Robustness against noise: The role of timing-synchrony measurement

In a previous report (Ghitza, 1987, [1]) we described a computational model based upon the temporal characteristics of the information in the auditory nerve fiber firing patterns, which produced an "auditory" spectral representation (the EIH) of the input signal. We also demonstrated that for speech recognition purposes, the EIH is more robust against noise compared to the traditional Fourier power spectrum. This paper reports on the first step towards understanding the role of different parameters in the EIH in achieving this performance. Both, the Fourier power spectrum measurement and the EIH measurement can be partitioned into two parts, a filter-bank followed by feature analyzer. In the Fourier power spectrum, the filter bank consists of uniformly shaped Hamming filters and the analyzer is based on power measurements. In the EIH, the filter bank consists of the cochlear filters and the analyzer is based on timing-synchrony measurements. The present study examines the relative importance of the filter-bank properties as compared to the analysis principle. For this purpose a modified EIH model has been created in which the cochlear filters have been replaced by the uniformly shaped Hamming filters. The output of the filter bank is processed by the timing-synchrony analyzer, as with the original EIH. The modified EIH and the Fourier power spectrum differs, therefor, only in the kind of analysis performed on the filter bank output. The modified EIH has been used as a front-end to a Dynamic Time Warp (DTW), using the same set-up as in Ghitza, 1987, [1]. A speaker dependent, isolated word recognition test has been conducted, on a database consisted of a 39 word alpha-digits vocabulary spoken by two male and two female speakers, in different levels of additive white noise. Compared to the Fourier-based front-end, the recognition scores have been slightly improved in clean environment but significantly improved in noisy environments. Furthermore, compared to the original EIH, the recognition scores have also been improved, both in clean and in noisy environments. These results demonstrate that the timing-synchrony measurement is significantly more robust against noise compared to the power measurement. They also show that the robustness is due to the timing-sync- hrony analyzer and not to the unique shape of the cochlear filters.