Introduction to HE-AAC
HE-AAC (which is often referred to as aacPlus) is a lossy compression system for digital audio and is short for High Efficiency Advanced Audio Encoding. It is optimized for use with streaming audio applications where low bit rates are required such as Internet Radio, streaming music services, etc. There are presently two versions of this compression scheme which are profiled as HE-AAC and HE-AAC V2. The second revision makes use of more enhanced features and is more standardized than the first version (HE-AAC).
Support for the HE-AAC Format
In digital music, there are several examples of how the HE-AAC format is supported and used. This includes:
- Software Media Players -- there is quite a variation in how different jukebox software applications support the HE-AAC format. For example, iTunes (version 9 and higher) and Winamp (pro version) support both HE-AAC encoding and decoding. However, many other popular programs such as Foobar2000, VLC Media Player, etc., only support the playback of HE-AAC encoded audio files.
- Streaming Music Services -- online music services like Pandora Radio and Rhapsody use the HE-AAC compression system to efficiently stream high quality audio to its users. Using HE-AAC, the bandwidth requirement for streaming is a lot lower than for other codec systems -- acceptable audio can be delivered as low as 32 Kbps in most cases.
- Internet Radio -- since HE-AAC is very efficient at low bit rates, this compression scheme is ideal for radio stations to webcast their broadcasts in real-time.
The First Version of HE-AAC
The developers of HE-AAC, Coding Technologies, first created the compression system by integrating Spectral Band Replication (SBR) into AAC LC (low complexity AAC) -- the trade name the company uses is CT-aacPlus. SBR (which Coding Technologies also developed) is used to enhance audio by efficiently coding higher frequencies. This coding enhancement technology, which is particularly good for streaming voice transmissions, works by reproducing higher frequencies by transposing lower ones -- these are stored at 1.5 Kbps.
In 2003 HE-AAC V1 was approved by the MPEG organization and included in their MPEG-4 document as an audio standard (ISO/IEC 14496-3:2001/Amd 1:2003).
The Second Version of HE-AAC
HE-AAC V2 which was also developed by Coding Technologies is an enhanced version of the previously released HE-AAC and was officially named by the company as Enhanced AAC+. This second revision includes an enhancement called Parametric Stereo.
As well as the combination of AAC-LC and SBR for efficiently coding audio as in the first revision of HE-AAC, this second version also has an added tool called, Parametric Stereo -- this focuses on efficiently compressing stereo signals. Rather than working in the frequency spectrum as in the case of SBR, the Parametric Stereo tool works by creating side information about the differences between the left and right channels. This side information can then be used to describe the spacial arrangement of the stereo image in the HE-AAC V2 based audio file. When the decoder uses this extra spacial information, the stereo can be faithfully (and efficiently) reproduced during playback while keeping the bitrate of the streaming audio to a minimum.
HE-AAC V2 also has other audio enhancements in its toolbox such as downmixing stereo to mono, error concealment, and spline resampling. Since its approval and standardization by the MPEG organization in 2006 (as ISO/IEC 14496-3:2005/Amd 2:2006), it has become commonly known as HE-AAC V2, aacPlus v2, and eAAC+.