Two types of DVD discs are produced commercially: DVD-Video discs, which store movies, and DVD-ROM discs, which store games, databases, and other computer data. All DVD-Video discs are also DVD-ROM discs, but not all DVD-ROM discs are DVD-Video discs. In effect, DVD-ROM discs correspond to CD-ROM discs, and DVD-Video discs correspond to CD-DA audio discs. DVD-Video discs can be played in DVD-Video players, which are common in home theater setups, or in a computer's DVD-ROM drive. DVD-ROM discs can be played only in a DVD-ROM drive. Like CDs, DVDs are produced commercially by a mechanical pressing process. The DVD-ROM standard is specified by ECMA-267 (http://www.ecma.ch/ecma1/STAND/ECMA-267.HTM) and ECMA-268 (http://www.ecma.ch/ecma1/STAND/ECMA-268.HTM).
12.1.1 DVD-ROM Types and Capacities
DVD-ROM discs are (or will be) available in numerous standardized types, most of which are uncommon or not used at all. Discs may be either of two physical sizes, and may have one or two sides, each of which may store data in a single or double layer. Like CDs, standard single-sided (SS) DVD-ROM discs are 1.2 mm thick. Double-sided (DS) discs are simply two thin (0.6 mm) discs glued back to back. Most DVD players and drives require manually flipping the disc to access the data on the other side. Each side may contain a single layer (SL) or a double layer (DL) of data. In the latter case, the top layer is semi-transparent, allowing the laser to read the second layer underneath it, at the expense of sacrificing some capacity from each layer. Double-sided mixed-layer (DS/ML) discs use a single data layer on one side and a double layer on the other, a compromise made necessary by the limitations of current DVD production methods. DS/DL discs are still a laboratory curiosity, and likely to remain so for some time. Table 12-1 lists the available DVD types and capacities. Capacities are always given in billions of bytes (109 bytes) rather than true gigabytes (230 bytes), but always advertised as "GB" nonetheless.
12.1.2 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 like DVD-Video. A sure sign that a drive is CAV is if it lists "max" in its speed rating.
As with CD drives, average access time is often not emphasized on DVD drives. The most common use of DVD drives is playing movies, which are sequential data, making both average access time and speeds higher than 1X unimportant. However, if you use a DVD drive to access databases or for other purposes where random access predominates, both average access time and throughput speed become more important.
12.1.3 Choosing a DVD-ROM Drive
In the past, DVD-ROM drives differed sharply in price and performance according to the generation to which they belonged. But DVD-ROM drives have rapidly become commoditized, much as CD-ROM drives did. Nowadays, even $60 entry-level ATAPI drives provide 16X read performance for DVDs and 40X or greater for CDs. You're unlikely to go wrong with a current-model DVD-ROM drive from any of the major Japanese producers including Hitachi, NEC, Panasonic, Sony, and Toshiba.
Use the following guidelines when choosing a DVD-ROM drive: