Dolby Digital

Dolby Digital is the marketing name for a series of lossy audio compression technologies by Dolby Laboratories.
== Versions ==
Dolby Digital includes several similar technologies.
=== Dolby Digital ===
Dolby Digital, or AC-3, is the common version containing up to 6 total channels of sound, with 5 channels for normal-range speakers (Right front, Center, Left Front, Right Rear and Left Rear) and one channel for the LFE, or subwoofer. The Dolby Digital format supports Mono and Stereo usages as well.
This codec has several aliases, which are different names for the same codec:
* Dolby Digital (promotion name, not accepted by the ATSC)
* DD (an abbreviation of above, often combined with channel count: DD 5.1)
* Dolby Surround AC-3 Digital (second promotional name, as seen on early film releases, and on home audio equipment till about 1995/6 or so)
* Dolby Stereo Digital (first promotional name, as seen on early releases, also seen on True Lies LaserDisc)
* Dolby SR-Digital (when the recording incorporates a Dolby SR-format recording for compatibility)
* SR-D (an abbreviation of above)
* Adaptive Transform Coder 3 (relates to the bitstream format of Dolby Digital)
* AC-3 (an abbreviation of above)
* Audio Codec 3, Advanced Codec 3, Acoustic Coder 3 (These are backronyms. However, Adaptive TRansform Acoustic Coding 3, or ATRAC3, is a separate format developed by Sony)
* ATSC A/52 (name of the standard, current version is A/52 Rev. B)
=== Dolby Digital EX ===
Dolby Digital EX is similar in practice to Dolby's earlier Pro-Logic format, which utilized Matrix technology to add a center and single rear surround channel to stereo soundtracks. EX adds an extension to the standard 5.1 channel Dolby Digital codec in the form of matrixed rear channels, creating 6.1 or 7.1 channel output. However, the format is not considered a true 6.1 or 7.1 channel codec because it lacks the capability to support a discrete 6th channel like the competing DTS-ES codec.
=== Dolby Digital Live ===
Dolby Digital Live (DDL) is a real-time encoding technology for interactive media such as video games. It converts any audio signals on a PC or game console into the 5.1-channel Dolby Digital format and transports it via a single S/PDIF cable.Dolby Digital Live. Dolby.com. The SoundStorm, used for the Xbox game console and certain nForce2-based PCs, used an early form of this technology. Dolby Digital Live is currently available in sound cards from manufacturers such as Turtle BeachMontego DDL. Turtlebeach.com. and Auzentech,HDA X-Plosion 7.1 DTS Connect. Auzentech.com. as well as on motherboards with codecs such as Realtek's ALC-882Danandtech.com. Jun 8, 2006. and ALC-888Realtek ALC888. A similar technology known as DTS Connect is available from competitor DTS.
=== Dolby Digital Surround EX ===
Whereas Dolby's Pro-Logic IIx format creates 6.1 and 7.1 channel output from stereo 2 channel (2.0), the Digital Surround EX codec adds a sixth and sometimes seventh channel to standard (non-EX) 5.1 channel Dolby Digital soundtracks, by using a phase matrix to extract a center-rear channel from the two surround channels.
=== Dolby Digital Plus ===
Dolby Digital Plus is an enhanced coding system based on the AC-3 codec. It offers increased bitrates (up to 6.144 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce compression artifacts, and backward compatibility with existing AC-3 hardware.
== Channel configurations ==
Although most commonly associated with the 5.1 channel configuration, Dolby Digital allows a number of different channel selections. The full list of available options is:
* Mono (Center only)
* 2-channel stereo (Left + Right), optionally carrying matrixed Dolby Surround
* 3-channel stereo (Left, Center, Right)
* 2-channel stereo with mono surround (Left, Right, Surround)
* 3-channel stereo with mono surround (Left, Center, Right, Surround)
* 4-channel quadrophonic (Left, Right, Left Surround, Right Surround)
* 5-channel surround (Left, Center, Right, Left Surround, Right Surround)
All of these configurations can optionally include the extra LFE channel. The last two with stereo surrounds can optionally use Dolby Digital EX matrix encoding to add an extra Rear Surround channel.
Many Dolby Digital decoders are equipped with downmixing functionality to distribute encoded channels to available speakers. This includes such functions as playing surround information through the front speakers if surround speakers are unavailable, and distributing the center channel to left and right if no center speaker is available. When outputting to separate equipment over a 2-channel connection, a Dolby Digital decoder can optionally encode the output using Dolby Surround to preserve surround information.
== Applications of Dolby Digital ==
Dolby Digital SR-D cinema soundtracks are optically recorded on a 35mm release print using sequential data blocks placed between every perforation hole on the sound track side of the film.
A CCD scanner in the projector picks up a scanned video image of this area, and a processor correlates the image area and extracts the digital data as an AC-3 bitstream.
These data are finally decoded into a 5.1 channel audio source.
Dolby Digital audio is also used on DVD Video and other purely digital media, like home cinema.
In this format, the AC-3 bitstream is interleaved with the video and control bitstreams.
The system is used in many bandwidth-limited applications other than DVD Video, such as digital TV.
According to the AC-3 standard, the maximum coded bit rate is 640 kbit/s. 35mm film prints use a fixed rate of 320 kbit/s. DVD-Video discs are limited to 448 kbit/s, although many players can succssfully play higher-rate bitstreams (which are non-compliant with the DVD specification.) Digital cable TV standards limit AC-3 to 448 kbit/s. ATSC limits AC-3 to 384 kbit/s, although a proposed addendum lifts that to 448. The Microsoft Xbox game console outputs an AC-3 signal at the maximum allowed rate, 640 kbit/s.
