Optical Drive Performance

Optical drive speeds are specified using an "X-factor." The earliest CD-ROM drives transferred data at a constant 150 KB/s, the same rate used by audio CDs, which is referred to as 1X. Later CD drives used variable speeds, changing the speed according to where the head was positioned on the CD. It's impossible to assign a single speed rating to such a drive, so manufacturers began specifying the maximum speed those drives used. For example, a CD-ROM drive that transfers data at a maximum rate 52 times the 150 KB/s audio CD rate, or 7,800 KB/s, is called a 52X Max drive.

DVD drives use the same kind of speed rating scheme, but the DVD "X-factor" is different. The 1X DVD rate is 1.321 MB/s, which is the data rate required to store 60 minutes of video on a 4.7 GB DVD disc, or about nine times faster than a 1X CD-ROM drive. For example, a 16X Max DVD drive transfers data as fast as about 21 MB/s, nearly three times the rate of a 52X CD drive.

To complicate matters further, optical drives do different tasks at different rates. For example, a typical early CD writer could write CD-R (write-once) discs at 4X, or 600 KB/s, but read discs at 24X, or 3,600 KB/s. When CD-RW (rewritable) discs were introduced, yet a third number was needed, because most CD writers wrote CD-R discs and CD-RW discs at different speeds. A typical modern CD writer might read CD discs at 52X, write CD-R discs at 52X, and rewrite CD-RW discs at 32X. Such a drive is referred to as a 52-52-32 drive.

SPEED LIMITS

Read speeds may also differ according to the type of disc being read. For example, a particular drive may read data CDs at 52X Max but audio CDs at only 24X, pressed data DVDs at 16X Max, but DVD-Video discs at only 8X, and DVD+R discs at 16X, but DVD+RW discs at only 8X. Similarly, a particular drive may write data CDs at 52X Max, but audio CDs at only 24X.

What About Blu-Ray and HD-DVD?

The next generation of DVD drives and discs will store much more data on a standard-size disc than do current DVD formats, by using a 405 nm blue laser rather than the 650 nm red laser used by current DVD drives and players. The shorter wavelength of the blue laser allows closer track spacing and higher data density within the track.

There is currently a format war going on between two proposed high-capacity DVD standards, which are mutually incompatible. Only one of these standards can prevail, and there are billions of dollars in license fees at stake. The Blu-Ray standard, promoted by the Blu-Ray Disc Association (http://www.bluraydisc.com), is backed by a group of technology manufacturers and movie studios, led by Philips and Sony. The HD-DVD standard is promoted by the DVD Forum (http://www.dvdforum.org) which owns the patents and trademarks for the original DVD standards and is backed by another group of technology manufacturers and movie studios, led by NEC and Toshiba. Although Microsoft has not committed exclusively to either standard, it has shown preference for HD-DVD, both by promising HD-DVD support in future versions of Windows and by announcing that future versions of its Xbox console will use HD-DVD.

The major advantage of Blu-Ray is its greater capacity. BD-ROM stores 23.3, 25, or 27 GB on a single-layer disc (as compared to 4.7 GB on a single-layer DVD-ROM), and 46.6, 50, or 54 GB on a dual-layer disc (as compared to 9.4 GB on a dual-layer DVD-ROM). HD DVD-ROM stores only 15 GB on a single-layer disc and 30 GB on a dual-layer disc. BD-ROM discs that use three or more layers are currently in development, and will expand storage capacity to 100 to 200 GB. Similarly, Toshiba has a three-layer HD DVD-ROM disc in development that will store 45 GB.

Both standards make provision for writable discs. BD-RE is the rewritable version of BD-ROM, analogous to DVD+RW. BD-RE recorders will initially ship in single-layer (25 GB) models, with dual-layer 50 GB models to follow. The HD-DVD equivalent is called HD DVD-Rewritable. Oddly, these discs store 20 GB per side rather than the 15 GB of the read-only version. BD-R is the write-once version of BD-ROM, analogous to DVD+R. BD-R recorders and drives will be available in 25 GB single-layer models initially with 50 GB dual-layer models to follow. The HD-DVD write-once standard is called HD DVD-R, and stores 15 GB in a single layer.

Looking at these specifications, you might wonder how HD-DVD has any chance at all against Blu-Ray. Fortunately for its supporters, HD-DVD has two things going for it. First, HD-DVD discs can be pressed on the existing equipment used to produce DVD-ROM discs, while Blu-Ray requires all new equipment. More important, HD-DVD gained the early support of several movie studios.

Movie studios want one standard, so they won't have to release their films on two different and incompatible types of discs. DVD rental companies like Blockbuster and Netflix want one standard, so they don't have to stock two different copies of the same title. Consumers want one standard, so they don't have to buy two expensive DVD players, one for each format. In fact, everyone wants one standard, but neither the Blu-Ray Disc Association nor the DVD Forum is willing to budge. There's simply too much money at stake.

From a PC perspective, none of this really matters, at least for now. As with any new technology, the prices will initially be astronomical. We expect the first high-capacity PC DVD burners to sell for $1,000, and the discs for $20 each. Then again, the first CD writer we used cost $20,000 and blank discs sold back then for $50 each. Prices will drop rapidly, although we don't expect high-capacity DVD writers for PCs to become mainstream components until at least 2007. And, we wonder how useful they'll be even then. We're not happy at the prospect of using a high-capacity DVD writer that's laden with DRM and produces serialized discs, but that may be the only alternative.

For now, and for the foreseeable future, we think the best choice is a dual-layer DVD+R/RW drive. Standard DVD+R and DVD+RW discs will remain available for many years, long after the high-capacity DVD format war has been decided.

So matters remained, until the first hybrid (or "combo") CD writer and DVD-ROM drive was introduced. At that point, a fourth number was needed to report the DVD-ROM read speed. A typical combo drive might write CD-R discs at 52X, rewrite CD-RW discs at 24X, read CDs at 40X, and read DVDs at 16X. Such a drive is referred to as a 52-24-40-16 drive.

Along came DVD writers, most of which read and write DVD-R, DVD-RW, DVD+R, and DVD+RW, often at different speeds. Most recent DVD writers can also write dual-layer DVD+R/DL and DVD-R/DL discs, also at different speeds. So, the apparently simple question, "Which drive is faster?" often has no simple answer.

Actual read and write speeds also vary from drive to drive, even if the drives have identical speed ratings. In addition to native differences between drives, performance depends on the brand of discs, the firmware version installed, and so on. For example, Drive A, rated to write DVD+R discs at 16X, may write those discs faster than Drive B, which is also rated for 16X DVD+R writes. Conversely, although both drives may be rated for 8X DVD+RW writes, Drive B may write DVD+RW discs faster than Drive A. But if you use a different brand of media or update the firmware in one or both of the drives, the positions may be reversed.

Finally, random access time may matter to you. In general, optical writers have more complex and heavier heads than read-only optical drives. Consequently, optical writers have noticeably slower access times than most read-only drives. For example, the random access time of a fast DVD-ROM drive might be 85 milliseconds, while that of a DVD writer might be twice that. Fast access times don't matter for sequential operations such as burning a disc or watching a DVD video. They do matter when you randomly access data from an optical disc, such as occurs when you play a DVD based game. That's why serious gamers usually have two optical drives in their gaming systems an optical writer for general use and a fast DVD-ROM drive for loading games.

Disk or Disc?

With Seagate as the sole exception, hard drive makers refer to their products as "disk" drives. The "disc" spelling is used universally to refer to optical discs.

More about Optical Drives