Unified Speech and Audio Coding (USAC) is an audio compression format and codec for both music and speech or any mix of speech and audio using very low bit rates between 12 and 64 kbit/s. It was developed by Moving Picture Experts Group (MPEG) and was published as an international standard ISO/IEC 23003-3 (a.k.a. MPEG-D Part 3) and also as an MPEG-4 Audio Object Type in ISO/IEC 14496-3:2009/Amd 3 in 2012.
It uses time-domain linear prediction and residual coding tools (ACELP-like techniques) for speech signal segments and transform coding tools (MDCT-based techniques) for music signal segments and it is able to switch between the tool sets dynamically in a signal-responsive manner. It is being developed with the aim of a single, unified coder with performance that equals or surpasses that of dedicated speech coders and dedicated music coders over a broad range of bitrates. Enhanced variations of the MPEG-4 Spectral Band Replication (SBR) and MPEG-D MPEG Surround parametric coding tools are integrated into the USAC codec.
The MPEG-D USAC standard (ISO/IEC 23003-3) defines the xHE-AAC profile (Extended High Efficiency AAC), which contains all of the tools of the HE-AAC v2 profile plus the mono/stereo capabilities of the Baseline USAC profile. As a result, a decoder built according to the xHE-AAC profile is able to also decode the bit streams created for the previous members of the AAC family profile(s). The xHE-AAC profile was designed for applications relying on a consistent performance at low data rates while being able to decode all existing AAC-LC, HE-AAC and HE-AACv2 content. xHE-AAC extends the operating range of the codec from 12 to 300 kb/s for stereo signals and allows seamless switching between bitrates over this range for adaptive bitrate delivery (using standards such as MPEG-DASH or HLS for example). xHE-AAC also includes MPEG-D DRC mandatory loudness control to playback content at a consistent volume and offers new dynamic range control profiles for listening in noisy situations.
In April 2016, Via Licensing announced the launch of a xHE-AAC patent pool licensing program for 2016. In 2018, xHE-AAC was included in Via Licensing's AAC patent pool at no additional cost.
Opus (codec) – a royalty free alternative, low latency codec for a similar usage
- MPEG. "Unified Speech and Audio Coding". The Moving Pictures Experts Group. Retrieved 2016-11-11.
- "ISO/IEC DIS 23003-3 - Information technology -- MPEG audio technologies -- Part 3: Unified speech and audio coding". 2011-02-15. Retrieved 2011-07-18.
- "ISO/IEC 14496-3:2009/PDAM 3 - Transport of unified speech and audio coding (USAC)". 2011-06-30. Retrieved 2011-07-18.
- Neuendorf; et al. (2013-12-20), The ISO/MPEG Unified Speech and Audio Coding Standard—Consistent High Quality for All Content Types and at All Bit Rates, retrieved 2015-06-13
- Neuendorf; et al. (2012-04-26), MPEG Unified Speech and Audio Coding-The ISO/MPEG standard for high-efficiency audio coding of all content types, retrieved 2015-06-13
- Neuendorf, Max; Multrus, Markus; Rettelbach, Nikolaus; Fuchs, Guillaume; Robilliard, Julien; Lecomte, Jérémie; Wilde, Stephan; Bayer, Stefan; Disch, Sascha; Helmrich, Christian; Lefebvre, Roch; Gournay, Philippe; Bessette, Bruno; Lapierre, Jimmy; Kjörling, Kristofer; Purnhagen, Heiko; Villemoes, Lars; Oomen, Werner; Schuijers, Erik; Kikuiri, Kei; Chinen, Toru; Norimatsu, Takeshi; Chong, Kok Seng; Oh, Eunmi; Kim, Miyoung; Quackenbush, Schuyler; Grill, Bernhard (2013-12-01). "The ISO/MPEG Unified Speech and Audio Coding Standard - Consistent High Quality for all Content Types and at all Bit Rates". Journal of the Audio Engineering Society. 61 (12): 956–977. ISSN 0004-7554.
- "Fraunhofer's xHE-AAC Audio Codec Software Extends Native AAC Support In Android P For Better Quality At Low Bitrates". Fraunhofer Institute for Integrated Circuits IIS. Retrieved 2020-07-11.
- "Technical Info | Digital Radio Mondiale". www.drm.org. Retrieved 2016-08-02.
- "xHE-AAC". Fraunhofer Institute for Integrated Circuits IIS. Retrieved 2016-08-02.
- xHE-AAC in Digital Radio Mondiale (DRM) (PDF). Fraunhofer IIS. 2015.
- "Via Licensing Announces Extended High Efficiency AAC Patent Pool - Via Corp". www.via-corp.com. Retrieved 2016-08-02.
- "Via Adds MPEG-D DRC To Advanced Audio Coding Patent Pool – ViaCorp". Retrieved 2020-07-11.
- "Apple recommends xHE-AAC for streaming of all audio assets – Fraunhofer Audio Blog". Retrieved 2020-07-11.
- "What's new in streaming audio for Apple Watch - WWDC 2020 - Videos". Apple Developer. Retrieved 2020-07-11.
- "Fraunhofer IIS licenses xHE-AAC audio codec software to Microsoft – Fraunhofer Audio Blog". Retrieved 2020-07-11.