Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | Digital Audio Compression (AC-3), Revision A (ATSC Doc. A/52A)
AC-3 and Dolby Digital (common names) |
| Description | 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. |
| Production phase | Generally used for final-state, end-user delivery. |
| Relationship to other formats | |
| Used by | 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 |
| LC experience or existing holdings | None |
| LC preference |
| 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. |
| 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 patent claims | General licensing is managed by the Dolby Laboratories Licensing Corporation. 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.). |
| 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. |
| Normal rendering | Good support. |
| Fidelity (support for 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." Several commentators note the existence of a running debate over which is better for surround sound in theaters (including home theaters): AC-3 or DTS; see Wikipedia entry Dolby vs. DTS. |
| Support for multiple sound channels | AC-3_A supports 5.1 surround sound. |
| Support for downloadable or user-defined sounds, samples, and patches | Not investigated at this time. |
| Functionality beyond normal rendering | Synchronizes with moving images. |
| Tag type | Value | Note |
| Filename Extension | ac3 | From the The File Extension Source. |
| Internet Media Type | audio/ac3 | From the The File Extension Source. |
| General | QT AC3 Codec provides information about a software component that permits AC-3 playback in QuickTime players. |
| History | See The Company's Founding: Dolby and the Digital Age. |
URLs
• ATSC Standard: Digital Audion Compression (AC-3), Revision A (http://www.atsc.org/standards/a_52a.pdf)
• Digital Audio Compression Standard, ATSC document A/52 prior to Revision A (http://www.sparta.lu.se/~bjorn/whitney/references/ac3spec.pdf)
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URLs
• Wikipedia entry for Dolby Digital (http://en.wikipedia.org/wiki/Dolby_Digital)
• The Company's Founding: Dolby and the Digital Age (http://www.dolby.com/about/who_we_are/history_5.html)
• QuickTime AC3 Codec (http://www.insaneness.com/ac3.html)
• The File Extension Source (http://filext.com/)
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