USAC (Unified Speech and Audio Coding, MPEG-D) is the standard that underlies xHE-AAC, and its design goal is in the name: one codec that handles speech and music equally well across an extremely wide bitrate range. It achieves this by combining AAC-style frequency-domain tools with speech-coding tools borrowed from the LPC world and switching between them frame by frame as the content demands, plus enhanced SBR and parametric stereo for low rates. The result is graceful quality from very low bitrates up to transparency, which makes its application form, xHE-AAC, well suited to adaptive streaming and broadcast where the same encode must serve both talk and music on weak and strong links.
