MPEG-2/MPEG-4 - AAC

What is MPEG-2 AAC ?

MPEG Advanced Audio Coding (AAC), also known as MPEG-2 NBC (Non-Backward Compatible) represents the actual state of the art in natural audio coding.

It is able to handle a lot more channels than MP2 or MP3 (48 full audio channels and 16 low frequency enhancement ones compared to 5 full audio channels and 1 low frequency enhancement one for MP2 or MP3), and it can handle higher sampling frequencies than MP3 (up to 96kHz compared to 48kHz). MPEG formal listening tests have demonstrate that for 2 channels it is able to provide slightly better audio quality at 96 kb/s than layer-3 at 128 kb/s or layer-2 at 192 kb/s.

How does MPEG-2 AAC work ?

AAC uses the coding tools already present in MP3, but uses them in a better way:

• The filter bank is a pure MDCT and not an hybrid filter bank like in MP3
• Long windows are nearly twice as long as MP3 ones, providing better frequency resolution
• Short windows are smaller than MP3 ones, providing better transcients handling and less pre-echo
• Ability to toggle middle/side stereo on a subband basis instead of entire frame basis
• Ability to toggle intensity stereo on a subband basis instead of using it only for a contiguous group of subband.

This first part is mainly a removal of MP3 limitations, as AAC standard doesn't have to preserve compatibility. AAC also introduces some new tools over previous coding schemes:

• Temporal Noise Shaping (TNS) is a tool designed to control the location, in time, of the quantization noise by transmission of filtering coefficients
• Prediction is a tool designed to enhance compressibility of stationnary signals

MPEG-4 AAC

The MPEG-4 standard adds some new tools in AAC, in order to improve coding quality at low bitrates:

• Perceptual Noise Substitution (PNS) allows to replace coding of noise-like parts of the signal by some noise generated on the decoder side
• Long Term Prediction (LTP) is once again a prediction tool.This one requires less computation power than the one used in MPEG-2 AAC, while providing comparable coding performance

Implementations and applications

AAC has been selected for use within the Digital Radio Mondial (DRM) system. Due to its superior performance, AAC should also play a major role for the delivery of high-quality music via the Internet.
High quality AAC implementations is available for licensing from Fraunhofer-Gesellschaft. An AAC encoder plug-in is available from Ahead.

© 1998-2003 Gabriel Bouvigne for MP3'Tech - www.mp3-tech.org