Making the case for optical storage

An all-encompassing look at today's optical formats.


Storing files on optical discs rather than on tape may give IT departments greater control in some applications such as day-to-day retrieval, physical distribution, archival data protection, and in some instances, backup and restore.

Magnetic tape systems boomed in the 1990s, and manufacturers continue to boost areal densities, speed up transfer rates, synthesize tougher substrates, optimize the transport machinery, and shrink costs. But tape has two potential shortcomings: a confusing array of incompatible formats and a relatively short recording life.

Moreover, tape can be vulnerable to environmental-especially magnetic-threats. Within even the most expensive and rugged tape media, magnetic coercivity inevitably weakens over time, and if recorded cartridges are not accessed regularly, the polarity of spots on tightly wound tape layers can bleed through and corrupt the polarity of spots on adjacent layers. That's why the well-established "best practice" among storage managers is to re-record files onto fresh tape every year or so. Calculated per gigabyte, tape storage is less expensive than any disk-based alternative, magnetic or optical. But having to buy new cartridges every year raises the real-world cost of tape.

In fairness, optical disc formats are also proliferating right now. But every format of optical drives and media, except one, is manufactured and supported by multiple companies. (The exception is 12-inch WORM, from Plasmon LMS).

One potential advantage of storing data on optical discs, even on rewritable media, is that for all practical purposes, the data is permanently recorded: It has a readable life measured in decades and can be erased or over-written only by a drive and not by accident.

MO: Still going strong

By the mid-1990s, 5.25-inch magneto-optic (MO) disk systems took over market leadership from 12-inch write once/read many (WORM) and held that position until recently. In MO drives (as in hard or floppy drives), an electromagnetic head polarizes magnetic spots in the recording layer. But whereas the polarity of a floppy or hard drive spot can be changed at room temperature, a spot on an MO disk can't be changed unless it is much hotter. So heat from a laser diode on one side of the disk heats the spot to that critical temperature (called the "Curie point"), whereupon a magnetic head on the other side of the disk polarizes it. When the disk rotates that spot past the laser, the spot cools and retains its polarity until or unless it is re-heated and remagnetized. Rubbing against a refrigerator magnet or even passing through an MRI scanner has no effect on an MO disk.

Sony and Fujitsu were just two of six companies making 650MB 5.25-inch MO systems then, and they also made a version with 3.5-inch disks with a capacity of 128MB. Today, only Fujitsu makes 3.5-inch MO drives, but several companies, including Verbatim, still make the media, and disk capacity has reached 2.6GB. However, the format has never been popular outside of Japan, largely because of competition from the recordable compact disc (CD-R).

Write-once, 650MB CD-R systems have proliferated rapidly, due to disc compatibility with read-only CD-ROM drives and CD-audio players. Within two years of CD-R's appearance in 1990, the price of a drive was less than half that of 3.5-inch MO and one-third that of 5.25-inch MO.

Click here to enlarge image

The manufacturers of 5.25-inch MO fought back by dramatically boosting capacities without significantly raising prices. All of them had previously agreed to standards and a common "road map" for development. So it was relatively easy for MO manufacturers to nearly double disk capacities every 18 months or so, to 1.3GB, 2.6GB, 5.2GB, and (now) to 9.4GB. And every new MO drive could-and still does-read the previous three generations of MO media and write to at least the previous two generations.

Users and storage managers regard this commitment to investment protection as MO's greatest advantage, and they have proven remarkably loyal to the format. Although there has been a shake-out among drive manufacturers (only Sony and MaxOptix remain), those drives are branded and supported by Hewlett-Packard, IBM, and Plasmon, and MO media is available from Imation, Maxell, TDK, and Verbatim.

Where MO really shines is in jukeboxes, because the robotic pickers cannot only insert, remove, and refile disks, but they can also just as easily flip them over. In addition, MO jukeboxes from HP, DISC, MaxOptix, and Plasmon come in almost any size/capacity point, from 10 to more than 1,000 disks.

