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The five basic types of DVD are referred to by their approximate capacity in gigabytes.
DVD uses 650 nm wavelength laser diode light as opposed to 780 nm for CD or 405 nm for Blu-ray Disc. This permits a smaller spot on the media surface (1.32 µm for DVD versus 2.11 µm for CD) compared to CDs.
Writing speeds for DVD were 1×, that is 1350 kB/s (1318 KiB/s), in the first drives and media models. 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), 9 times slower.
(The following text is an edited extract from http://en.wikipedia.org/wiki/Dvd)
DVD-Video is a standard for storing video content on DVD media. In the U.S., weekly DVD-Video rentals first out-numbered weekly VHS cassette rentals in June 2003, illustrating the rapid adoption rate of the technology in the marketplace.
Though many resolutions and formats are supported, most consumer DVD-Video discs use either 4:3 or anamorphic 16:9 aspect ratio MPEG-2 video, stored at a resolution of 720×480 (NTSC) or 720×576 (PAL) at 29.97 or 25 FPS. Audio is commonly stored using the Dolby Digital (AC-3) or Digital Theatre System (DTS) formats, ranging from 16-bits/48 kHz to 24-bits/96 kHz format with monaural to 7.1 channel "Surround Sound" presentation, and/or MPEG-1 Layer 2. Although the specifications for video and audio requirements vary by global region and television system, many DVD players support all possible formats. DVD-Video also supports features like menus, selectable subtitles, multiple camera angles, and multiple audio tracks.
High capacity data discs known as DVD-ROMs are now commonly used by the software industry (in particular game developers) to hold large software titles.
The single layer format is most common, although dual layer DVD-ROMs will soon become the standard format as game developers seek to extend their titles into higher resolution, more complex data.
The existence of DVD-ROM does create one potential problem for disc manufacturers however. A DVD-Video title, if incorrectly formatted (e.g. without UDF or universal disc formatting), will seem like a DVD-ROM title at the glass mastering stage. Unless the operator is specifically looking for a video format, an ISO image disc may be replicated, rendering the title completely unplayable on standard DVD players. It is for this reason that developers must test their titles on standard set top DVD players, and never test the final master in a computer that may read and decode an ISO image.
DVD recordable and rewritable (DVD-R, DVD-RW)
HP initially developed recordable DVD media from the need to store data for back-up and transport.
DVD recordables are now also used for consumer audio and video recording. Three formats were developed: DVD-R/RW (minus/dash), DVD+R/RW (plus), DVD-RAM.
Dual Layer recording allows DVD-R and DVD+R discs to store significantly more data, up to 8.5 gigabytes per side, per disc, compared with 4.7 gigabytes for single-layer discs. 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 semi-transparent layer. The layer change mechanism in some DVD players can show a noticeable pause, as long as two seconds by some accounts. This caused more than a few 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, though the blank media remains more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media.
DVD-Audio is a format for delivering high-fidelity audio content on a DVD. It offers many channel configuration options (from mono to 5.1 surround sound) at various sampling frequencies (up to 24-bits/192 kHz versus CDDAs 16-bits/44.1 kHz). Compared with the CD format, the much higher capacity DVD format enables the inclusion of considerably more music (with respect to total running time and quantity of songs) and/or far higher audio quality (reflected by higher linear sampling rates and higher vertical bit-rates, and/or additional channels for spatial sound reproduction).
Despite DVD-Audio's superior technical specifications, there is debate as to whether the resulting audio enhancements are distinguishable in typical listening environments. DVD-Audio currently forms a niche market, probably due to the very sort of format war with rival standard SACD that DVD-Video avoided.
DVD-Audio discs employ a robust copy prevention mechanism, called Content Protection for Prerecorded Media (CPPM) developed by the 4C group (IBM, Intel, Matsushita, and Toshiba).
To date, CPPM has not been "broken" in the sense that DVD-Video's CSS has been broken, but ways to circumvent it have been developed. By modifying commercial DVD(-Audio) playback software to write the decrypted and decoded audio streams to the hard disk, users can, essentially, extract content from DVD-Audio discs much in the same way they can from DVD-Video discs.
(The following text is an edited extract from http://en.wikipedia.org/wiki/Dvd)
There were two plausible successors to DVD being developed by different consortia. Sony and Panasonic's Blu-ray Disc (BD) and DVD Forum's HD DVD; the "official" successor designed by Toshiba.
The two formats were engaged in a format war from 2006 to 2007, but in January and February, 2008, nearly all studios and large stores dropped support for HD DVD, and on February 19, Toshiba announced that they would discontinue the development of HD DVD. However, unlike previous format changes (i.e. vinyl records to compact disc, VHS videotape to DVD), there is no immediate indication that production of standard DVD will be immediately discontinued.
Both Blu-ray Disc and HD DVD players will play standard DVDs, and DVD release news websites such as TVShowsonDVD.com continue to list upcoming standard DVD releases well into 2009. In addition, in announcing its abandonment of HD DVD, Toshiba indicated it will still continue to produce standard DVD players and recorders.
Meanwhile, Wal-Mart, in announcing its decision to stop selling HD DVD in favour of Blu-ray Disc a few days before Toshiba's announcement, specifically singled out standard DVD in its announcement, indicating the mega-retailer has no plans to stop selling the older format as it adopts the new one.
Blu-ray Disc uses a "blue" (technically violet) laser operating at a wavelength of 405 nm to read and write data. Conventional DVDs and CDs use red and near infrared lasers at 650 nm and 780 nm respectively.
The blue-violet laser's shorter wavelength makes it possible to store more information on a 12 cm CD/DVD sized disc. The minimum "spot size" on which a laser can be focused is limited by diffraction, and depends on the wavelength of the light and the numerical aperture of the lens used to focus it. By decreasing the wavelength, increasing the numerical aperture from 0.60 to 0.85 and making the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller spot. This allows more information to be stored in the same area. In addition to the optical improvements, Blu-ray Discs feature improvements in data encoding that further increase the capacity. (See Compact disc for information on optical discs' physical structure.)
Because the Blu-ray Disc data layer is closer to the surface of the disc, compared to the DVD standard, it was at first more vulnerable to scratches. The first discs were housed in cartridges for protection. Advances in polymer technology eventually made the cartridges unnecessary.
TDK was the first company to develop a working scratch protection coating for Blu-ray Discs. It was named Durabis. In addition, both Sony and Panasonic's replication methods include proprietary hard-coat technologies. Sony's rewritable media are sprayed with a scratch-resistant and antistatic coating. Verbatim's recordable and rewritable Blu-ray Disc discs use their own proprietary hard-coat technology called ScratchGuard.