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Technical information on DVD players

Technical information

A DVD can contain:

The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity. As of 2004, the double sided formats have almost disappeared from the marketplace.

The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are usually gold-colored, while single-layered sides are silver-colored, like a CD.

Each medium can contain any of the above content and can be any layer type (double layer DVD-R is announced for 2004, while double layer DVD+R discs are already on the market, though scarce and expensive).

The DVD Forum has created the official DVD-R(W) standards. But as the licensing cost for this technology is very high, another group was founded: the DVD+RW Alliance who created the DVD+R(W) standard with lower licensing costs. At first, DVD+R(W) media were typically more expensive than DVD-R(W) media, but the prices have become very comparable.

The "+" (plus) and "-" (dash) are two similar technical standards that are partially compatible. As of 2004, both formats are equally popular, with about half of the industry supporting "+", and the other half "-". It is open to debate whether either format will push the other out of the market, or whether they will co-exist indefinitely. All DVD readers are supposed to read both formats (though real-world compatibility lies around 90% for both formats), and most current DVD writers can write both formats.

Unlike compact discs, where sound (CDDA, Red Book) is stored in a fundamentally different fashion than data (Yellow book et al.), a properly authored DVD will always contain data in the UDF filesystem.

The data transfer rate of a DVD drive is given in multiples of 1350 kB/s, which means that a drive with 16x speed designation allows a data transfer rate of 16 x 1350 = 21600 kB/s (21.09 MB/s). As CD drive speeds are given in mulitples of 150 kB/s, one DVD "speed" equals nine CD "speeds", i.e. 8x DVD drive should have data transfer rate similar to 72x CD drive (which do not exist). In physical rotation terms (spins per second), one DVD "speed" equals three CD "speeds", so the amount of data that are read during one rotation is three times larger for DVD than for CD and 8x DVD drive has the same rotational speed as 24x CD drive.

Note that both CD and DVD disks and drives usually have constant rotational speed while reading and data density on the track is also constant; as linear (meters per second) track speed grows at outer parts of the disk proportionally to the radius, the maximum data rate specified for the drive/disk is achieved only at the end of the disk's track (disks are written from inside). Average speed of the drive therefore equals to only about 50-70% of the maximum nominated speed.