Source of detailed technical information about audio encoding with MP3 and similar formats. Features...www.mp3-tech.org/
This system is based on masking properties of the human ear:When you look at the sun and if a bird passes ahead, you do not see it because of the too predominant light of the sun. In audio, it is similar. During strong sounds, you do not hear the weakest sounds. Take as an example a piece of organ: when the organist does not play, you hear the breath in the piping, and when he plays, you no longer hear it because it is masked.
It is therefore not necessary to code all the sounds. This is the first property used by the MP3 format to earn some space. For this the MP3 encoder uses a psychoacoustic model modeling the behavior of the human ear.
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