The MDCT coefficients are predicted from the two preceding frames, using a separate LMS-adapted (Least Mean Square) predictor for every frequency band. This improves coding efficiency for stationary signals. Residuals after the prediction are non-uniformly quantized and coded using one of 12 different Huffman codes.

In MPEG-2 AAC there are a lot of optional extra features. One of the most
interesting is Temporal Noise Shaping (TNS), which works well for
transient signals. The idea is, that a tonal signal in the time domain has
transient peaks in the frequency domain. The dual of this, is that a
signal which is transient in the time domain is ``tonal'' in the frequency
domain, i.e consists mainly of a few sines. ``Tonal'' sounds are easily
predicted using a LPC approach. Thus, a simple linear predictor is used to
predict the next *spectral* sample (going from low frequencies to high)
from it lower-frequency neighbors.

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Center for Computer Research in Music and Acoustics (CCRMA), Stanford University

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