Dolby Digital

Alias names

• Dolby Digital (promotion name, not accepted by the ATSC), often combined with channel count (DD 5.1)
• DD
• Dolby SR-Digital (for more than 2 channels)
• SR-D
• Audio Codec 3
• AC-3
• ATSC A/52 (name of the standard, current version is A/52 Rev. A)

Dolby Digital is the trademark for Dolby Laboratories[?]' AC-3 lossy audio compression system. It supports anywhere from 1.0 channels (mono) to 5.1 channels (full surround) and also dual channel (1+1). "5.1" surround sound consists of the 5 full-range (10 Hz...22 kHz) channels (3 screen channels and 2 surround channels) plus the limited range (10 Hz...120 Hz) low frequency effect channel LFE[?].

The last version is Dolby Digital EX. an extension of dolby digital that provides 6.1 and 7.1 channel configurations by matrixing the channels (Dolby 5.1 otherwise uses discrete channel encoding.) This configuration offers more rear[?] channels, and is less frequently used in home systems than in theaters.

The codec

The term AC-3 applies to the encoding process that creates the Dolby Digital files. Dolby Digital relies on perceptual coding[?] to remove audio information that is disguised by other, louder frequencies in the waveform. In this sense it is an example of lossy data compression as these frequencies are not restored on playback.

Applications of Dolby Digital

Dolby Digital SR-D cinema sound tracks[?] 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. This data is 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, such as digital TV.

Bitrates range is from 32 to 640 kbits. For DVD data rates up to 448 kbps are allowed. AAC outperforms AC-3 at any bitrate, but is much more complex. Advantages of AAC become clearly audible at <400 kbps for 5.1 channels, at <180 kbps for 2.0 channels.

External links

Dolby laboratories (http://www.dolby.com)