Dolby Digital

From Open Encyclopedia

Dolby Digital is the trademarked marketing name for Dolby Laboratories' "lossy" AC-3 codec. The common version contains six 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.

Dolby Digital EX is very 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 Surround EX: Whereas Dolby's Pro-Logic IIx format creates 6.1 and 7.1 channel output from stereo 2 channel (2.0). Dolby formats, the Digital Surround EX codec adds a sixth and sometimes seventh channel to standard (non-EX) 5.1 channel Dolby Digital soundtracks.

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 is the next generation lossless format by Dolby, designed specifically for use in High-Definition Disc-based Media. It provides a 100 percent lossless coding technology, up to 18 Mbit/s, over eight full-range channels of 24-bit/96 kHz audio (HD DVD and Blu-ray Disc standards currently limit their maximum number of audio channels to eight), support for High-Definition Media Interface and support for extensive metadata including dialogue normalization and dynamic range control. It has been selected as the mandatory format for HD DVD and as an optional format for Blu-ray Disc.

1 Alias names
2 Applications of Dolby Digital
3 Dolby Technologies in Packaged Media Formats
4 See also
5 References
6 External links

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Alias names

Dolby Digital (promotion name, not accepted by the ATSC), often combined with channel count (DD 5.1)
DD (an abbreviation of above)
Dolby SR-Digital (when the recording incorporates a Dolby Surround-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. A)

These are all different names for the same codec.

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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. DVD-Video players typically hit a ceiling of 448 kbit/s (due to manufacturer-imposed limitations), although they could technically handle the maximum bit rate. Digital cable TV standards limit AC-3 to 448 kbit/s. ATSC limits AC-3 to 384 kbit/s. 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-generation DVD standards. As of May 2005, DD-Plus is a "mandatory codec" for HD DVD. This means all HD DVD hardware will be capable of decoding audio-content compressed by DD-Plus. DD-Plus is also an "optional codec" for Blu-ray Disc.

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Dolby Technologies in Packaged Media Formats

HD DVD				Image:Blu-ray Disc.svg			Image:DVD logo.png			DVD-Audio
Codec	Status	Channels	Max Bit Rate	Status	Channels	Max Bit Rate	Status	Channels	Max Bit Rate	Status	Channels	Max Bit Rate
Image:Dolby digital.png	Mandatory	5.1	448 kbps	Mandatory	5.1	640 kbps	Mandatory	5.1	448 kbps	Optional in video zone for playback compatibility on DVD-Video players	5.1	448 kbps
Dolby Digital Plus		7.1	3 Mbps	Optional	7.1	1.7 Mbps	N/A
Image:Meridian lossless.png		6	9.6 Mbps	N/A						Mandatory	6	9.6 Mbps
Dolby TrueHD		8	18 Mbps	Optional	8	18 Mbps	N/A