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. 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 and Auzentech, as well as on motherboards with codecs such as Realtek's ALC-882D and ALC-888 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.