3GPP, the organisation behind third-generation mobile phone network standards, has selected MPEG 4's High Efficiency AAC (HE-AAC) as its codec of choice for 3G-delivered audio content.

HE-AAC builds on AAC (Advanced Audio Codec) by adding a spectral band replication system from codec specialist Coding Technologies. This essentially reconstructs high frequency signals from hints incorporated in the encoded file. That allows the high frequencies to be removed from the file, which encodes only the lower frequencies using regular AAC.

The upshot is an increase in the codec's efficiency by a factor of two - essentially you can encode music at half the bitrate of a regular AAC without a loss of audio quality.

Coding claims HE-AAC delivers CD-quality stereo at 48Kbps - much less than the sub-CD 128Kbps AAC Apple uses for its iTunes Music Store, for example. Smaller files mean quicker downloads, and more songs can be squeezed into the target handset's memory.

3GPP's interest in the codec arises from the need to define a standard mechanism for streaming and downloading quality audio to 3G handsets. The organisation's blessing doesn't mandate HE-AAC's usage, but it's nonetheless a weighty recommendation for the format.

In the UK, network operators Vodafone and O2 use HE-AAC as the basis for their music download services. AOL uses it too. Unlike the MP3 format, AAC was designed with DRM in mind, and for the format incorporates space within each file for DRM data.

The technology behind the HE-AAC codec was incorporated into the MPEG 4 patent pool in July 2004, having already been selected by the DVD Forum as the mandated format for audio files stored in future DVD Audio discs' 'ROM zone' partitions. ®

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