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High-Efficiency Advanced Audio Coding

An audio coding format for lossy data compression of digital audio defined as an MPEG-4 Audio profile in ISO/IEC 14496-3.

High-Efficiency Advanced Audio Coding (AAC-HE) is an audio coding format for lossy data compression of digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC (AAC-LC) optimized for low-bitrate applications such as streaming audio. The usage profile AAC-HE v1 uses spectral band replication (SBR) to enhance the modified discrete cosine transform (MDCT) compression efficiency in the frequency domain. The usage profile AAC-HE v2 couples SBR with Parametric Stereo (PS) to further enhance the compression efficiency of stereo signals.

AAC-HE is used in digital radio standards like HD Radio, DAB+, and Digital Radio Mondiale.

The progenitor of AAC-HE was developed by Coding Technologies by combining MPEG-2 AAC-LC with a proprietary mechanism for spectral band replication (SBR), to be used by XM Radio for their satellite radio service. Subsequently, Coding Technologies submitted their SBR mechanism to MPEG as a basis of what ultimately became AAC-HE.

AAC-HE v1 was standardized as a profile of MPEG-4 Audio in 2003 by MPEG and published as part of the ISO/IEC 14496-3:2001/Amd 1:2003 specification.

The AAC-HE v2 profile was standardized in 2006 as per ISO/IEC 14496-3:2005/Amd 2:2006.

Parts of the AAC-HE specification had previously been standardized and published by various bodies in 3GPP TS 26.401, ETSI TS 126 401 V6.1.0, ISO/IEC 14496-3:2001/Amd.1:2003 and ISO/IEC 14496-3:2001/Amd 2:2004.

At the time, Coding Technologies had already begun using the trade names AAC+ and aacPlus for what is now known as AAC-HE v1, and aacPlus v2 and eAAC+ for what is now known as AAC-HE v2.

Perceived quality

Testing indicates that material decoded from 64 kbit/s AAC-HE does not quite have the similar audio quality to material decoded from MP3 at 128 kbit/s using high-quality encoders. The test, taking bitrate distribution and RMSD into account, is a tie between mp3PRO, AAC-HE, and Ogg Vorbis.

Further controlled testing by 3GPP during their revision 6 specification process indicates that AAC-HE and AAC-HE v2 provide "Good" audio quality for music at low bit rates (e.g., 24 kbit/s).

In 2011, a public listening test comparing the two best-rated AAC-HE encoders at the time to Opus and Ogg Vorbis indicated statistically significant superiority at 64 kbit/s for Opus over all other contenders, and second-ranked Apple's implementation of AAC-HE as statistically superior to both Ogg Vorbis and Nero AAC-HE, which were tied for third place.

MPEG-2 and MPEG-4 AAC-LC decoders without SBR support will decode the AAC-LC part of the audio, resulting in audio output with only half the sampling frequency, thereby reducing the audio bandwidth. This usually results in the high-end, or treble, portion of the audio signal missing from the audio product.

Key Terms

aac he
aac he v1
aac lc
audio
high efficiency advanced audio coding
iso iec
mpeg
ogg vorbis
sbr
technologies

Additional Resources

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Acronymn

AAC-HE

Synonymns

High-Efficiency Advanced Audio Coding
(none found)

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Sources & Credits

Last modified on May 25 2020
Content adapted from Wikipedia
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