MPEG Advanced Audio Coding
MPEG doc#: N7704
Date: October 2005
Author: S. Quackenbush
What it does:
Advanced Audio Coding (AAC) is a multi-channel perceptual audio coder that provides excellent compression of music signals while achieving transparent quality relative to a stereo Compact Disc original for audio material when coded at 128 kb/s.
What it is for:
AAC is appropriate for applications involving storage or transmission of mono, stereo or multi-channel music or other audio signals where quality of the reconstructed audio is paramount.
Description of MPEG Advanced Audio Coding
Advanced Audio Coding (AAC) is a wideband perceptual audio coding algorithm that provides state of the art levels of compression for audio signals.
AAC is primarily available as MPEG AAC Profile technology, which comprises the AAC Low Complexity technology.
The MPEG-2 AAC standardization effort was focused on providing EBU broadcast quality compression performance for 5-channel music signals at a total bit rate of 320 kb/s. In order to achieve this level of performance, the work item was not constrained to be in any way backward compatible with MPEG-1 audio technology.
MPEG-4 AAC incorporates MPEG-2 AAC, with the addition of the Perceptual Noise Substitution (PNS) tool.
Overview of technology
AAC achieves coding gain primary through three strategies. First, it uses a high-resolution transform (a 1024-frequency-bins) to achieve redundancy removal. This is the invertible removal of information based on purely statistical properties of a signal. Second, it uses a continuously signal-adaptive model of the human auditory system to determine a threshold for the perception of quantization noise and thereby achieve irrelevancy reduction. This is the irretrievable removal of information based on the fact that it is not perceivable Third, entropy coding is used to match the actual entropy of the quantized values with the entropy of their representation in the bitstream. Additionally, AAC provided tools for the joint coding of stereo signals and other coding tools for special classes of signals.
The following figure shows a block diagram of the AAC encoder, in which those modules that provide the primary coding gain are highlighted.
AAC has seen considerable adoption by industry. It has application in compression for PC-based and portable devices, compression for terrestrial digital audio broadcast, streaming of compressed media for both Internet and mobile telephone channels. Additionally if functions as a core coder for other MPEG technology (i.e. HE-AAC) that has application in both satellite-delivered digital audio broadcast and mobile telephony audio streaming.