PC Hardware in a Nutshell, 3rd Edition

DVD-ROM Speed

Like CD drives, DVD drives use the “X-factor” to specify throughput. Confusingly, DVD “X” doesn’t mean the same thing as CD “X”. A 1X CD drive transfers data at 150 KB/s (0.146 MB/s), but a 1X DVD drive transfers data at 11.08 million bits/sec (1.321 MB/s), or about nine times the 1X CD rate. In theory, then, the X-rating for a DVD drive when playing a CD disc would be about nine times its DVD rating. In practice, that’s not the case. Early-model DVD-ROM drives typically provide 20X to 32X max CD performance. Current DVD-ROM drives generally provide 40X to 48X max CD performance. DVD throughput faster than 1X is unimportant for DVD-Video, which always plays at 1X, but does provide faster and smoother searching. High throughput is more important if you use the drive to read DVD-ROM discs for playing games or for accessing DVD-ROM data.

Early DVD-ROM drives used CLV, spinning the disc more slowly on outer tracks and faster near the center, to maintain the constant data rate needed for DVD-Video. Current DVD-ROM drives use CAV, spinning the disc at a constant speed and using a buffer to maintain a constant data rate for sequential applications such as DVD-Video. A sure sign that a drive is CAV is if it lists “max” in its speed rating.

Tip

The actual throughput of a DVD-ROM drive depends on the type of disc it is reading. For example, a 16X drive may provide 16X max on single-layer discs, but only 10X max on double-layer discs.

As with CD drives, average access time is often ...