“…Parameter values of these models are estimated from samples of speech in the testing environments, and either the features of the incoming speech or the internally-stored representations of speech in the system are modified. Typical structural models for adaptation to acoustical variability assume that speech is corrupted either by additive noise with an unknown power spectrum (Porter & Boll, 1984;Ephraim, 1992;Erell & Weintraub, 1990;Gales & Young, 1992;Lockwood, Boudy, et al, 1992;Bellegarda, de Souza, et al, 1992), or by a combination of additive noise and linear filtering (Acero & Stern, 1990). Much of the early work in robust recognition involved a re-implementation of techniques developed to remove additive noise for the purpose of speech enhancement, as reviewed in section 10.3.…”