DVD is an optical disc storage media format that is used for playback of movies with high video and sound quality and for storing data. DVDs are similar in appearance to compact discs.
Two DVDs with different bottom sides.
DVD pick-up head and drive.
During the early 1990s there were two high density optical storage standards in development; one was the Multimedia Compact Disc (MMCD), backed by Philips and Sony, and the other was the Super Disc (SD), supported by Toshiba, Time-Warner, Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer, Thomson and JVC. IBM led an effort to unite the various companies behind a single standard, anticipating a repeat of the costly format war between VHS and Betamax in the 1980s. The result was the DVD format, announced in September of 1995. The official DVD specification was released in Version 1.0 in September, 1996. It is maintained by the DVD Forum, formerly the DVD Consortium, consisting of the ten founding companies and over 220 additional members. The first DVD players and discs were available in November of 1996 in Japan and in March of 1997 in the United States.
By the northern spring of 1999, the price of a DVD player had dropped below the $300 mark. At that point Wal-Mart began to offer DVD players for sale in its stores. When Wal-Mart began selling DVDs in stores, DVDs only represented a small part of their video inventory; VHS tapes of movies made up the remainder. As of 2004, the situation is now reversed. Most retail stores mainly offer DVDs for sale, and VHS copies of movies make up a minority of the sales. The price of a DVD player has dropped to below the level of a typical VCR; a low-end player can be purchased for as little as $40 in a number of retail stores.
In 2000, Sony released its PlayStation 2 console in Japan. In addition to playing video games developed for the system, it was also able to play DVD movies. In Japan, this proved to be a huge selling point due to the fact that the PS2 was much cheaper than many of the DVD players available there. As a result, many electronic stores that normally didn't carry video game consoles carried PS2s. Following on with this tradition Sony have decided to implement one of DVD's possible successors, Blu Ray, into their next PlayStation console currently known as the PlayStation 3
"DVD" was originally an initialism for "digital video disc"; some members of the DVD Forum believe that it should stand for "digital versatile disc", to indicate its potential for non-video applications. Toshiba, which maintains the official DVD Forum site, adheres to the interpretation of "digital versatile disc." The DVD Forum never reached a consensus on the matter, however, and so today the official name of the format is simply "DVD"; the letters do not "officially" stand for anything. (http://www.dvddemystified.com/dvdfaq.html)
A DVD can contain:
- DVD-Video (containing movies (video and sound))
- DVD-Audio (containing high-definition sound)
- DVD-Data (containing data)
The disc medium can be:
- DVD-ROM (read only, manufactured by a press)
- DVD+R/RW (R=Recordable once, RW = ReWritable)
- DVD-R/RW (R=Recordable once, RW = ReWritable)
- DVD-RAM (random access rewritable; after-write checking of data integrity is always active.)
The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity. As of 2004, the double sided formats have almost disappeared from the marketplace.
- DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB)
- DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
- DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
- DVD-14: double sided, double layer on one side, single layer on other, 13.2 GB (12.3 GiB)
- DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)
The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are usually gold-colored, while single-layered sides are silver-colored, like a CD.
Each medium can contain any of the above content and can be any layer type (double layer DVD-R is announced for 2004, while double layer DVD+R discs are already on the market, though scarce and expensive).
The DVD Forum has created the official DVD-R(W) standards. But as the licensing cost for this technology is very high, another group was founded: the DVD+RW Alliance who created the DVD+R(W) standard with lower licensing costs. At first, DVD+R(W) media were typically more expensive than DVD-R(W) media, but the prices have become very comparable.
The "+" (plus) and "-" (dash) are two similar technical standards that are partially compatible. As of 2004, both formats are equally popular, with about half of the industry supporting "+", and the other half "-". It is open to debate whether either format will push the other out of the market, or whether they will co-exist indefinitely. All DVD readers are supposed to read both formats (though real-world compatibility lies around 90% for both formats), and most current DVD writers can write both formats.
Unlike compact discs, where sound (CDDA, Red Book) is stored in a fundamentally different fashion than data (Yellow book et al.), a properly authored DVD will always contain data in the UDF filesystem.
