xHE-AAC

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Extended High-Efficiency AAC (xHE-AAC) is an MPEG profile of USAC (Unified speech and audio coding) defined in ISO/IEC 23003-3. xHE-AAC contains all of the tools of the High Efficiency AAC v2 Profile and as such capable of decoding all AAC family profile streams.

MPEG USAC standard employs the proven MDCT based transform coding techniques known from MPEG-4 Audio and combined them with specialized speech coder elements like ACELP. This achieves equally good quality for coding audio and speech.

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 are 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

In the MPEG USAC standard, two profiles are defined:
1) Extended HE-AAC Profile
2) Baseline USAC Profile

The Extended High Efficiency AAC Profile is a USAC profile that contains all of the tools of the High Efficiency AAC v2 Profile and as such capable of decoding all AAC family profile streams. In order to exploit the consistent performance across content types at low bit rates the profile additionally incorporates mono/stereo capability of the Baseline USAC Profile as outlined below.

As a result the Extended High Efficiency AAC Profile represents a natural evolution of one of the most successful families of profiles in MPEG Audio: the AAC Family. The structure of the profiles ensures that (a) an AAC decoder plays AAC LC (Low Complexity), (b) an HE-AAC decoder plays AAC LC and SBR, (c) an HE-AAC v2 decoder plays AAC LC, SBR, and PS, and (d) an xHE-AAC decoder plays HE-AAC v2, AAC LC, SBR, and PS.

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