MO vendors acknowledge, however, that 9.4GB (4.7GB/side) is probably the upper limit for 5.25-inch MO disks. There isn't any more "undeveloped real estate" closer in to the hub or farther out toward the edge. Also, the recording spots cannot be shrunk smaller or squeezed closer together without incurring an unacceptably high error rate. Capacity would go up (it would just about quadruple!) if the wavelength of the laser diode could be halved from that of red light to that of blue light. But manufacturers have not been able to make blue laser diodes either small enough, cheap enough, or long-lived enough to satisfactorily replace red lasers.

CD: A burning desire

When the first recordable CD systems arrived, the drives cost more than $5,000 and the media well over $20 apiece, but the discs could be read in any CD-ROM drive or CD-audio player.

The technology was swiftly standardized and licensed to a dozen manufacturers. By the mid-1990s, drive prices had tumbled to less than $500, and the media to less than $2. At that point, Philips and Sony introduced a rewritable version (CD-RW) and persuaded all the manufacturers of read-only drives to tuck in the extra "MultiRead" chipset. Today, a basic CD-RW drive (a "CD burner") retails for less than $200, rewritable media for only about $1, and write-once media for less than $0.50.

As with MO, standards help to protect investments in equipment. But a small glitch has arisen recently in the otherwise standardized specs for rewritable media. CD-RW discs certified for high-speed recording (i.e., by newer drives that write at 12x speed and above) may not work in older drives with a top write speed of 8x or less. So check the drive specs before upgrading to newer, faster media-or stick with write-once CD-R, which has always been recordable at faster speeds than CD-RW.

An intriguing CD hybrid from Eastman Kodak, called CD-PROM, combines stamped tracks and recordable (CD-R) tracks. With CD-PROM, users can customize distribution discs for specific recipients, and users can modify some files but not others. However, if there is a killer app, it's not yet apparent.

The installed base of CD approaches ubiquity: There is practically no computer now that does not come with a CD-ROM reader or that cannot be equipped with a CD burner-either internal or external. So any application involving the physical transfer of files (up to about 650MB per disc) can be met with CD media.

DVD: The next phase

A few CD-Rs have been tweaked to hold 700MB or a little more, but they're not universally readable. And today's applications are increasingly laden with graphics, digital still, and video images that consume hundreds of megabytes.

About six years ago, there was dissention in the Optical Storage Technology Association-the industry's de facto standards group-between Sony and Philips on one hand, and Toshiba, Hitachi, and Matsushita on the other. At issue was a new, higher-capacity optical disc system with the same (4.72-inch) form factor as CD media. When the smoke cleared, the Toshiba alliance "won," which is why we call the new format by their name for it: DVD.

DVD recording employs phase-change technology, which has been around for years (one of the now-defunct 12-inch WORM systems used it) but it didn't take center stage until it was applied to CD-RW. Heat from a laser diode renders a recording spot either highly reflective ("crystalline") or not ("amorphous"), which is read by a second laser as logical 1s or 0s. (CD-R, by contrast, is write-once because it uses "ablative" technology to make a permanent, physical mark in the recording layer.)

MO manufacturers have criticized phase-change recording for years, claiming that the media can sustain only about one-tenth as many rewrite cycles as MO. But the numbers are "hundreds of thousands" versus "millions," and that imposes no practical limitation on ordinary storage applications. Files are more often read than written, and reading has no effect on phase-change spots at all. Besides, files-even in jukeboxes-are rarely erased or rewritten more than a few hundred times before the media is relegated to the archives or discarded.

It's important to remember, however, that DVD capacity was set not by the data-storage industry but by Holly wood: A disc had to hold two hours' worth of full-motion video and deliver an image size and quality comparable to that of broadcast television. With a fairly tight compression scheme (MPEG-2), the minimum capacity was targeted at 4.7GB per recording layer, which is now the standard.

Like CDs, DVDs can be factory-replicated, and those molded-and-stamped DVDs are guaranteed to be readable in any drive that accepts DVD media, including video players. Recordable DVD formats emerged about five years ago, but they have not replaced CDs as universal distribution media, partly because DVD is so new and CD so well-entrenched. They have not replaced CDs mainly because there are competing DVD recording formats, only one of which-the write-once version called DVD-R-is universally readable.