The data transfer rate of a DVD drive is given in multiples of 1350 kB/s, which means that a drive with 16x speed designation allows a data transfer rate of 16 x 1350 = 21600 kB/s (21.09 MB/s). As CD drive speeds are given in mulitples of 150 kB/s, one DVD "speed" equals nine CD "speeds", i.e. 8x DVD drive should have data transfer rate similar to a 72x CD drive. In physical rotation terms (spins per second), one DVD "speed" equals three CD "speeds", so the amount of data that are read during one rotation is three times larger for DVD than for CD and 8x DVD drive has the same rotational speed as 24x CD drive.
Note that both CD and DVD disks and drives usually have constant rotational speed while reading and data density on the track is also constant; as linear (meters per second) track speed grows at outer parts of the disk proportionally to the radius, the maximum data rate specified for the drive/disk is achieved only at the end of the disk's track (disks are written from inside). Average speed of the drive therefore equals to only about 50-70% of the maximum nominated speed.
DVD-Video discs require a DVD-drive with a MPEG-2 decoder (eg. a DVD-player or a DVD computer drive with a software DVD player). Commercial DVD movies are encoded using a combination of MPEG-2 compressed video and audio of varying formats (often multi-channel formats as described below). Typical data rates for DVD movies range from 3-10 Mbit/s, and the bitrate is usually adaptive. A high number of audio tracks and/or lots of extra material on the disk will usually result in a lower bitrate (and image quality) for the main feature.
The audio data on a DVD movie can be of the format PCM, DTS, MPEG audio, or Dolby Digital (AC-3). In countries using the NTSC standard any movie should contain a sound track in (at least) either PCM or Dolby AC-3 formats, and any NTSC player must support these two; all the others are optional. This ensures any standard compatible disc can be played on any standard compatible player. The vast majority of commercial NTSC releases today employ AC-3 audio.
Initially, in countries using the PAL standard (e.g. most of Europe) the sound of DVD was supposed to be standardized on PCM and MPEG-2 audio, but apparently against the wishes of Philips, under public pressure on December 5, 1997, the DVD Forum accepted the addition of Dolby AC-3 to the optional formats on discs and mandatory formats in players. The vast majority of commercial PAL releases employ AC-3 audio by now.
DVDs can contain more than one channel of audio to go together with the video content. In many cases, sound tracks in more than one language track are present (for example the film's original language as well as a dubbed track in the language of the country where the disc is being sold).
With several channels of audio from the DVD, the cabling needed to carry the signal to an amplifier or TV can occasionally be somewhat frustrating. Most systems include an optional digital connector for this task, which is then paired with a similar input on the amplifier. The selected audio signal is sent over the connection, typically over RCA jacks or TOSLINK, in its original format to be decoded by the audio equipment. When playing compact discs, the signal is sent in S/PDIF format instead.
Video is another issue which continues to present problems. Current players typically output analog video only, both composite video on an RCA jack, as well as S-Video in the standard connector. However neither of these connectors were intended to be used for progressive video, so yet another set of connectors has started to appear in the form of component video, which keeps the three components of the video, one luminance signal and two color difference signal, as stored on the DVD itself, on fully separate wires (whereas s-video uses two wires, uniting and degrading the two color signals, and composite only one, uniting and degrading all three signals). Additionally, the connectors are further confused by using a number of different physical connectors on different player models, RCA or BNC, as well as using VGA cables in a non-standard way (VGA is normally analog RGB, not component). Even worse, there are often two sets of component outputs, one carrying interlaced video, and the other progressive. In Europe and other PAL areas, SCART connectors are typically used, which carry both composite and analog RGB intelaced video signals, as well as analog 2-channel sound on a single multiwire cable, and which offer a reasonable compromise between video quality -- which is superior to S-Video though inferior to progressive component video -- and cost.
DVD Video may also include one or more subtitle tracks in various languages, including those made especially for the hearing impaired. They are stored as images with transparent background which are overlayed over the video during playback. Subtitles are restricted to four colors (including transparency) and thus tend to look cruder than permanent subtitles on film.
