HE-AAC

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.

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.

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. [http://www.ebu.ch/CMSimages/en/tec_doc_t3296_tcm6-10497.pdf] . 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 [ [http://www.rjamorim.com/test/64test/results.html Results of 64kbit/s Listening Test ] ] [ [http://www.hydrogenaudio.org/forums/index.php?showtopic=50888 Multiformat Listening Test @ 48 kbps - FINISHED - Hydrogenaudio Forums ] ] [ [http://www.hydrogenaudio.org/forums/index.php?showtopic=35438 80 kbps personal listening test (summer 2005) - Hydrogenaudio Forums ] ] [ [http://forum.hardware.fr/hardwarefr/VideoSon/MP3-WMA-AAC-OGG-qualite-kbit/s-evaluation-sujet-84950-1.htm • MP3 • WMA • AAC • OGG • qualité à 96 kbps (évaluation) - Traitement Audio - Video & Son - FORUM HardWare.fr ] ] . 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.

upport

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. [] , content owners are not required to pay license fees to distribute content in HE AAC.

tandards

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. http://www.codingtechnologies.com/products/assets/CT_aacPlus_whitepaper.pdf]

The current version of the HE-AAC standard is ISO/IEC 14496-3:2005 (with 14496-3:2005/Amd.2. for HE-AAC v2ISO/IEC 14496-3:2005/Amd.2 [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=43026] ] )

AacPlus v2 by Codingtechnologies 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.

Versions

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

ee also

* WMA Professional

References

External links

* [http://www.ebu.ch/en/technical/trev/trev_305-moser.pdf 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.
* [http://www.tuner2.com/ Tuner2.com] list of Internet radio stations using aacPlus
* [http://www.digitalradiotech.co.uk/ digitalradiotech.co.uk] information on systems that use AAC+, such as DAB+, DVB-H, Internet radio
* [http://www.radiofeeds.co.uk/other.asp RadioFeeds UK & Ireland (AAC+)] UK terrestrial radio stations streaming online in aacPlus format
* [http://www.codingtechnologies.com/products/aacPlus.htm aacPlus explained]
* [http://www.audiocoding.com/ Open Source AAC codec] FAAC (encoder) and FAAD2 (decoder)
* [http://teknoraver.net/software/mp4tools/ Reworked HE-AAC encoder] aacplusenc
* [http://www.winamp.com/ 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).
* [http://www.digit-life.com/articles/mp3pro/ mp3PRO vs MP3] - includes graphs comparing high-frequency performance for MP3pro (similar to HE-AAC+)