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High-Efficiency Advanced Audio Coding

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This is an old revision of this page, as edited by 70.187.189.51 (talk) at 23:08, 1 July 2007 (edited top paragraph to more accurately reflect the development of HE AAC. Also changed standard reference to refer to the specific ISO documents for HE AAC v1 and HE AAC v2.). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

High Efficiency AAC (HE-AAC) is a lossy data compression scheme for digital audio. It is an extension of Low Complexity AAC (AAC LC) optimized for low-bitrate applications such as streaming audio. HE-AAC version 1 (HE-AAC v1) uses Spectral Band Replication (SBR) to enhance the compression efficiency in the frequency domain. HE-AAC version 2 (HE-AAC v2) couples SBR with Parametric Stereo (PS) to enhance the compression efficiency of stereo signals.

HE-AAC version 1 was standardized in 2003 by MPEG and published as part of MPEG-4 in document ISO/IEC 14496-3, Amd.1:2003. HE AAC version 2 was standardized in 2004 by MPEG and published as part of MPEG-4 in document ISO/IEC 14496-3, Amd.2:2004

The progenitor of HE-AAC was developed by Coding Technologies under the trade name CT-aacPlus. CT-aacPlus combined MPEG-2 AAC LC with the Coding Technologies invented Spectral Band Replication (SBR). CT-aacPlus is the codec used by XM Radio for their satellite radio service. Subsequently, Coding Technologies submitted their SBR to MPEG as a basis of HE-AAC.

Later, but prior to the standardization of HE AAC v2 by MPEG, Coding Technologies submitted the combination of HE-AAC v1 coupled with Parametric Stereo to 3GPP under the name Enhanced AAC+. As a result, aacPlus, aacPlus v2, AAC+, and EAAC+ are now common trade names which refer to the versions of HE-AAC.

Perceived quality

Scientific testing by the European Broadcasting Union has indicated that HE-AAC at 48kbit/s was ranked as "Excellent" quality using the MUSHRA scale. [1]. MP3 in the same testing received a score less than half that of HE-AAC and was ranked "Poor" using the MUSHRA scale. Data from this testing also indicated that some individuals confused 48kbit/s encoded material with an unencoded original.

Other testing indicates that material decoded from 64 kbit/s HE-AAC does not have similar audio quality to material decoded from MP3 at 128 [[kbit/s][2]. However, this testing was conducted in an uncontrolled manner and cannot be deemed conclusive.

Further conrolled testing by 3GPP during their revision 6 specification process indicates that HE-AAC and its derivative MPEG-4 HE-AAC v2 provide "Good" audio quality for music at low bit rates (e.g. 24Kbps).

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 ony half the sampling frequency, thereby reducing the audio bandwidth. No known tests have been conducted which compare the audio quality of such decoded material.

Support

HE-AAC is supported in the open source FAAD/FAAD2 decoding library (and all players incorporating it), Winamp, foobar2000, and Sony's latest SonicStage version 4.

HE-AAC is also used by AOL Radio clients to deliver high-fidelity music at low bitrates.

As of 2007, neither iTunes, nor iPod supports HE-AAC. Only MPEG 2 AAC (MPEG 2 Audio - part 3) is supported so the only players for Mac OS X are VLC (without metadata/title streaming) and Songbird.

Nero has released a free of charge command line HE-AAC encoder, and also support HE-AAC inside the Nero software suite.

Sorenson Media’s Squeeze Compression Suite includes an HE-AACv2 encoder and is available for Mac OS X as well as Windows.

The 3GPP consortium released source code of a reference HE-AACv2 encoder that appears to be quite competitive regarding quality[3]. No community-supported Free Software encoder is available yet.

Promotion aspects

Commercial trademarks

HE-AAC is sometimes marketed by vendors using trademarks such as aacPlus, aacPlusV2, AAC+, eAAC+, and NeroDigital Audio.

Licensing and patents

As with the MP3 format [4], no licenses or payments seem to be required to be able to stream or distribute content in AAC format.[5] This assertion, however, requires some definitive evidence to back it up, and so should not be relied on without additional and conscientiously retrieved corroborating evidential materials.

However, a patent license is required for all manufacturers or developers of AAC codec extensions such as MPEG 4 Audio HE-AAC profile or MPEG 4 Audio AAC-BSAC.[6]

All AAC extensions such as MPEG 4 Audio HE-AAC profile, MPEG 4 Audio AAC-BSAC require a patent license, and thus uses proprietary technology. But contrary to popular belief, and of great interest, it is not the property of a single company, having been developed in a standards-making organization by several companies [7].

Standards

HE-AAC was first standardized in ISO/IEC 14496-3:2001/Amd.1. HE-AAC v2 (AAC with Parametric Stereo) was first specified in ISO/IEC 14496-3:2001/Amd.4. [8]

The current version of the HE-AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2[9])

AacPlus v2 by Codingtechnologies [8] is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005 for Satellite services to Handheld devices (DVB-SH) below 3 GHz.

References

  1. ^ http://www.ebu.ch/CMSimages/en/tec_doc_t3296_tcm6-10497.pdf
  2. ^ http://www.rjamorim.com/test/64test/results.html
  3. ^ http://www.mp3-tech.org/tests/aac_48/results.html
  4. ^ AudioMPEG. "MPEG Audio Licensing Programme".
  5. ^ Via Licensing. "MPEG-4 Audio Licensing FAQ Q7".
  6. ^ Via Licensing. "MPEG-4 Audio Licensing FAQ".
  7. ^ Via Licensing. "MPEG-4 Audio Licensors".
  8. ^ a b http://www.codingtechnologies.com/products/assets/CT_aacPlus_whitepaper.pdf
  9. ^ ISO/IEC 14496-3:2005/Amd.2 [1]
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