DVD Video may contain Chapters for easy navigation (and continuation of a partially watched film). If space permits, it is also possible to include several versions (called "angles") of certain scenes, though today this feature is mostly used -- if at all -- not to show different angles of the action, but as part of internationalization to e.g. show different language versions of images containing written text, if subtitles won't do.
A major selling point of DVD Video is that its storage capacity allows for a wide variety of extra features in addition to the feature film itself. This can include audio commentary that is timed to the film sequence, documentary features, unused footage, trivia text commentary, simple games and film shorts.
Most DVD-Video titles use Content Scrambling System (CSS) encryption, which is intended to discourage people from making perfect digital copies to another medium or from bypassing the region control mechanism (see below). Discs can also specify that the player use Macrovision, an analog anti-copying mechanism that prevents the consumer from copying the video onto a VCR tape by using a deliberately-defective signal which may also cause problems for some projection TV's as well as older television models. This alone would not prevent the duplication of DVDs in their entirety without decrypting the data, given suitable equipment, although "consumer-grade" DVD writers deny this ability by refusing to duplicate the tracks on the disc which contain the decryption keys.
The CSS system has caused problems for the inclusion of DVD players in strictly open source operating systems, since open source player implementations can not officially obtain access to the decryption keys or license the patents involved in the CSS system. Proprietary software players may also be difficult to find on some platforms. However at least one successful effort has been made to write a decoder by reverse engineering, resulting in DeCSS. This has led to long-running legal battles and the arrest of some of those involved in creating or distributing the DeCSS code, through the use of the U.S. Digital Millennium Copyright Act, on the grounds that such software could also be used to facilitate unauthorized copying of the data on the discs.
DVD movies can contain a region code, denoting which area of the world it is targeted at, which is completely independent of encryption. The commercial DVD-video player specification dictates that players must only play discs that contain their region code. This allows the film studios to set different retail prices in different markets and extract the maximum possible price from consumers. With region coding, studios can dictate release schedules and prices around the world. However, many DVD players allow playback of any disc, or can be modified to do so. Region coding pertains to regional lockout, which originated from the video game industry.
|0 ||Playable in all regions |
|1 ||United States, Canada, and U.S. territories |
|2 ||Western Europe, Greenland, South Africa, Lesotho, Swaziland, Japan, Egypt, and the Middle East |
|3 ||Southeast Asia, South Korea, Hong Kong, Macau, Indonesia, Philippines, Taiwan |
|4 ||Australia, New Zealand, Oceania, Mexico, Central America, South America |
|5 ||Russia, other Former Soviet Union countries, Eastern Europe, the Indian subcontinent, Mongolia, North Korea, the rest of Africa |
|6 ||People's Republic of China |
|7 ||Reserved for future use |
|8 ||International venues such as aircraft, cruise ships, etc. |
See a world map showing region codes (http://www.robertsdvd.com/world.gif)
European Region 2 DVDs may be sub-coded D1 through D4. "D1" identifies a UK-only release. "D2" and "D3" identify European DVDs that are not sold in the UK and the Republic of Ireland. "D4" identifies DVDs that are distributed throughout Europe.
Region 0 designates no actual region, but it is used as shorthand for a disc meant to be playable on all players. On such a disc, the actual region coding is R1/2/3/4/5/6. In the early days, region 0 players were created that would allow any region disc to be played in them, but studios responded by adjusting regioned discs to refuse to play if the player was determined 0 (since no player should anyway). This system is known as Regional Coding Enhancement or just RCE.
Many view region code enforcement as a violation of WTO free trade agreements; however, no legal rulings have yet been made in this area. However, many manufacturers of DVD players now freely supply information on how to disable the region code checking, and on some recent models, it appears to be disabled by default.