Dolby is part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and also considered the successor to MP3. AAC outperforms AC-3 at any bitrate, but is more complex. The advantages of AAC become clearly audible at less than 400 kbit/s for 5.1 channels, and at less than 180 kbit/s for 2.0 channels.
Dolby Digital Plus (DD-Plus) will likely be deployed in future entertainment equipment. It is already deployed in the HD DVD system. DD-Plus is also an "optional codec" for Blu-ray Disc.
== Dolby Technologies in Packaged Media Formats ==
 	HD DVD 	HD DVD 	HD DVD 	Blu-ray 	Blu-ray 	Blu-ray 	DVD 	DVD 	DVD 	DVD-Audio 	DVD-Audio	DVD-Audio
Codec 	Status 	Channels	Max Bit Rate	Status 	Channels	Max Bit Rate	Status 	Channels	Max Bit Rate	Status 	Channels 	Max Bit Rate
Dolby Digital 	Mandatory	5.1 	448 kbit/s 	Mandatory	5.1 	640 kbit/s 	Mandatory	5.1 	448 kbit/s 	Optional in video zone for playback compatibility on DVD-Video players	5.1 	448 kbit/s
Dolby Digital Plus	Mandatory	7.1 	3 Mbit/s 	Optional 	7.1 	1.7 Mbit/s 	N/A 	N/A 	N/A 	N/A 	N/A 	N/A
Dolby TrueHD 	Mandatory	8 	18 Mbit/s 	Optional 	8 	18 Mbit/s 	N/A 	N/A 	N/A 	N/A 	N/A 	N/A
==Gallery==
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== Releases ==
Release date 	Title 	Distributor 	Format 	Trailer
February 8, 1995 	True Lies 	Fox Video 	Laserdisc	
January 6, 1996 	Mortal Kombat 	Image EntertainmentNew Line Home Video 	Laserdisc	
February 18, 1997	12 MonkeysSignature Collection 	MCA/Universal Home Video 	Laserdisc	
March 18, 1997 	The Long Kiss Goodnight 	Image EntertainmentNew Line Home Video 	Laserdisc	
May 20, 1997 	Daylight 	MCA/Universal Home Video 	Laserdisc	
July 1, 1997 	DaylightSignature Collection 	MCA/Universal Home Video 	Laserdisc	
July 30, 1997 	Pink Floyd: The Wall 	MGM/UA Home Video 	Laserdisc	
August 19, 1997 	Dante's Peak 	MCA/Universal Home Video 	Laserdisc	
October 22, 1997 	Austin Powers: International Man of Mystery	Image EntertainmentNew Line Home Video 	Laserdisc	
July 29, 1998 	Dark City 	Image EntertainmentNew Line Home Video 	Laserdisc	
August 5, 1998 	Wag the Dog 	Image EntertainmentNew Line Home Video 	Laserdisc	
October 7, 1998 	Lost in Space 	Image EntertainmentNew Line Home Video 	Laserdisc	
October 14, 1998 	Friday 	Image EntertainmentNew Line Home Video 	Laserdisc	
November 24, 1998	The FrightenersSignature Collection 	MCA/Universal Home Video 	Laserdisc	
December 22, 1998	Blade 	Image EntertainmentNew Line Home Video 	Laserdisc	
March 2, 1999 	Rush Hour 	Image EntertainmentNew Line Home Video 	Laserdisc	
March 16, 1999 	Living Out Loud 	Image EntertainmentNew Line Home Video 	Laserdisc	
April 6, 1999 	American History X 	Image EntertainmentNew Line Home Video 	Laserdisc	
August 3, 1999 	Paulie 	Image EntertainmentDreamWorks Home Entertainment	Laserdisc	
August 10, 1999 	Antz 	Image EntertainmentDreamWorks Home Entertainment	Laserdisc	
August 17, 1999 	Amistad 	Image EntertainmentDreamWorks Home Entertainment	Laserdisc	
August 31, 1999 	Small Soldiers 	Image EntertainmentDreamWorks Home Entertainment	Laserdisc	
November 16, 1999	Austin Powers: The Spy Who Shagged Me 	Image EntertainmentNew Line Home Video 	Laserdisc	
April 7, 2000 	Star Wars: Episode I – The Phantom Menace 	20th Century Fox Home Entertainment 	Laserdisc	
October 11, 2005 	Kicking & Screaming 	Universal Studios Home Entertainment 	VHS 	
October 11, 2005 	Kicking & Screaming 	Universal Studios Home Entertainment 	DVD 	
November 1, 2005 	The Perfect Man 	Universal Studios Home Entertainment 	VHS 	
November 1, 2005 	The Perfect Man 	Universal Studios Home Entertainment 	DVD 	
November 15, 2005	Madagascar 	DreamWorks Home Entertainment 	DVD 	
December 6, 2005 	Cinderella Man 	Universal Studios Home Entertainment 	VHS 	
December 6, 2005 	Cinderella Man 	Universal Studios Home Entertainment 	DVD 	
==See also==
*Dolby Laboratories - company history and technology development.
*Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
*Dolby Pro Logic - analogue surround sound system.
* Dolby TrueHD - lossless codec for HD DVD and Blu-ray Disc
*Home cinema
*Loudspeaker
*SoundStorm - a real-time AC3 encoder for nForce2-based computers.
*DTS - a competing surround sound format.
*THX
==Trivia== 
*to be added
==References==
==External links==
*Dolby Laboratories 
*Dolby Chart
*AC3Filter - open source DirectShow AC3 decoder
*Open source liba52 dolby digital decoder 
*ATSC standards, from which a PDF file of the A/52B "Digital Audio Compression (AC-3) Standard" may be downloaded
Category:Audio codecs
Category:Film sound production
Category:High-definition
Category:Sound technology