Description

Dolby Digital is the trademark for Dolby Laboratories' AC-3 audio coding system. It is a 'lossy' audio compression system based on principles of perceptual coding. Its main application is for multichannel audio, but it supports anywhere from 1.0 channels (mono) to 5.1 channels (full surround) and also dual channel stereo (1+1).

5.1 surround sound is the most common mode, made up of five full-range (10 Hz-22 kHz) channels (left, right, center, left surround, right surround), while the .1 refers to a limited range (10 Hz-120 Hz) Low Frequency Effect (LFE) channel, that carries deep bass sound effects for the subwoofer.

Dolby Digital 5.1 EX is an extension that provides a centre surround channel. The channel is created by using a matrix decoder on the left and right surround channels in a similar fashion to Dolby Pro Logic's creation of its front centre channel. Since it works by additionally processing the decoded 5.1 channels, it is not a true 6.1 channel format, but is fully backward compatible with 5.1 systems.

Dolby Digital Plus is an enhanced coding system based on the AC-3 codec. It offers increased bitrates (up to 3Mbit/sec), 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.

Related articles

• Dolby Laboratories - company history and technology development.
• Dolby noise reduction systems for analogue recorded magnetic tape cassettes.
• Dolby Pro Logic - analogue surround sound system.
• Dolby vs DTS
• Home cinema
• Loudspeaker

External links

• Dolby Laboratories
• Open Source liba52 dolby digital decoder
• ATSC standards, from which a PDF file of the A/52A "Digital Audio Compression (AC-3) Standard" may be downloaded