Noise Control Loop

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 noise.

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.