INTERNATIONAL ORGANISATION FOR STANDARDISATION
ORGANISATION INTERNATIONALE DE NORMALISATION
ISO/IEC JTC1/SC29/WG11
CODING OF MOVING PICTURES AND AUDIO

ISO/IEC JTC1/SC29/WG11 N12022
March 2011, Geneva, Switzerland

Source	Audio Subgroup
Status	Approved
Title	Unified Speech and Audio Coder Common Encoder Reference Software

ISO/IEC 23003-3 Unified Speech and Audio Coding (USAC) is a new audio coding standard that is able to code speech, audio or any mix of speech and audio with a consistent quality over a wide range of bitrates. It supports single and multi-channel coding at high bitrates providing perceptually transparent quality. At the same time, it enables highly efficient coding at very low bitrates while delivering a full bandwidth audio signal. A block diagram of the USAC encoder is shown below.

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. In order to achieve equally good quality for coding audio and speech, USAC employs proven MDCT based transform coding techniques as known from MPEG-4 Audio integrated with state-of-the-art  speech coding techniques like ACELP. Enhanced variations of the MPEG-4 Spectral Band Replication (SBR) and MPEG-D MPEG Surround parametric coding tools are tightly integrated into the codec. This combination of tools permits high performance compression down to the lowest bit rates.

The Unified Speech and Audio Coder Common Reference Software Encoder project aims to develop, as a collaborative effort amongst MPEG experts, a reference-quality USAC encoder. The code base is available on the MPEG SVN server. As the project matures, MPEG will assess its performance via subjective listening tests.

Figure 1 – Block diagram of the USAC encoder