Sub-band coding can be thought of a

Sub-band coding can be thought of as a method of controlling and distributing quantization noise across the signal spectrum. Quantization is a nonlinear
operation which produces distortion products that are typically broad in spectrum. The human ear does not detect the quantization distortion at all frequencies equally well. It is therefore possible to achieve substantial improvement in
quality by coding the signal in narrower bands. In a sub-band coder, speech is
typically divided into four or eight sub-bands by a bank of filters, and each subband is sampled at a bandpass Nyquist rate (which is lower than the original
sampling rate) and encoded with different accuracy in accordance to a perceptual
criteria. Band-splitting can be done in many ways. One approach could be to
divide the entire speech band into unequal sub-bands that contribute equally to
the articulation index. One partitioning of the speech band according to this
method as suggested by Crochiere, et al, [Cr076] is given below.