dvds

DVD (sometimes explained as "digital video disc" or "digital versatile disc") is a digital optical disc storage format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs can be played in many types of players, including DVD players. DVDs offer higher storage capacity than compact discs while having the same dimensions.

Pre-recorded DVDs are mass-produced using molding machines that physically stamp data onto the DVD. Such discs are known as DVD-ROM, because data can only be read and not written or erased. Blank recordable DVD discs (DVD-R and DVD+R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW, DVD+RW, and DVD-RAM) can be recorded and erased multiple times.

DVDs are used in DVD-Video consumer digital video format and in DVD-Audio consumer digital audio format, as well as for authoring DVD discs written in a special AVCHD format to hold high definition material (often in conjunction with AVCHD format camcorders). DVDs containing other types of information may be referred to as DVD data discs.

History
Development
Before the advent of DVD in 1995, Video CD (VCD) became one of the first formats for distributing digitally encoded films on standard 120 mm (4.7 in) optical discs. (Its predecessor, CD Video, used analog video encoding.) VCD was on the market in 1993. In the same year, two new optical disc storage formats were being developed. One was the Multimedia Compact Disc (MMCD), backed by Philips and Sony, and the other was the Super Density (SD) disc, supported by Toshiba, Time Warner, Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer, Thomson, and JVC. A much earlier consumer optical disc data storage device, known as LaserDisc, was developed in the United States. The original LaserDisc technology was invented by David Paul Gregg and James Russell in 1958 and first patented in 1961, with its first public demonstration in 1972. It first came to market in Atlanta, Georgia in 1978, but used much larger discs than the later formats. Due to the high cost of players and discs, consumer adoption of LaserDisc was very low in both North America and Europe, and was not widely used anywhere outside Japan and the more affluent areas of Southeast Asia, such as Hong Kong, Singapore, Malaysia and Taiwan.

Representatives from the SD camp asked IBM for advice on the file system to use for their disc, and sought support for their format for storing computer data. Alan E. Bell, a researcher from IBM's Almaden Research Center, got that request, and also learned of the MMCD development project. Wary of being caught in a repeat of the costly videotape format war between VHS and Betamax in the 1980s, he convened a group of computer industry experts, including representatives from Apple, Microsoft, Sun Microsystems, Dell, and many others. This group was referred to as the Technical Working Group, or TWG.

The TWG voted to boycott both formats unless the two camps agreed on a single, converged standard. They recruited Lou Gerstner, president of IBM, to pressure the executives of the warring factions. In one significant compromise, the MMCD and SD groups agreed to adopt proposal SD 9, which specified that both layers of the dual-layered disc be read from the same side-instead of proposal SD 10, which would have created a two-sided disc that users would have to turn over. As a result, the DVD specification provided a storage capacity of 4.7 GB for a single-layered, single-sided disc and 8.5 GB for a dual-layered, single-sided disc. The DVD specification ended up similar to Toshiba and Matsushita's Super Density Disc, except for the dual-layer option (MMCD was single-sided and optionally dual-layer-whereas SD was single-layer, but optionally double-sided) and EFMPlus modulation.

Philips and Sony decided that it was in their best interests to avoid another format war over their Multimedia Compact Disc, and agreed to unify with companies backing the Super Density Disc to release a single format, with technologies from both. After other compromises between MMCD and SD, the computer companies through TWG won the day, and a single format, now called DVD, was agreed upon. The TWG also collaborated with the Optical Storage Technology Association (OSTA) on the use of their implementation of the ISO-13346 file system (known as Universal Disk Format) for use on the new DVDs.

Adoption
Movie and home entertainment distributors adopted the DVD format to replace the ubiquitous VHS tape as the primary consumer digital video distribution format. They embraced DVD because it produces superior moving pictures and sound, provides superior data lifespan, and can be interactive. Interactivity on LaserDiscs had proven desirable to consumers, especially collectors. When LaserDisc prices dropped from approximately $100 per disc to $20 per disc at retail, this luxury feature became available for mass consumption. Simultaneously, the movie studios decided to change their home entertainment release model from a rental model to a for purchase model, and large numbers of DVDs were sold.

At the same time, a demand for interactive design talent and services was created. Movies in the past had uniquely designed title sequences. Suddenly every movie being released required information architecture and interactive design components that matched the film's tone and were at the quality level that Hollywood demanded for its product.

DVD as a format had two qualities at the time that were not available in any other interactive medium: enough capacity and speed to provide high quality, full motion video and sound, and low cost delivery mechanism provided by consumer products retailers. Retailers would quickly move to sell their players for under $200, and eventually for under $50 at retail. In addition, the medium itself was small enough and light enough to mail using general first class postage. Almost overnight, this created a new business opportunity and model for business innovators to re-invent the home entertainment distribution model. It also gave companies an inexpensive way to provide business and product information on full motion video through direct mail.

Specifications
The DVD specifications created and updated by the DVD Forum are published as so-called DVD Books (e.g. DVD-ROM Book, DVD-Audio Book, DVD-Video Book, DVD-R Book, DVD-RW Book, DVD-RAM Book, DVD-AR Book, DVD-VR Book, etc.).

