Unified Speech and Audio Coding 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 and was published as an international standard ISO/IEC 23003-3 and also as an MPEG-4 AudioObject Type in ISO/IEC 14496-3:2009/Amd 3 in 2012. It uses time-domain linear prediction and residual coding tools for speech signal segments and transform coding tools 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-4Spectral Band Replication and MPEG-DMPEG Surround parametric coding tools are integrated into the USAC codec.
xHE-AAC
The MPEG-D USAC standard defines the xHE-AAC profile, 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. 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. 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. xHE-AAC is a mandatory audio codec in the Digital Radio Mondiale standard. 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. xHE-AAC is supported in Android since Android P and in iOS since iOS 13. It has been announced that it will be added to watchOS 7 and has been licensed to Microsoft.
