PC Hardware in a Nutshell, 3rd Edition

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 of 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 semitransparent, 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 are 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 are always advertised as “GB” nonetheless.

Table 12-1. DVD-ROM types and capacities

Type	Diameter	Layers	Actual GB	Stated GB	Video
DVD-1	80 mm	SS/SL	1.36	1.45	0.5 hr
DVD-2	80 mm	SS/DL	2.47	2.65	~ 1.3
DVD-3	80 mm	DS/SL	2.72	2.9	~ 1.4
DVD-4	80 mm	DS/DL