Barix and AAC+

Overview AAC (Advanced Audio Coding)

AAC (Advanced Audio Coding) is a perceptual coding method used to compress digital audio files for efficient storage and transmission. Upon playback, the expanded files can provide sound quality nearly indistinguishable from the original sources. In principle, AAC is similar to MP3, but offers a number of advantages designed to improve audio quality. These include higher-efficiency compression, more channels, and better handling of audio frequencies above 16 kHz. The improved efficiency of AAC files makes AAC a particularly good choice for streaming audio over the Internet.

AAC, developed in part by Dolby Laboratories, is one of several audio coding systems defined by ISO MPEG standards, where it was first specified as MPEG-2 AAC, and then enhanced and extended within MPEG-4. Apple's popular iTunes ® music service employs the AAC format.

AAC was designed to provide high quality audio at lower bit-rates than previous MPEG audio compression formats. AAC was further refined through the MPEG-4 standardization process and has subsequently been enhanced with bandwidth extension technology yielding High Efficiency AAC (HE AAC or AAC+), and with the addition of parametric stereo, resulting in High Efficiency AAC version 2 (HE AAC v2). 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.

The "AAC Family" is a set of backwards-compatible audio coding technologies: MPEG-4 AAC LC decoders can playback MPEG-2 AAC LC bit-streams, MPEG-4 HE AAC decoders can playback both MPEG-4 and MPEG-2 AAC LC bit-streams, etc. In this way the AAC family can support a wide variety of applications ranging from extremely low bit-rates required for music delivery over cellular phone networks, to “transparent” quality (indistinguishable from the original source material) for the most discriminating listeners.

Industry Support

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. 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. XLD, an OS X audio encoding program, offers encoding from any of its supported formats to HE-AAC.

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.

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 with a much lower quality and many errors, so its use is not recommended.

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.

Licensing and patents

Companies holding patents for HE AAC v1 have formed a patent pool under Via Licensing to provide a single point of license for product makers. Patents owned by Coding Technologies and Philips covering the Parametric Stereo used in HE AAC v2 are not included in the Via Licensing pool and are licensed separately by Coding Technologies.

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

Versions

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

Barix implementation and support for AAC encoding and decoding

Barix supports AAC, HE-AAC (AAC+) and HE-AACv2 decoding in the Exstreamer 100 only. These decoders are not supported in the Annuncicom 100, Instreamer 100 or Exstreamer 1000 products. Barix does not provide encoding devices for these technologies.

The Exstreamer 100 is unique in the industry with a high level of capability, a wide range of supported decoders and an extensive customisation potential to suit particular applications.