DVD-Audio is a new format to deliver high-fidelity audio content on a DVD. It offers many channels (from mono to 5.1 surround sound) at various sampling frequencies and sample sizes. Audio on a disc can be 16, 20, or 24 bit and can be at sampling rates of 44.1, 48, 88.2, 96, 176.4, or 192 kHz (the highest sampling rates of 176.4 and 192 kHz are limited to stereo only). In addition, different sampling sizes and frequencies can be used on a single disc. Audio is stored on the disc in LPCM format or is losslessly compressed with Meridian Lossless Packing. The DVD-Audio player may downmix surround sound to stereo if the listener does not have surround sound. DVD-Audio may also feature menus, still images, slideshows, and video. Also, DVD-Audio discs usually contain Dolby Digital or DTS versions of the audio (with lossy compression, usually downsampled to lower sampling sizes and frequencies) in the DVD-Video section. This is done to ensure compatibility with DVD-Video players.
The introduction of the DVD-Audio format angered many early-adopters of the DVD format. While the DVD-Audio discs do have higher fidelity, there is debate as to whether or not the difference is distinguishable to typical human ears. DVD-Audio currently forms a niche market, probably due to requiring new and rather expensive equipment. DVD-Audio is currently (as of 2003) in a format war with SACD. Most market observers believe the winner of the war will eventually supplant the Compact Disc due to its superior playback capabilities, unless a new and superior format takes over from either.
A DVD-RAM is easily recognized due to the numerous rectangles on its surface.
- DVD-ROM discs are pressed similarly to CDs. The reflective surface is silver or gold coloured. They can be single-sided single-layered, single-sided double-layered, double-sided single-layered and double-sided double-layered. As of 2004, new double sided discs have become quite rare.
- DVD recorders started to become available in Japan during 1999, and in the rest of the world soon after, with a familiar battle for format dominance beginning. As with the adoptance of USB, Apple computer was one of the early adopters of the technology. DVD recorders require a special unit to write and can use 1 or 2 disc sides (the disc capacity is measured in GB/side):
- DVD-R discs can record up to 4.7 GB in a similar fashion to a CD-R disc. It is supported by the DVD Forum. Once recorded and finalized it can be played by most DVD-ROM players.
- DVD-RW discs can record up to 4.7 GB in a similar fashion to a CD-RW drive. Supported by the DVD Forum.
- DVD-RAM (current specification is version 2.1) require a special unit to play 4.7 or 9.4GB recorded discs (DVD-RAM disc are typically housed in a cartridge). 2.6GB discs can be removed from their caddy and used in DVD-ROM drives. Top capacity is 9.4GB (4.7GB/side). Supported by the DVD Forum.
- DVD+R discs can record up to 4.7 GB single-layered, single-sided DVD+R disc. This is currently up to 16x speed. Like DVD-R you can record only once. Supported by the DVD+RW Alliance.
- DVD+RW discs can record up to 4.7 GB with up to 4x speed. Since it is rewritable it can be overwritten several times. It does not need special "pre-pits" or finalization to be played in a DVD-Player. Supported by the DVD+RW Alliance.
- DVD+R DL is a derivate of DVD+R that uses dual layer recordable discs to store up to 8.5 GB of data. Supported by the DVD+RW Alliance.
All above formats are also available as 8 cm (3 inch) sized DVD mini discs (not mini-DVD, which describes DVD data on a CD) with a disc capacity of 1.5 GB.
DVD players and recorders
Modern recorders often support additional formats, including DVD+/-R/RW, CD-R/RW, MP3, SVCD, JPEG, PNG, SVG, KAR and MPEG4 (DivX/XviD). Some also include USB ports or flash memory readers. Many are priced at under $/€ 100.
Competitors and successors
There are two successors to DVD being developed by two different consortiums: The Blu-ray Disc and HD-DVD.
On November 18, 2003, the Chinese news agency Xinhua reports the final standard of the Chinese government-sponsored Enhanced Versatile Disc (EVD) and several patents around it.
On November 19, 2003 the DVD Forum decided with eight to six votes that HD-DVD is the HDTV successor of the DVD.
- DVD Demystified, Jim Taylor; McGraw-Hill Professional; ISBN 0071350268 (2nd edition, December 22, 2000)
- DVD Authoring and Production, Ralph Labarge; CMP Books; ISBN 1578200822 (August 2001)