Digital Video Discs (DVDs)

girl holding dvd disc
Writeable DVDs, unlike compact discs, were
immediately seen as a way to store
information for computers...not just as a way
to store video footage.
A DVD (Digital Versatile Disc or Digital Video Disc) works much the same way a Compact Disc (CD) does. A single track runs in a spiral around the disc, starting at the center and working its way out. The track contains a series of pits and lands (bumps and flat spots) the same way the track on a CD does, except those on a DVD are more information can fit onto the disc, even though it's physically the same size as a CD.

The track on a DVD is 320nm (nanometers – one millionth of a millimeter) wide and the pits are 120nm deep. The distance between each track line is 740nm (compared to 1600nm on a CD). The laser that reads a DVD uses a 650nm wavelength (the wavelength on a CD player is 780nm—so the laser used by the DVD player can see a smaller spot than the one on a CD player).

The different types of DVDs include:

DVD-Rs are recordable DVDs using a similar dye that was used in the CDRs. The laser writes data on the track that was imprinted when it was manufactured. It does this with a special light frequency that will change the colors of the dye, so that the pits and lands are read the same way bumps would be read by the DVD player. Once someone records information on a DVD-R, the data is final. They would be able to add data (provided the disc isn't full) but not erase or change the data that's already there.


DVD+Rs are also recordable, and uses mostly the same technologies used by the DVD-Rs. The differences would not be noticed by the consumer, but the DVD+R is more reliable in multi-sessions (when someone records something on a disc, and then records more information on it later). DVD+Rs (like DVD-Rs) can only be recorded on once. Someone could continue to add data, but the data that's already written cannot be changed or erased.

DVD-RWs are much like the CD-RWs. They can be written to, erased, and rewritten to over and over again. They use a metallic alloy (instead of the dye used by DVD-R) to make the pits and lands readable by the DVD player. Some DVD players will not read a DVD-RW. The only big difference between the plus and minus versions is that DVD+RWs do not have to erase the entire disc before rewriting over old data (as the DVD-RWs do).

DVD-RAMs are very much like DVD-RWs, with one big difference: the tracks on DVD-RAMs are concentric, meaning a DVD-RAM has many tracks, each one being a complete circle, instead of having one single track that spirals around the disc.

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