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.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
USAC is the reason one AAC-family stream can carry both a talk show and a concert at low bitrates. Volume 4 covers USAC and the rest of the codec family in detail, from AAC-LC and HE-AAC to speech codecs and Opus, along with bitrate choices and loudness metadata. Written by Nikolay Sapunov, CEO at Fora Soft.
Deciding whether USAC (xHE-AAC) is worth it for your low-bitrate audio streams?
Tell us your bitrates, devices and content mix, and we will compare USAC against AAC and Opus for your case and recommend a codec ladder with device support notes. 250+ video products shipped since 2005.
