Modulation Processing for Speech Enhancement

Abstract

Many of the traditionally speech enhancement methods reduce noise from corrupted speech by processing the magnitude spectrum in a short-time Fourier analysis-modification-synthesis (AMS) based framework. More recently, use of the modulation domain for speech processing has been investigated, however early efforts in this direction did not account for the changing properties of the modulation spectrum across time. Motivated by this and evidence of the significance of the modulation domain, we investigated the processing of the modulation spectrum on a short-time basis for speech enhancement. For this purpose, a modulation domain-based AMS framework was used, in which the trajectories of each acoustic frequency bin were processed frame-wise in a secondary AMS framework. A number of different enhancement algorithms were investigated for the enhancement of speech in the short-time modulation domain. These included spectral subtraction and MMSE magnitude estimation. In each case, the respective algorithm was used to modify the short-time modulation magnitude spectrum within the modulation AMS framework. Here we review the findings of this investigation, comparing the quality of stimuli enhanced using these modulation based approaches to stimuli enhanced using corresponding modification algorithms applied in the acoustic domain. Results presented show modulation domain based approaches to have improved quality compared to their acoustic domain counterparts. Further, MMSE modulation magnitude estimation (MME) is shown to have improved speech quality compared to Modulation spectral subtraction (ModSSub) stimuli. MME stimuli are found to have good removal of noise without the introduction of musical noise, problematic in spectral subtraction based enhancement. Results also show that ModSSub has minimal musical noise compared to acoustic Spectral subtraction, for appropriately selected modulation frame duration. For modulation domain based methods, modulation frame duration is shown to be an important parameter, with quality generally improved by use of shorter frame durations. From the results of experiments conducted, it is concluded that the short-time modulation domain provides an effective alternative to the short-time acoustic domain for speech processing. Further, that in this domain, MME provides effective noise suppression without the introduction of musical noise distortion.