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Dolby Digital

Dolby Digital is the marketing name for a series of lossy audio compression technologies developed by Dolby Laboratories.

Versions

Dolby Digital includes several similar technologies, which include Dolby Digital, Dolby Digital EX, Dolby Digital Live, Dolby Digital Surround EX, Dolby Digital Plus, and Dolby TrueHD.

Dolby Digital

Dolby Digital, or AC-3, is the common version containing up to six discrete channels of sound. The most elaborate mode in common usage involves five channels for normal-range speakers (20 Hz – 20,000 Hz) (right front, center, left front, right rear and left rear) and one channel (20 Hz – 120 Hz) for the subwoofer driven low-frequency effects. Mono and stereo modes are also supported. AC-3 supports audio sample-rates up to 48kHz. Batman Returns was the first film to use Dolby Digital technology when it premiered in theaters in Summer 1992. The Laserdisc version of Clear and Present Danger featured the first Home theater Dolby Digital mix in 1995.

This codec has several aliases, which are different names for the same codec:

Dolby Digital (promotional 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 until about 1995/1996)
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 unlike the competing DTS-ES codec.

Dolby Digital Surround EX

The Cinema Version of "Dolby Digital EX" is called Dolby Digital Surround Ex and works the same way. Dolby Digital Surround EX was co-developed by Dolby and Lucasfilm THX in time for the release in May 1999 of Star Wars Episode I: The Phantom Menace. It provides an economical and backwards-compatible means for 5.1 soundtracks to carry a sixth, center back surround channel for improved localization of effects. The extra surround channel is matrix encoded onto the discrete Left Surround and Right Surround channels of the 5.1 mix, much like the front center channel on Dolby Surround encoded stereo soundtracks. The result can be played without loss of information on standard 5.1 systems, or played in 6.1 or 7.1 on systems equipped with Surround EX decoding and additional speakers. Dolby Digital Surround EX has since been used for the Star Wars prequels on the DVD versions and also the remastered original Star Wars trilogy. A number of DVDs have Dolby Digital Surround EX audio option.

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. A similar technology known as DTS Connect is available from competitor DTS.

Dolby Digital Live is currently available in sound cards from manufacturers such as Creative Labs, Turtle Beach, HT OMEGA SYSTEM, Auzentech and Asus using C-Media chipsets. The SoundStorm, used for the Xbox game console and certain nForce2-based PCs, used an early form of this technology.

DDL is also available on motherboards with codecs such as Realtek's ALC882D, ALC888DD and ALC888H.

DDL is also supported by all Creative X-Fi based sound cards, but is intentionally disabled in the drivers by Creative on all but the Auzentech Prelude. A programmer named Daniel Kawakami has re-enabled this feature and fixed other bugs in the Windows Vista drivers in a series of modified drivers that he made available. Creative Labs has alleged that Daniel has violated their intellectual property and has demanded he cease distributing his modified drivers . Creative has since released the X-Fi Titanium sound card which fully supports Dolby Digital Live.

In September 2008 Creative released the "Dolby Digital Live" pack which enables Dolby Digital Live on Creative's X-Fi sound cards. The product can be bought from Creative directly.

An important benefit of this technology is that it enables the use of digital multichannel sound with consumer sound cards, which are otherwise limited to PCM stereo or multichannel analog.

Dolby Digital Plus

E-AC-3, more commonly known as 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.

Dolby TrueHD

Dolby TrueHD, developed by Dolby Laboratories, is an advanced lossless audio codec based on Meridian Lossless Packing. Support for the codec is mandatory for HD DVD and optional for Blu-ray Disc hardware. TrueHD supports 24 bit, 96 kHz audio channels at up to 18 Mbit/s over 14 channels (HD DVD and Blu-ray Disc standards currently limit the maximum number of audio channels to eight). It also supports extensive metadata, including dialog normalization and Dynamic Range Control.

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 quadraphonic (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 Low Frequency Effect (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.

The '.1' in 5.1, 7.1 etc. refers to the LFE channel, which is also a discrete channel.

Applications of Dolby Digital

Dolby Digital SR-D cinema soundtracks are optically recorded on a 35 mm 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. The AC-3 standard allows a maximum coded bit rate of 640 kbit/s. 35 mm film prints use a fixed rate of 320 kbit/s. HD-DVD and DVD-Video discs are limited to 448 kbit/s, although many players can successfully play higher-rate bitstreams (which are non-compliant with the DVD specification). ATSC and Digital cable standards limit AC-3 to 448 kbit/s. Blu-ray Disc, the Sony PlayStation 3 and the Microsoft Xbox game console can output an AC-3 signal at a full 640 kbit/s. Some Sony PlayStation 2 console games are also capable to output AC-3 standard audio as well.

Dolby is also part of a group of organizations involved in the development of AAC (Advanced Audio Coding), part of MPEG specifications, and considered the successor to MP3. AAC outperforms AC-3 at any bitrate, but is more complex.

Dolby Digital Plus (DD-Plus) is supported in HD DVD, as a mandatory codec, and in Blu-ray Disc, as an optional codec.

Dolby Technologies in packaged media formats

	HD DVD			Blu-ray			DVD			DVD-Audio			Laserdisc
Codec	Player support	Channels (max)	Max Bit Rate	Player support	Channels (max)	Max Bit Rate	Player support	Channels (max)	Max Bit Rate	Player support	Channels (max)	Max Bit Rate	Player support	Channels (max)	Max Bit Rate
Dolby Digital	Mandatory	5.1	504 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	Optional	5.1	384 kbit/s
Dolby Digital Plus		7.1	3 Mbit/s	Optional	7.1	1.7 Mbit/s	N/A
Dolby TrueHD		8	18 Mbit/s	Optional	8	18 Mbit/s	N/A

Technical details

The data layout of AC-3 is described by simplified "C-like" language in official specifications. An AC-3 stream is made up by a series of synchronization frames, which are composed of six audio blocks. Each audio block contains 256 audio samples per channel. Note 6×256 = 1536 = Audio frame size. Below is a simplified AC-3 header intended to give an introduction into the data syntax. A detailed description of the header can be found in the ATSC "Digital Audio Compression (AC-3) Standard", section 5.4.

Field Name	# of bits	Description
syncword	16	0x0B77 Transmission of data is left bit first also known as Big Endian.
CRC	16
Sampling frequency	2	'11'=reserved '10'=32 kHz '01'=44.1 '00'=48
Frame Size Code	6
Bit Stream Identification	5
Bit Stream Mode	3	'000'=main audio service
Audio Coding Mode	3	'010'=left, right channel ordering
Center Mix level	2
Surround Mix Level	2
Dolby Surround Mode	2	'00'=not indicated '01'=Not surround encoded '10'=Yes, surround encoded.