Speaker verification in a time-feature space
Sarel van Vuuren · 1999
The goal of this dissertation is to determine the relative importance of components of the modulation spectrum for automatic speaker verification and to use this knowledge to improve the performance of an automatic speaker verification system. It is proposed that the power spectrum of a time sequence of logarithmic energy, called the modulation spectrum, provide information that may be used to reduce the effects of adverse environments. The proposed strategy is to attenuate spectral components that are not particularly useful for speaker verification. The aim is to reduce system sensitivity to telephone handset variability without reducing verification accuracy. By computing the effect of carbon-button and electret microphone transducers on the modulation spectrum of telephone speech, it is found that handset transducer variability accounts for a substantial portion of the total variability at moderate to high modulation frequencies. This is shown to be the case also at very low modulation frequencies, where variability is ascribed to the effect of a convolutional channel. This result is substantiated with verification results on the Switchboard corpora as used in 1997–1998 NIST speaker recognition evaluations. The main conclusion is that components of the modulation spectrum between 0.1 Hz and 10 Hz contain the most useful information for speaker verification. To deal with adverse environments, it is proposed that the time sequences of logarithmic energy be lowpass filtered. When compared to other filtering techniques such as cepstral mean subtraction that may retain components up to 50 Hz or RASTA processing that retains components between 1 Hz and 13 Hz, lowpass filtering to 10 Hz is found to significantly reduce verification error in conditions where handset transducers differ between training and testing. It is furthermore proposed that the feature stream be sampled down from a 100 Hz sampling rate to as low as a 25 Hz sampling rate after lowpass filtering. Using this processing, a relative reduction in error of about 10% is shown for the 1997 and 1998 NIST speaker recognition evaluations. Additional contributions of the dissertation include the design and implementation of a modular, high-performance speaker recognition toolkit.