Sustainability of Digital Formats: Planning for Library of Congress Collections

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AC-3 Compressed Audio (Dolby Digital), Revision A

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name Digital Audio Compression (AC-3), Revision A (ATSC Doc. A/52A) (formal name); AC-3 and Dolby Digital (common names)

Lossy format designed for the efficient encoding of surround sound, developed to support motion picture presentations in theaters and at home. This Web page describes AC-3 as standardized by the Advanced Television Systems Committee (ATSC); the trademarked Dolby Digital codec family includes AC-3 and other variants.

The maximum bitrate in the ATSC AC-3 specification is 640 kb/s. In DVD applications and in digital cable television, however, player and distribution limitations keep the maximum to 448 kb/s. According to the Wikipedia (entry for Dolby Digital, as viewed in October 2005), Dolby Digital Plus (rates to 3 Mb/s) will be used in the emerging high definition DVD formats, and this encoding is reported to be backward compatible with AC-3 compliant players.

Files with an ac3 extension are used in the course of authoring a DVD, at which point they are multiplexed ("muxed") with the video stream. Some ac3 files are downloadable from Web sites, suggesting that they have a life separate from DVD production; comments welcome.

Note: Revision B was published in 2010 and added a new annex, “Enhanced AC-3 Bit Stream Syntax” which specifies an additional syntax that offers additional coding tools and features. Revision B is not described at this Web site at this time.

Production phase Generally used for final-state, end-user delivery.
Relationship to other formats
    Has extension AES3_SMPTE, AES3 Digital Audio Interface, SMPTE Extensions. Specification for the inclusion of AC-3 audio in MPEG-2 streams (and other matters).
    Used by VOB (MPEG-2 Multiplex for DVD) may use AC-3 Elementary Stream, not documented at this time
    Used by ATSC digital broadcast television, not documented at this time
    Has earlier version ATSC document A/52 (prior to Revision A), not documented at this time
    Has later version Revision B and 2010 version (published as part of ATSC A/52:2010: Digital Audio Compression (AC-3) (E-AC-3) Standard), not documented at this time. See History Notes below.

Local use Explanation of format description terms

LC experience or existing holdings None
LC preference  

Sustainability factors Explanation of format description terms

Disclosure Fully documented. Specification developed by the Audio Specialist Group of the ATSC.
    Documentation ATSC Doc. A/52A, August 2001. ATSC Standard: Digital Audio Compression (AC-3), Revision A. This version is no longer available from ATSC. It has been supplanted by (and incorporated as part of) A/52:2010: Digital Audio Compression (AC-3) (E-AC-3) Standard (link available through Internet Archive).
Adoption Widespread adoption in theater, DVD, and digital television contexts. Extent of adoption of files as self-standing and disseminated entities is unknown, although files with both ac3 and wav extensions may be downloaded from Web sites; these apparently require a player with Dolby Digital capabilities, e.g., a DVD player or a Sound Blaster card.
    Licensing and patents General licensing is managed by the Dolby Laboratories Licensing Corporation; see (link available through Internet Archive). There appear to be additional or separate licenses pertaining to the use of AC-3 in DVD video recorders; these are managed by Philips (Koninklijke Philips Electronics N.V.).
Transparency Depends upon algorithms and tools to read; will require sophistication to build tools.
Self-documentation The specification does not suggest that the format permits the incorporation of any descriptive (intellectual) metadata. Regarding technical metadata, the specification indicates the following: An AC-3 serial coded audio bitstream is made up of a sequence of synchronization frames, of which contains 6 coded audio blocks, each of which represent 256 new audio samples per channel. There are frame headers that contains information about bit-rate, sample rate, number of encoded channels, etc. A synchronization information (SI) header at the beginning of each frame contains information needed to acquire and maintain synchronization. A bit stream information (BSI) header follows SI, and contains parameters describing the coded audio service. This data is required to synchronize to and decode the encoded bit stream. Error detection codes are inserted in order to allow the decoder to verify that a received frame of data is error free.
External dependencies Surround sound requires appropriate amplifier and loudspeakers or headphone.
Technical protection considerations Not applicable; depends up context of use.

Quality and functionality factors Explanation of format description terms

Normal rendering Good support.
Fidelity (high audio resolution)

Very good to excellent, given that this is a format for compression. The Wikipedia entry for Dolby Digital (October 2005) reports: "Dolby is part of a group of organizations involved in the development of AAC (Advanced Audio Coding) [see AAC_MP4] . . . . 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."

There is a running debate among specialists and aficionados over which is better--AC-3 or DTS--for surround sound in theaters, especially home theaters.

Multiple channels AC-3_A supports 5.1 surround sound.
Support for user-defined sounds, samples, and patches Not investigated at this time.
Functionality beyond normal rendering Synchronizes with moving images.

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension ac3
From the The File Extension Source.
Internet Media Type audio/ac3
From the The File Extension Source.

Notes Explanation of format description terms

General QT AC3 Codec provides information about a software component that permits AC-3 playback in QuickTime players.
History See Dolby History (link available through Internet Archive). The following notes were taken from the 2010 specification cited below: "A/52 approved 10 November 1994; Annex A approved 12 April 1995, Annex B and Annex C approved 20 December 1995; A/52A revision approved 20 August 2001 (Revision A corrected some errata in the detailed specifications, revised Annex A to include additional information about the DVB standard, removed Annex B that described an interface specification [superseded by IEC and SMPTE standards], and added a new annex, 'Alternate Bit Stream Syntax,' which contributes [in a compatible fashion] some new features to the AC-3 bit stream); A/52B revision approved 14 June 2005 (Revision B corrected some errata in the detailed specifications, and added a new annex, 'Enhanced AC-3 Bit Stream Syntax' which specifies a non-backwards compatible syntax that offers additional coding tools and features. Informative references were removed from the body of the document and placed in a new Annex B); A/52:2010 approved 22 November 2010 (A/52:2010 clarified several areas of ambiguity in A/52B identified by CEA working group R4.3 WG12. Additional items were identified by TSG/S6 members and subsequently addressed)."

Format specifications Explanation of format description terms

Useful references


Last Updated: 08/04/2021