USAC (Unified speech and audio coding)

USAC (Unified speech and audio coding) is an MPEG codec speech and audio codec defined in ISO/IEC 23003-3. USAC also defines a profile known as the Extended High-Efficiency AAC (xHE-AAC) audio codec which is the latest member of the MPEG AAC codec family.

In order to achieve equally good quality for coding audio and speech, USAC employs the proven MDCT (modified discrete cosine transform) based transform coding techniques known from MPEG-4 Audio and combined them with specialized speech coder elements like ACELP.

Parametric coding tools such as MPEG-4 Spectral Band Replication (SBR) and MPEG-D Parametric Stereo were enhanced and tightly integrated into the codec. The result delivers highly efficient coding and operates down to the lowest bit rates (as low as 8 kbps).

Where previous audio codecs had specific strengths in coding either speech or audio content, USAC is able to encode all content equally well regardless of the content type. The codec supports single and multi-channel coding at high bit rates and provides perceptually transparent quality. At the same time, it enables very efficient coding at very low bit rates while retaining the full audio bandwidth.

The main focus of this codec is for use in applications in the field of typical broadcast scenarios, multi-media download to mobile devices, user-generated content such as podcasts, digital radio, mobile TV, audio books, etc

Two USAC profiles are defined in MPEG:

1) Extended HE-AAC Profile (xHE-AAC)
2) Baseline USAC Profile

Baseline USAC Profile provides a clear stand-alone profile for applications where a universally applicable codec is required but the capability of supporting the existing MPEG-4 AAC profiles is not relevant.

On the other hand, the Extended High Efficiency AAC Profile contains all the tools of the High Efficiency AAC v2 Profile and as such capable of decoding all AAC family profile streams.

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