What Is the HE-AAC Audio Format?

High-Efficiency Advanced Audio Encoding (HE-AAC), which is often referred to as aacPlus, is a lossy compression system for digital audio. HE-AAC is optimized for use with streaming audio applications in which low bit rates are required such as internet radio and streaming music services. There are two versions of this compression scheme: HE-AAC and HE-AAC v2. The second version uses enhanced features and is more standardized than the first version.

In digital music, the HE-AAC format is supported and used in different ways. These include:

• Software media players. Different jukebox software applications support the HE-AAC format in varying ways. For example, iTunes and Winamp pro version support both HE-AAC encoding and decoding. However, many other popular programs such as Foobar2000 and VLC Media Player support only the playback of HE-AAC encoded audio files.
• Streaming music services. Online music services such as Pandora use the HE-AAC compression system to stream high-quality audio to users efficiently. Using HE-AAC, the bandwidth requirement for streaming is much lower than for other codec systems. Acceptable audio can be delivered as low as 32Kbps in most cases.
• Internet radio. Since HE-AAC is efficient at low bit rates, this compression scheme is ideal for radio stations to webcast their broadcasts in real time.

Coding Technologies, the developers of HE-AAC, 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.

In 2003, HE-AAC v1 was approved by the MPEG organization and included in their MPEG-4 document as an audio standard.

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.

Parametric stereo 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 audio file. When the decoder uses this extra spatial 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 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, it has become commonly known as HE-AAC v2, aacPlus v2, and eAAC+.