Sustainability of Digital Formats: Planning for Library of Congress Collections

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RealAudio, AAC Codec

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name RealAudio, AAC Codec
Description Audio compression codec recommended by RealMedia for use when delivered bitrates greater than 128 kb/s. RealMedia promotional literature states "for consumer music services, as well as easy transfer and playback compatibility to the extensive range of portable and mobile devices that support AAC." The Wikipedia article on RealAudio (consulted February 15, 2011) states that the codec is MPEG-4 HE-AAC (see Notes below).
Production phase Generally a final-state (end-user delivery) format.
Relationship to other formats
    Used by RealAudio_10, RealAudio, Version 10

Local use Explanation of format description terms

LC experience or existing holdings See RealAudio_10
LC preference See RealAudio_10

Sustainability factors Explanation of format description terms

Disclosure See RealAudio_10
    Documentation See RealAudio_10
Adoption See RealAudio_10
    Licensing and patents See RealAudio_10
Transparency See RealAudio_10, and AAC_MP4 Advanced Audio Coding, MPEG-4.
Self-documentation See RealAudio_10
External dependencies See RealAudio_10
Technical protection considerations See RealAudio_10

Quality and functionality factors Explanation of format description terms

Sound
Normal rendering Good support
Fidelity (high audio resolution) Good; promotional literature says that this codec provides "the highest quality listening experience for consumer music services."
Multiple channels See AAC_MP4
Support for user-defined sounds, samples, and patches Not applicable

File type signifiers and format identifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable. 

See RealAudio_10

Internet Media Type Not applicable. 

See RealAudio_10

Magic numbers Not applicable. 

See RealAudio_10

Microsoft FOURCC raac
For the LC-AAC codec (RealAudio 9); this fourcc code will appear in the RealAudio_10 file wrapper
Microsoft FOURCC racp
For the HE-AAC codec (RealAudio 10); this fourcc code will appear in the RealAudio_10 file wrapper.

Notes Explanation of format description terms

General There are two HE-AAC profiles, as described in the Wikipedia article on AAC (consulted in February 2011): (1) High Efficiency AAC Profile defined in 2003, uses AAC-LC, SBR, and (2) High Efficiency AAC v2 Profile, defined in 2006, uses AAC-LC, SBR, PS. The compiler of this description does not know which HE profile is used by RealAudio, unless the FOURCC codes for "HE" and "LC" simply refer to these two profiles.
History  

Format specifications Explanation of format description terms


Useful references

URLs


Last Updated: 02/21/2017