A DVD, also known as Digital Video Disc, is an optical disc used to store media format. It was invented when Philips, Sony, Toshiba, and Time Warner came together in 1995. DVDs are the same size as CDs, but are capable of storing almost seven times more data.
In 1993 two optical disc storage formats were being developed. One was the MultiMedia Compact Disc, backed by Philips and Sony, and the other was the Super Density Disc, supported by Toshiba, Time Warner, Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer, Thomson, and JVC. After years of researching, developing, and combining ideas, the DVD specification was finalized for the DVD movie player and DVD-ROM computer applications in December 1995.
The basic types of DVD are 12 cm in diameter, single-sided or homogeneous double-sided, and are referred to by an approximation of their capacity in gigabytes. Each DVD sector contains 2,418 bytes of data. Of these bytes, 2,048 bytes of them are formatted to store user data. Writing speeds for DVD were 1350 kB/s in the first models. More recent models have 20 times that speed.
The structure of a DVD is fairly complex. In reference to a standard DVD, the disc spin motor is a gray cylinder with a black resilient drive ring on top. A magnetic clamp secures the disc after the disc tray stops moving inward. The gray metal framework of a DVD is shock-mounted at its four corners to reduce sensitivity to external shocks, and to lessen drive noise when running fast. Brass-colored washers are at the four corners, with soft shock mount grommets beneath them. Screws run though the grommets to fasten them to the black plastic frame underneath. Two parallel precision guide rods run from upper left to lower right, and carry the sled and the moving optical read-write head. A dark gray disc with two holes on either side has a blue lens surrounded by silver-colored metal. This is the lens closest to the disc that reads and writes by focusing the laser light to a very small spot. A resourceful actuator comprising permanent magnets and coils that move the lens up and down to maintain focus on the data layer is underneath the disc. As well, the actuator moves the lens slightly toward and away from the spin-motor spindle to keep the spot on track. The focus and tracking are relatively fast and very precise. To select tracks, a stepping motor rotates a coarse-pitch lead screw to move the “sled” throughout its total travel range.
Recordable DVDs are now used for consumer audio and video recording. These types of DVDs come in three formats: DVD-R/RW, DVD+R/RW, and DVD-RAM. While most DVD writers can write both the DVD+R/RW and DVD-R/RW formats, the “plus” and the “dash” formats use different writing specifications. Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store up to 8.54 gigabytes of data per disc, which is almost double that of single-layer discs. A dual-layer disc differs from a standard DVD by employing a second physical layer within the actual disc. The drive with dual-layer capability accesses the second layer by shining the laser through the first one. DVD recordable discs supporting this technology are backward-compatible with some DVD players and drives. The recording speeds reached by dual-layer media are still well below those of single-layer media.
DVD-Video is a standard for content on DVD media. The format initially went on sale in Japan in 1996, and reached the United States six months later. DVD eventually became the primary form of home video distribution in the United States when weekly DVD rentals began outnumbering weekly VHS cassette rentals. It is now the primary form of home video distribution worldwide.
While many resolutions and formats are supported, most consumer DVDs use either 4:3 or 16:9 aspect ratios that are stored at a resolution of 720/704Ã—480 or 720/704Ã—576. Audio is commonly stored using the Digital Theater System format, ranging from 16-bits/48 kHz to 24-bits/96 kHz. Although the provisions for video and audio requirements vary in different regions, many DVD players support all possible formats. DVD Video also supports features such as menus, selectable subtitles, different camera angles, and several audio tracks.
DVD Audio is a format that allows consumers to play high quality audio media on DVD. It offers many channel configuration selections at multiple different sampling frequencies. DVD formats have a much higher capacity than CD formats and can include significantly more music. DVDs can also be used for a backup medium. There are two considerations for a backup medium: obsolescence and durability. If the conditions of the DVD format are not preserved and there is no device that can read the medium, it can be difficult to re-create devices that can read DVDs, making it very hard to retrieve the data.
Durability of DVDs is measured by how long the disc itself can be stored until the data is finally lost. There are five factors that affect durability: sealing method, reflective layer, organic dye makeup, where it was manufactured, and storage practices. The endurance of the ability to read from a DVD+R or DVD-R is largely reliant on manufacturing quality ranging from 2 to 15 years. Thus, it is considered an unreliable medium for backup unless they have quality storage conditions and handling.