Not to be confused with MPEG-4 SLS, which is branded HD-AAC.

Hierarchical structure of AAC Profile, HE-AAC Profile and HE-AAC v2 Profile, and compatibility between them. The HE-AAC Profile decoder is fully capable of decoding any AAC Profile stream. Similarly the HE-AAC v2 decoder can handle all HE-AAC Profile streams as well as all AAC Profile streams. Based on the MPEG-4 Part 3 technical specification.^[1]

Evolution from MPEG-2 AAC LC (Low Complexity) Profile and MPEG-4 AAC LC Object Type to HE-AACv2 Profile.^[2]

High-Efficiency Advanced Audio Coding (HE-AAC) is a lossy data compression scheme for 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. HE-AAC version 1 profile (HE-AAC v1) uses spectral band replication (SBR) to enhance the compression efficiency in the frequency domain. HE-AAC version 2 profile (HE-AAC v2) couples SBR with Parametric Stereo (PS) to enhance the compression efficiency of stereo signals. It is a standardized and improved version of the AACplus codec.

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

[edit] History

The HE-AAC version 1 was standardized as a profile of MPEG-4 Audio in 2003 by MPEG and published as part of MPEG-4 in document ISO/IEC 14496-3:2001/Amd 1:2003.^[3] The HE-AAC version 2 profile was standardized in 2006 as ISO/IEC 14496-3:2005/Amd 2:2006.^[1][4] A coding tool (Parametric Stereo) used in HE-AAC v2 was standardized in 2004 by MPEG and published in document ISO/IEC 14496-3:2001/Amd 2:2004.^[5]

HE-AAC version 2 was also standardized under the name Enhanced aacPlus by 3GPP for 3G UMTS multimedia services in September 2004 (3GPP TS 26.401). It is based on the AAC LC, SBR and Parametric Stereo coding tools defined in the MPEG-4 Audio standard (and it refers to ISO/IEC 14496-3:2001/Amd.1:2003 - Bandwidth Extension, ISO/IEC 14496-3:2001/Amd.2:2004 - Parametric Coding for High Quality Audio, ISO/IEC 14496-3:2001 and ISO/IEC 14496-3:2001/Amd.1:2003/DCOR1). In addition it includes further tools such as error concealment, spline resampler, and stereo-to-mono downmix.^[6] It was also published by ETSI as TS 126 401 V6.1.0 in December 2004.^[6]

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 that refer to HE-AAC v2. AAC+ and aacPlus are common trade names that 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. ^[7]. 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^{[8][9][10][11]}. The test, taking bitrate distribution and RMSD into account, is a tie between MP3pro, HE AAC and 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.

[edit] Support

[edit] Encoding

iTunes 9 supports HE-AAC encoding and playback.^[12][13] Nero has released a free-of-charge command line HE-AAC encoder^[14], 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.^[15] 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. ^[16] 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.^[17]

As of iTunes 9, iPhone OS 3.1 and Fall 2009 iPods, support for HE-AAC playback is included. Other third-party software featuring support for HE-AAC include FStream, VLC (without metadata/title streaming), Songbird, Snowtape, and recent development versions of Audacious Media Player.

Older versions of Apple iTunes, iPod Touch, and iPhone will play HE-AAC files at reduced fidelity because they ignore the spectral-band replication and parametric stereo information, instead playing them as though they were standard AAC-LC files without the high-frequency, or "treble," information that is only present in the SBR part of the signal.^{[citation needed]} These will report the track length as twice its actual length.^{[citation needed]}

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.^[18]

[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 ^[19] to provide a single point of license for product makers.

Patent licenses are required for end-product companies that make hardware or software products that include HE AAC encoders and/or decoders.^[20] Unlike the MP3 format ^[21], content owners are not required to pay license fees to distribute content in HE AAC.

[edit] Standards

HE-AAC profile was first standardized in ISO/IEC 14496-3:2001/Amd 1:2003.^[22] HE-AAC v2 profile (HE-AAC with Parametric Stereo) was first specified in ISO/IEC 14496-3:2005/Amd 2:2006.^[1][4][23] The Parametric Stereo coding tool used by HE-AAC v2 was standardized in 2004 and published as ISO/IEC 14496-3:2001/Amd 2:2004.^[5][6]

The current version of the MPEG-4 Audio (including HE-AAC standards) is published in ISO/IEC 14496-3:2009.

Enhanced aacPlus is required audio compression format in 3GPP technical specifications for 3G UMTS multimedia services and should be supported in IP Multimedia Subsystem (IMS), Multimedia Messaging Service (MMS), Multimedia Broadcast/Multicast Service (MBMS) and Transparent end-to-end Packet-switched Streaming Service (PSS).^{[24] [25] [26][27]} HE-AAC version 2 was standardized under the name Enhanced aacPlus by 3GPP for 3G UMTS multimedia services in September 2004 (3GPP TS 26.401).^[28]

HE-AAC and HE-AAC v2 audio coding for DVB applications is standardized by TS 101 154.^[29][30] AacPlus v2 by Coding Technologies^[31] is also standardized by the ETSI as TS 102 005 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.264 with audio AAC LC on main program(single or multi) and video H.264 with audio HE-AACv2 in the 1Seg mobile sub-program.

[edit] Versions

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

[edit] See also

• Advanced Audio Coding

[edit] References

[edit] External links

• MPEG-4 HE-AAC v2 — audio coding for today's digital media world, article in the EBU technical review (01/2006) giving explanations on HE-AAC.
• Tuner2.com list of Internet radio stations using aacPlus
• RadioFeeds UK & Ireland (AAC+) UK terrestrial radio stations streaming online in aacPlus format
• aacPlus explained
• Open Source AAC codec FAAC (encoder) and FAAD2 (decoder)
• Reworked HE-AAC encoder aacplusenc (Windows binary aacplusenc.exe only goes to 51kbps.)
• SNG 1.3 Front-end for Nero Encoder e.g. aac-lc, aac-he, aac-hev2 e.g.(Van Halen's Panama HE-AAC maxed at 205kbps)
• neroAacEnc.exe - the Nero Encoder back-end for the SNG 1.3 front-end.
• Winamp.com Audio player/ripper that allows you to rip CDs into HE-AAC and convert other audio files into HE-AAC (with a free add-on).
• mp3PRO vs MP3 - includes graphs comparing high-frequency performance for MP3pro (similar to HE-AAC+)
• Official MPEG web site
• Via Licensing Corporation - AAC and HE AAC patent pool
• RFC 4281 - The Codecs Parameter for "Bucket" Media Types