High-Efficiency Advanced Audio Coding (AAC-HE) is an audio coding format for lossy data compression of digital audio defined as a MPEG-4 Audio profile in ISO/IEC 14496-3. It is an extension of Low Complexity AAC (AAC-LC) optimized for low-bitrate applications such as streaming audio. The usage profile AAC-HE v1 uses spectral band replication (SBR) to enhance the modified discrete cosine transform (MDCT) compression efficiency in the frequency domain. The usage profile AAC-HE v2 couples SBR with Parametric Stereo (PS) to further enhance the compression efficiency of stereo signals.
AAC-HE is used in digital radio standards like HD Radio, DAB+, and Digital Radio Mondiale.
The progenitor of AAC-HE was developed by Coding Technologies by combining MPEG-2 AAC-LC with a proprietary mechanism for spectral band replication (SBR), to be used by XM Radio for their satellite radio service. Subsequently, Coding Technologies submitted their SBR mechanism to MPEG as a basis of what ultimately became AAC-HE.
AAC-HE v1 was standardized as a profile of MPEG-4 Audio in 2003 by MPEG and published as part of the ISO/IEC 14496-3:2001/Amd 1:2003 specification.
The AAC-HE v2 profile was standardized in 2006 as per ISO/IEC 14496-3:2005/Amd 2:2006.
Parts of the AAC-HE specification had previously been standardized and published by various bodies in 3GPP TS 26.401, ETSI TS 126 401 V6.1.0, ISO/IEC 14496-3:2001/Amd.1:2003 and ISO/IEC 14496-3:2001/Amd 2:2004.
At the time, Coding Technologies had already begun using the trade names AAC+ and aacPlus for what is now known as AAC-HE v1, and aacPlus v2 and eAAC+ for what is now known as AAC-HE v2.
Testing indicates that material decoded from 64 kbit/s AAC-HE does not quite have the similar audio quality to material decoded from MP3 at 128 kbit/s using high-quality encoders. The test, taking bitrate distribution and RMSD into account, is a tie between mp3PRO, AAC-HE, and Ogg Vorbis.
Further controlled testing by 3GPP during their revision 6 specification process indicates that AAC-HE and AAC-HE v2 provide "Good" audio quality for music at low bit rates (e.g., 24 kbit/s).
In 2011, a public listening test comparing the two best-rated AAC-HE encoders at the time to Opus and Ogg Vorbis indicated statistically significant superiority at 64 kbit/s for Opus over all other contenders, and second-ranked Apple's implementation of AAC-HE as statistically superior to both Ogg Vorbis and Nero AAC-HE, which were tied for third place.
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. This usually results in the high-end, or treble, portion of the audio signal missing from the audio product.