Some specifications for mechanical, physical and optical characteristics of DVD optical discs can be downloaded as freely available standards from the ISO website. There are also equivalent European Computer Manufacturers Association (ECMA) standards for some of these specifications, such as Ecma-267 for DVD-ROMs. Also, the DVD+RW Alliance publishes competing recordable DVD specifications such as DVD+R, DVD+R DL, DVD+RW or DVD+RW DL. These DVD formats are also ISO standards.

Some of DVD specifications (e.g. for DVD-Video) are not publicly available and can be obtained only from the DVD Format/Logo Licensing Corporation for a fee of US $5000. Every subscriber must sign a non-disclosure agreement as certain information in the DVD Book is proprietary and confidential.

DVD recordable and rewritable
HP initially developed recordable DVD media from the need to store data for backup and transport. DVD recordables are now also used for consumer audio and video recording. Three formats were developed: DVD-R/RW, DVD+R/RW (plus), and DVD-RAM. DVD-R is available in two formats, General (650 nm) and Authoring (635 nm), where Authoring discs may be recorded with CSS encrypted video content but General discs may not.

Although most DVD writers can nowadays write the DVD+R/RW and DVD-R/RW formats (usually denoted by "DVD±RW" and/or the existence of both the DVD Forum logo and the DVD+RW Alliance logo), the "plus" and the "dash" formats use different writing specifications. Most DVD readers and players play both kinds of discs, though older models can have trouble with the "plus" variants.

Dual-layer recording
Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data-up to 8.5 gigabytes per disc, compared with 4.7 gigabytes for single-layer discs. Along with this, DVD-DLs have slower write speeds as compared to ordinary DVDs. When played, a slight transition can sometimes be seen in the playback when the player changes layers. DVD-R DL was developed for the DVD Forum by Pioneer Corporation; DVD+R DL was developed for the DVD+RW Alliance by Philips and Mitsubishi Kagaku Media (MKM).

A dual-layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc itself. The drive with dual-layer capability accesses the second layer by shining the laser through the first semitransparent layer. In some DVD players, the layer change can exhibit a noticeable pause, up to several seconds. This caused some viewers to worry that their dual-layer discs were damaged or defective, with the end result that studios began listing a standard message explaining the dual-layer pausing effect on all dual-layer disc packaging.

DVD recordable discs supporting this technology are backward-compatible with some existing DVD players and DVD-ROM drives. Many current DVD recorders support dual-layer technology, and the price is now comparable to that of single-layer drives, although the blank media remain more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media.

Dual layer DVDs are recorded using Opposite Track Path (OTP).[25] DVD-ROM discs mastered for computer use are produced with track 0 starting at the inside diameter (as is the case with a single layer). Track 1 then starts at the outside diameter. DVD video discs are mastered slightly differently. The video is divided between the layers such that layer 1 can be made to start at the same diameter that layer 0 finishes. This speeds up the transition as the layer changes because although the laser does have to refocus on layer 1, it does not have to skip across the disc to find it.

Capacity
The basic types of DVD (12 cm diameter, single-sided or homogeneous double-sided) are referred to by a rough approximation of their capacity in gigabytes. In draft versions of the specification, DVD-5 indeed held five gigabytes, but some parameters were changed later on as explained above, so the capacity decreased. Other formats, those with 8 cm diameter and hybrid variants, acquired similar numeric names with even larger deviation.

The 12 cm type is a standard DVD, and the 8 cm variety is known as a MiniDVD. These are the same sizes as a standard CD and a mini-CD, respectively. The capacity by surface (MiB/cm2) varies from 6.92 MiB/cm2 in the DVD-1 to 18.0 MiB/cm2 in the DVD-18.

As with hard disk drives, in the DVD realm, gigabyte and the symbol GB are usually used in the SI sense (i.e., 109, or 1,000,000,000 bytes). For distinction, a gigabyte is normally used in the binary sense (i.e., 10243 (230), or 1,073,741,824 bytes).

DVD drives and players
DVD drives are devices that can read DVD discs on a computer. DVD players are a particular type of devices that do not require a computer to work, and can read DVD-Video and DVD-Audio discs.

Laser and optics
DVD uses 650 nm wavelength laser diode light, as opposed to 780 nm for CD. This shorter wavelength etches a smaller pit on the media surface compared to CDs (0.74 μm for DVD versus 1.6 μm for CD), allowing in part for DVD's increased storage capacity.

In comparison, Blu-ray Disc, the successor to the DVD format, uses a wavelength of 405 nm, and one dual-layer disc has a 50 GB storage capacity.

Transfer rates
Read and write speeds for the first DVD drives and players were of 1,385 kB/s (1,353 KiB/s); this speed is usually called "1×". More recent models, at 18× or 20×, have 18 or 20 times that speed. Note that for CD drives, 1× means 153.6 kB/s (150 KiB/s), about one-ninth as swift.

Source: Wikipedia