Also called the Outer Iteration Loop, this loop shapes the quantization
noise. Each band from the Filter Bank has a scalefactor (sort of like
a gain, or volume control for that individual band of data). If the scalefactor
is 1.0 (the default value), there is no change to that band. However,
if the quantization noise in a band exceeds the masking threshold (from
the perceptual model), the scalefactor adjusts to reduce the quantization
Each time the scalefactors are adjusted the Rate Loop has to be repeated,
which is why it is nested within the Noise Control Loop. We do this because
reducing the quantization noise requires a larger number of quantization
steps, and in turn, this demands a higher bit rate--the Rate Loop controls
this bit rate (see the section on the Rate Loop).
Noise is computed by subtracting
the original spectrum from the quantized spectrum, when this value
goes below the masking threshold (set by the
perceptual model) for every scalefactor band the loop can stop.