Speaker Normalization with All-pass Transforms Center for Language and Speech Processing 72 Speaker Normalization with All-pass Transforms

Speaker normalization is a process in which the short-time features of speech from a given speaker are transformed so as to better match some speaker independent model. Vocal tract length normalization (VTLN) is a popular speaker normalization scheme wherein the frequency axis of the short-time spectrum associated with a particular speaker’s speech is rescaled or warped prior to the extraction of cepstral features. In this work, we develop a novel speaker normalization scheme by exploiting the fact that frequency domain transformations similar to that inherent in VTLN can be accomplished entirely in the cepstral domain through the use of conformal maps. We propose a class of such maps, designated all-pass transforms for reasons given hereafter, and rigorously investigate their properties. Theoretical results are provided relating to the transformation of cepstral sequences under these maps. Additionally, all relations necessary to determine maximum likelihood estimates of the mapping parameters are derived for both speaker normalization and adaptation.