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High-Efficiency Advanced Audio Coding (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 thefrequency domain. HE-AAC version 2 (HE-AAC v2) couples SBR with Parametric Stereo (PS) to enhance the compression efficiency of stereo signals.

[edit]History

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 v2 and eAAC+ are now common trade names which refer to HE-AAC v2. AAC+ and aacPlus are common trade names which refer to HE-AAC v1.

[edit]Perceived quality

Scientific testing by the European Broadcasting Union has indicated that HE-AAC at 48 kbit/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 48 kbit/s encoded material with an uncompressed original.

Testing indicates that material decoded from 64 kbit/s HE-AAC does not yet have similar audio quality to material decoded from MP3 at 128 kbit/s using high quality encoders^{[2][3][4] [5]}. The test, taking bitrate distribution and RMSD into account, is a tie between MP3pro, HE AAC and the open-source Ogg Vorbis.

Further controlled 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. 24 kbit/s).

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. No known tests have been conducted which compare the audio quality of such decoded material.

[edit]Support

[edit]Encoding

Nero has released a free-of-charge command line HE-AAC encoder, and also supports HE-AAC inside the Nero software suite. Sorenson Media’s Squeeze Compression Suite includes an HE-AACv1 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 offer competitive quality.^[6] Winamp Pro also supports ripping music to HE-AAC. Using a transcoding plugin for Winamp's media library, any file can be transcoded to HE-AAC. ^[7] XLD, an OS X audio encoding program, offers encoding from any of its supported formats to HE-AAC. Nokia PC Suite may encode audiofiles to eAAC+ format before transmitting them to mobile phone.

[edit]Decoding

HE-AAC is supported in the open source FAAD/FAAD2 decoding library (and all players incorporating it): VLC media player, Winamp, foobar2000, Audacious Media Player, and Sony's latest SonicStage version 4. HE-AAC is also used by AOL Radio clients to deliver high-fidelity music at low bitrates.

Adobe's Flash Player 9 supports HE-AACv2.^[8]

Only MPEG 2 AAC (MPEG-4 Audio - part 3) is supported, so the only players for Mac OS X are FStream, VLC (without metadata/title streaming), Songbird, and recent development versions of Audacious Media Player. iTunes will play HE-AAC files, but ignores the spectral-band replication, and parametric stereo information, playing them as though they were standard AAC-LC files.

Dolby will soon be releasing Dolby Pulse decoders. As HE-AAC v2 is the core of Dolby Pulse, it is assumed these decoders will be able to decode HE-AAC v2 files as well.^[9]

[edit]Promotion aspects

[edit]Commercial trademarks and labeling

HE-AAC is marketed under the trademark aacPlus(tm) by Coding Technologies and under the trademark Nero Digital(tm) by Nero AG. Sony Ericsson, Nokia and Samsung use AAC+ to label support for HE AAC v1 and eAAC+ to label support for HE-AAC v2 on their phones. Motorola uses AAC+ to indicate HE AAC v1 and "AAC+ Enhanced" to indicate HE AAC v2.

[edit]Licensing and patents

Companies holding patents for HE AAC have formed a patent pool administered by Via Licensing Corporation ^[10] to provide a single point of license for product makers.

Patent licenses are required for end-product companies making hardware or software products which include HE AAC encoders and/or decoders.^[11] Unlike the MP3 format^[12], content owners are not required to pay license fees to distribute content in HE AAC.

[edit]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. ^[13]

The current version of the HE-AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2^[14])

AacPlus v2 by Coding Technologies ^[13] is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005 for Satellite services to Handheld devices (DVB-SH) below 3 GHz.

In December 2007, Brazil started broadcasting terrestrial DTV standard called International ISDB-Tb that implements video coding H.264AVC with audio AAC-LC on main program(single or multi) and video H.264AVC with audio HE-AACv2 in the 1Seg mobile sub-program.

[edit]Versions

The following is the summary of the different version of HE-AAC: