19. A digital system comprising: a microphone; a memory configured to store a plurality of acoustic models and a plurality of instructions; and a processor operatively coupled to the memory and configured to receive a portion of a speech signal from the microphone and to execute the plurality of instructions to perform a method for adapting at least one acoustic model of the plurality of acoustic models, the method comprising: estimating noise in the portion of the speech signal; determining a first estimated variance scaling vector using an estimated 2-order polynomial and the noise estimation, wherein the estimated 2-order polynomial represents a priori knowledge of a dependency of a variance scaling vector on noise; determining a second estimated variance scaling vector using statistics from prior portions of the speech signal; determining a variance scaling factor using the first estimated variance scaling vector and the second estimated variance scaling vector; and using the variance scaling factor to adapt the at least one acoustic model.
19. A digital system comprising: a microphone; a memory configured to store a plurality of acoustic models and a plurality of instructions; and a processor operatively coupled to the memory and configured to receive a portion of a speech signal from the microphone and to execute the plurality of instructions to perform a method for adapting at least one acoustic model of the plurality of acoustic models, the method comprising: estimating noise in the portion of the speech signal; determining a first estimated variance scaling vector using an estimated 2-order polynomial and the noise estimation, wherein the estimated 2-order polynomial represents a priori knowledge of a dependency of a variance scaling vector on noise; determining a second estimated variance scaling vector using statistics from prior portions of the speech signal; determining a variance scaling factor using the first estimated variance scaling vector and the second estimated variance scaling vector; and using the variance scaling factor to adapt the at least one acoustic model. 20. The digital system of claim 19 , wherein the method further comprises selecting an empirically determined weighting factor, wherein the determining a variance scaling factor further comprises using the weighting factor in the determination of the variance scaling factor.