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 six discrete channels of sound, with 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. The Dolby Digital format supports mono and stereo usages as well. Batman Returns was the first film to use the Dolby Digital technology when it premiered in theaters in Summer 1992. The LaserDisc version of Clear and Present Danger featured the very first Home Theater Dolby Digital mix in 1995.

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 until 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 unlike 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.^[1] 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 Beach^[2] and Auzentech^[3], as well as on motherboards with codecs such as Realtek's ALC-882D^[4] and ALC-888^[5] A similar technology known as DTS Connect is available from competitor DTS.

Dolby Digital Surround EX

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 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.

Dolby TrueHD

Dolby TrueHD, from 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 8 channels (although HD DVD and Blu-ray Disc standards currently limit their maximum number of audio channels to eight, Dolby TrueHD can output up to 14 sound channels simultaneously). 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 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 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 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. The AC-3 standard allows a maximum coded bit rate of 640 kbit/s. 35mm 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 TV standards limit AC-3 to 448 kbit/s. Both Blu-Ray Disc, the Sony Playstation3 and the Microsoft Xbox game console can output an AC-3 signal at a full 640Kbps. Some Sony Playstation2 console games are also capable to output AC-3 standard audio as well.

In certain console and PC games, for example, games from the Sony Playstation2 console platform, is capable to output real time Dolby Pro Logic or Dolby Pro Logic II audio during game play.

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) is supported in HD DVD, as a mandatory codec, and in Blu-ray Disc, as an optional codec.

Dolby Technologies in packaged media formats

See also

• dialnorm - Dolby Digital metadata parameter controlling decoder gain.
• Dolby Laboratories - company history and technology development.
• Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
• Dolby Stereo - first cinema analogue surround sound system
• Dolby Pro Logic - consumer version of the Dolby Stereo analogue surround sound system.
• Dolby TrueHD - lossless codec for HD DVD and Blu-ray Disc
• Home cinema
• Loudspeaker
• List of Dolby Digital films
• SoundStorm - a real-time AC3 encoder for nForce2-based computers.
• C-Media - Producer of DDL audio chipsets used in many sound cards and motherboards.

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.

References

1. ^ Dolby Digital Live. Dolby.com.
2. ^ Montego DDL. Turtlebeach.com.
3. ^ HDA X-Plosion 7.1 DTS Connect. Auzentech.com.
4. ^ anandtech.com. Jun 8, 2006.
5. ^ Realtek ALC888.

Wikipedia information about Dolby Digital This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Dolby Digital". It may not have been reviewed by professional editors (see full disclaimer). Help contribute to Americola and edit this article.