AC3 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 allotted audio) for the subwoofer driven low-frequency effects. Mono and stereo modes are also supported. AC-3 supports audio sample-rates up to 48 kHz. 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.
AC3 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.
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)
Audio Coding 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)
