The influence of pitch and noise on the discriminability of filterbank features

Most features used for speech recognition are derived from the output of a filterbank inspired by the auditory system. The two most commonly used filter shapes are the triangular filters used in MFCC (mel-frequency cepstral coefficients) and the gammatone filters that model psychoacoustic critical bands. However, for both of these filterbanks there are free parameters that must be chosen by the system designer. In this paper, we explore the effect that different parameter settings have on the discriminability of speech sound classes. Specifically, we focus our attention on two primary parameters: the filter shape (triangular or gammatone) and the filter bandwidth. We use variations in the noise level and the pitch to explore the behavior of different filterbanks. We use the Fisher linear discriminant to give us insight about why some filterbanks perform better than others. We observe three things: 1) there are significant differences even among different implementations of the same filterbank, 2) wider filters help remove the non-informative pitch information, and 3) the Fisher criteria helps us understand why. We validate the Fisher measure with speech recognition experiments on the Aurora-4 speech corpus.