CD Algebra: 12X = 10X = 8X ?

For most people, CD performance is measured in X's. What's an X, you ask, and why? In CD-ROMese, 1X refers to the original CD-ROM drives, which could transfer data at 150 KBps. Then we got double-speed drives, abbreviated as 2X, which spun twice as fast as the 1X devices. As drive rotational and data transfer speeds increased, the X nomenclature stuck, so we now have 12X drives, and 16X units will be out soon. Theoretically, this X rating means that a 12X drive spins 12 times as fast as 1X units and transfers data at 1.8 MBps.

The 12X drives are capable of operating at up to 12X rotational rates, but strictly speaking, there's no such thing as a "pure" 12X drive. In the re al world, 12X CD-ROM drives attain these speeds only under optimum conditions. Even a CLV drive w ill fall back from 12X to 8X -- and subsequently to 4X and 1X -- if disc imperfections get in the way of read performance.

One advantage of CAV is that fallback to slower speeds tends to occur in much smaller increments. This keeps the data transfer rate close to its 1.8-MBps maximum.

Acknowledging this situation, some vendors are now beginning to move beyond simple-minded terminology. They are now describing spin rates in terms of ranges or averages.