FREQUENCY WARPING USING ALL-PASS TRANSFORMATIONS
Recall the bilinear transform:
which implements a nonlinear warping of the frequency axis: This can be implemented as a series of all-pass transformations:


The cepstral coefficients are input and the output are frequency-warped cepstral coefficients. This is an interesting way to implement speaker-specific warpings (e.g., male vs. female speakers).