Dolby Digital
A digital audio encoding system from Dolby used in movie and home theaters. First used in 1995, Dolby Digital employs Dolby's AC-3 (Audio Coding-3) coding and compression technology and is the standard for DVD-Video and HDTV.
5.1 Channels
Dolby Digital provides five discrete channels of audio plus a subwoofer channel for low frequency effects (the "1" in the 5.1). The five channels are front left, right and center and surround left and right.
6.1 Channels
Co-developed with Lucasfilm THX, Dolby Digital Surround EX adds a center surround channel. The first film to use this enhanced version of Dolby Digital was "Star Wars: Episode I - The Phantom Menace," in 1999.
Dolby Digital is the trademarked marketing name for Dolby Laboratories' 'lossy' AC-3 codec. The common version contains 5.1 channels (five primary speakers and an LFE channel), but the 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 a 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 3 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.
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.
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 bit rate is 640 kbit/s. DVD-Video limits AC-3 to 448 kbit/s. Digital cable TV standards limit AC-3 to 448 kbit/s. ATSC limits AC-3 to 384 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.
