What Is the HE-AAC Audio Format?

High-Efficiency Advanced Audio Encoding (HE-AAC)—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 or 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.

Support for the HE-AAC Format

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 various ways. For example, iTunes (now Apple Music) supports both HE-AAC encoding and decoding. However, many other popular programs such as 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. Using HE-AAC, the bandwidth requirement for streaming is lower than with other codec systems. Acceptable audio can be delivered as low as 32 Kbps 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.

First Version of HE-AAC

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, reproduces 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.

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 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 describes the spatial 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 bit rate of the streaming audio to a minimum.

HE-AAC v2 has other audio enhancements in its toolbox such as down-mixing 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+.