Pioneer pioneered DVD-R with a $17,000 drive and $50 media; today, DVD-R costs about $800 and $10, respectively. At about the same time (1997), Matsushita introduced a rewritable format called DVD-RAM that cost a little less than $1,000 with $25 media that's now retailing for $300/$5.

Compared to MO-at the same 4.7GB/side capacity point-DVD-RAM media and drives are smaller and lighter and cost less than one-third the price. DVD-RAM media is available from Maxell, Optodisc, and TDK, among others. Jukebox manufacturers such as Asaca, DISC, JVC, Kubota, NSM, and Plasmon have also turned to DVD-RAM because they can continue to use the same frames and robotics as for CDs, changing only the drives and firmware.

While DVD discs have far smaller capacities than tape cartridges, they are far less expensive per unit, and their per-gigabyte cost is almost on par with that of tape. Unless file sizes routinely exceed disk capacity, DVD systems are competitive against tape systems. Besides, disc jukeboxes are less expensive than all but the very lowest-end tape libraries, and with discs' random access versus tapes' linear access, most file-seeking times are shorter.

Both Pioneer and Matsushita/ Toshiba/ Hitachi have made their latest DVD recorders multi-functional: Pioneer added rewritability in a format called DVD-RW, and the Matsushita group implemented write-once DVD-R capability. As you might expect, the rewritable media are mutually incompatible; only DVD-R discs can be read or written to universally.

But one more proprietary rewritable format has now emerged. Sony, Philips, and four other manufacturers have been promising and demonstrating DVD+RW ever since Matsushita and its allies launched DVD-RAM. However, only now are DVD+RW drives and media being shipped.

HP's offering is the first commercial brand of DVD+RW, and Dell is the first PC maker to offer it. Apple is now OEMing the Pioneer DVD-R drive. DVD-RAM doesn't have comparable OEM support from computer companies, but it enjoys the largest installed base and has ongoing commitments from jukebox makers. DVD-RAM is the most widely available DVD format: As an external peripheral, it is branded by Matsushita (Pana sonic), Hitachi, LaCie, Toshiba, and others. Verbatim makes all four recordable DVD media: DVD-R, DVD-RAM, DVD-RW, and DVD+RW.


Write-once DVD-R remains the most widely readable DVD format of all and is thus best-positioned for applications involving physical file distribution. But it's fair to ask, "If there's a war over rewritable DVD formats, which of the three is most likely to 'win?' "

The DVD+RW camp claims that today's DVD-ROM drives and DVD-video players can read DVD+RW discs just as easily as DVD-R. If true, that may give the format a leg up-but mainly among home and small-business users. DVD-RAM has had several years' head start in the marketplace, and being that much further down the manufacturing learning curve, it is-and will remain-the least expensive. DVD-RW is new, but Pioneer's savvy combination of DVD-R, DVD-RW, CD-R, and CD-RW really puts the "multi" into multi-functionality, and the all-in-one approach may prove attractive.

Before committing to optical discs, it's also fair to ask if there are any newer, superior storage technologies on the horizon. In the long run, they will come. But IT managers should not expect to see systems with dramatically greater capacities any time soon. DVDs can be stamped with two data layers per side but cannot yet accommodate two recordable layers per side. That enhancement probably won't come to market for another year or two.

DataPlay, a new write-once format due for release late this year, is specified to store up to 500MB on discs the size of a quarter. The first DataPlay drives and media will come from Samsung and Imation, but the format's physical size and capacity point suggest that target applications will focus more on "play" (meaning audio entertainment) than on "data."

And don't expect exotic storage concepts to bear fruit soon. The blue laser diode remains elusive. Quinta and TeraStor made big noises a few years ago about optical-magnetic hybrid technologies known as "near field" and "far field" recording, but neither has yet moved onto the factory floor. And there will have to be fundamental breakthroughs in materials science before any holographic system can store the much-vaunted terabyte-in-a-sugar-cube.

Hal Glatzer is a freelance writer in San Francisco, CA. He can be reached at halglatzer@sprintmail.com.

Vendors mentioned in this article

www.panasonic.com (Matsushita)

This article was originally published on November 01, 2001