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Disks Partitions and Volumes

The initials stand for Digital Versatile Disc, the second generation of photomechanical data storage. Using two-sided technology developed by Toshiba, each DVD disc can store 4.3 gigabytes of information on a disc that's outwardly identical to an ordinary CD. Originally targeted as a medium for movies, the format accommodates digital data of all types including interactive multimedia productions.

Where the CD started as a single-purpose device and later grew to accommodate applications its developer could never have foreseen, DVD starts from the beginning as a multipurpose technology. It is meant to replace just about every consumer media storage and playback system. The developers of the DVD believe that it will quickly replace videocassettes, audio CDs, and all the various CD-based computer storage systems-and probably serve several more purposes no one has yet guessed. Although anyone can make such speculations, in the case of the DVD developers they carry weight. After all, the developers make the same VCRs, CD players, and other devices they expect their new wonderchild to replace. Although the first DVD devices have not included DVD recorders, that technology is expected to follow the initial product releases.

DVD represents a grand union of standards, originally adopted in December 1995 as an amalgam of two competing proposals; one from Toshiba, the other from the original CD developers, Sony and Philips. Everyone (and particularly Sony) had learned a hard lesson from the Betamax-VHS marketplace wars. Settling on one standard from the beginning makes the entire industry better off-and even benefits the consumer. Consequently, the DVD standard incorporates technology from both proposals but most heavily relies on the Toshiba technology.

As with CDs, each application for the DVD will have its own sub-title. These include DVD-Video for video applications such as the distribution of motion pictures; DVD-Audio, as a high quality audio disc with capabilities far beyond today's 16-bit discs; DVD-ROM for the distribution of computer software and other data; DVD-RAM, the recordable system akin to CD-R;

Physical Specifications

DVD medium itself resembles ordinary CDs. They are the same 120 millimeters (about 4.75 inches) in diameter and 1.2 millimeters (about 0.05 inch) thick. Unlike conventional Compact Discs, the DVD is made from two platters, each 0.6 mm thick, cemented together. Each can be a complete disc, recorded on two sides. The resulting sandwich has two layers per side or four separate recording surfaces.

Distinguishing individual layers requires only focusing the laser. Each layer is a different distance from the lens of the optical head, so when one layer is in focus the other is out of focus. When the beam is out of focus, it scans such a wide area that the out-of-focus pattern simply doesn't register. The beam reads only the layer that's sharply focused.

The track spiral on DVD is more closely spaced to achieve its higher capacity, adjacent grooves located at 0.74 micrometer increments (the recording pitch). The laser that reads the discs has a wavelength of 635 to 650 nanometers, a medium red.

The disc spins at a constant linear velocity of 3.49 meters per second, which is actually slower than CDs. As a result, disc spin rate varies from 600 RPM at the outer edge to about 1200 RPM at the inner edge of the recordable area. The data bits themselves are about 0.4 micrometers long, so the raw data rate of the drive is 11.08 megabits per second. The actual user data rate, less overhead, amounts to about 9.8 megabits per second. In CD terms, the base DVD speed is about 7x to 8x.

Standards

Table below lists the DVD standards that will most commonly be used in multimedia applications, the number of side and layers used by each and the approximate storage capacity. The table relates the digital capacity to an approximate playing time based on an example data rate of 4.7Mbits/sec for both image and sound, enough for three simultaneous audio channels and subtitles.

Disc Capacity Under Common DVD Standards

Initial releases conform to the DVD-5 standard. This format was tailored to the needs of the motion picture and videocassette industry. It allows a standard Hollywood-style movie to fit on a single disc. Unlike videocassettes, however, the movie will have digital quality images and sound, and not just stereo sound, but full eight channel surround. In that the cost of duplicating DVDs is a fraction of that of videocassettes, the software industry will be urging you into the new medium as fast as it can. In that you should easily be able to see and hear the difference, you shouldn't need too much encouragement.

DVD-Video goes far beyond today's VHS and CD-Video systems. It allows both conventional style images with the 4:3 aspect ratio as well as those with the 16:9 ratio favored by high definition television systems. DVD players are required to have built-in filters to translate 16:9 images into the full width of a 4:3 aspect ratio screen, in other words, built-in letterbox format translation. The DVD players will also allow you to zoom in to fill the screen height with a 16:9 image and pan to either side of the picture. The MPEG-2 encoding delivers about four times the spatial resolution as MPEG-1 used by some CD systems and allows a high quality display with 480 lines of 720 pixel each to fit into a four-megabyte-per-second data stream. As with any video compression technique, the exact data rate depends on the complexity of the image. Typically, a high quality image requires less data than a low quality one plagued by noise.

DVD Video also introduces the concept of subpictures, which are additional images of limited color depth that can be multiplexed with the main audio and video data. The DVD standard allows for up to 32 subpictures, which typically will be menus for control systems, subtitles for foreign language films, or production credits. Each subpicture can measure as large as 720 by 480 pixels in four colors.

DVD-Audio takes advantage of the vast storage space of the medium to store either higher quality audio or more channels. Where CDs are built around a 44.1 KHz sampling rate, DVD supports both this rate (and the whole Red Book standard) as well as 48 KHz, the same as professional audio systems, and a super high quality 96 KHz sampling rate. In addition to today's 16-bit digital audio, DVD will also support 24-bit audio as well as several compressed multichannel formats to accompany video. The multi-channel audio standards vary with the video standard used, with Dolby AC-3 (eight-channel audio) for NTSC video.

The DVD-ROM storage system allows the storage of files up to a maximum size of one gigabyte in the standard Micro UDF/ISO 9660 Bridge format. The system will accommodate any data type with the limits of the file system. For example, you can put MPEG-1 video files on a DVD, enough to play for six hours on a single DVD-5 disc. In that many software companies are hard pressed to fill the confines of an ordinary CD, what they will do with the vast resources of DVD can hardly be imagined.