A Digital Versatile Disc - Read Only Memory, or DVD-ROM, is a media storage disk that closely resembles a CD or compact disc. The major difference is that the DVD is formatted to hold far more data. A CD commonly has a capacity of 650 megabytes, while the smallest capacity DVD can store about seven times more data, or 4.38 gigabytes (GB).
There are various kinds of DVDs, but the DVD-ROM refers to a read-only disc, or a disc that cannot be written over. A DVD movie bought from the local video store is a good example. Blank DVDs with designations like "DVD-R" and "DVD+R" are formatted, recordable DVDs. The —R and +R refer to competing format standards, but both will record movies, audio, or other data.
The disc encodes data in the form of a spiraling trail of pits and lands separated by nanometers. The trail starts at the center of the DVD, and winds around times until it reaches the outer edge. In the case of a double layer disk, the trail continues on a second layer of material. If the disc is also double-sided, the trail of pits and lands extends to side two.
A laser beam in the DVD player tracks the beam as the disc spins, while a special device reads the intensity of the reflection as it bounces off the pits and lands. The reflective variance gets translated to bits of data which form bytes. As a result, DVDs can vary in capacity as follows:
- Single-sided single-layer disc — 4.38 GB
- Single-sided double-layer disc — 7.95 GB
- Double-sided single-layer disc — 8.75 GB
- Double-sided double-layer disc — 15.9 GB
The DVD-ROM has largely replaced the video cassette, being far more efficient and superior in most respects. For one, it stores data in digital form, while the video cassette uses less precise analog technology. Under normal conditions, a DVD remains error free and consistent, regardless of the amount of times it is viewed, while a video tape stretches with wear and eventually needs replacement. It can also hold more information in a higher format, and a viewer can skip to specific scenes without the need for fast-forwarding or rewinding. DVDs are much more compact and easier to store as well, and players can usually also play CDs.
When purchasing a DVD player, consumers should be sure to get one that can play all DVD-ROM formats, including double-sided, double-layered discs. For home theater systems, shoppers should look for models equipped with a 192 kilohertz (kHz), 24-bit digital/analog converter (DAC) for true theater quality. By comparison, standard DVD players use 96 kHz, 24-bit DACs. This is still a big improvement over CDs, however, which use 44.1 kHz, 16-bit sampling to produce audio. For this reason, people are moving towards DVDs to store music. An audio DVD can hold just over an hour of multi-channel music at 192 kHz, the highest bit rate; about two hours at 96 kHz; and close to seven hours at the standard CD sampling rate of 44.1 kHz.
Cassettes, video cassettes, and laser discs have become legacy technologies, and DVDs may be headed in the same direction. An increasing number of movies and TV programs are being released on Blu-ray™ discs, which have a higher capacity and quality. For home audio and data storage, flash drives have become quite popular. Recordable DVDs are still typically available wherever music and movies are sold, however, including office supply chains and discount marts.