
445 Indio Way, Sunnyvale, CA 94086; 800/347-5724; Fax 408/481-9488; mitsubishi-informationstorage.com
NEC Corporation
7-1, Shiba 5-chome, Minato-ku, Tokyo 108-01, JAPAN; +81 3 3454 1111; +81 3 3798 6511; Fax +81 3 3798 1510; dec-j.co.jp
Optical Storage Technology Association (OSTA)
311 East Carrillo Street, Santa Barbara, CA 93101; 805/963-3853; Fax 805/962-1541; osta.org
Philips Professional Solutions
Philips Electronics North America Corporation, 3200 North First Street, San Jose, CA 95134; 408/453-7010; Fax 408/487-8668; philips.com
Ricoh Corporation
3001 Orchard Parkway, San Jose, CA 95134-2088; 408/432-8800; 408/954-5349; Fax 408/944-3312; ricoh.com
Sony Electronics, Inc.
3300 Zanker Road, MD: SJ2A2, San Jose, CA 95134-1901; 408/432-1600; Fax 408/955-5111; sony.com
Toshiba America Information Systems
Disk Products Division, 9740 Irvine Boulevard, P.O. Box 19724, Irvine, CA 92713-9724; 714/457-0777; 714/583-3146; Fax 714/583-3133; toshiba.com
Yamaha Systems Technology, Inc.
Yamaha Corporation of America, 100 Century Center Court, San Jose, CA 95112; 800/543-7457; 408/467-2300; Fax 408/437-9741; yamahasyst.com
Dana J. Parker, contributing editor for EMedia Professional, also writes the Standard Deviations column. She is a Denver-based independent consultant, writer, speaker, industry analyst, and co-author of four books on compact disc technologies. She is currently working on a book about DVD technology, to be published by Prentice Hall Professional Technical Reference. |
