Investigating Amplitude Modulation for Speech Signal Processing

It is known that when signals interfere in filter-bank channels the resultant signal shows changes in both amplitude envelope and frequency dependant upon the dominance of one signal over another. These effects are particularly prevelent when using gamma-tone filter-banks where the cochlear filters have a particularly high bandwidth. By investigating the effects of interfering signals in these situations it is hoped that useful features for speech signal processing will result.
This investigation begins with experiments on the interaction of dual tones in a single filter showing 'beating' in the resultant waveform envelope and deviations in the instantaneous frequency (IF) shown to be related to the relative amplitudes of the source tones. This effect was shown to be theoretically predictable, and reversable by analysis of IF deviation and amplitude of the resultant frequency. Further investigation of IF deviation on speech like signals required an investigation of interfernce across the whole filter-bank. This required using an extension of the normal spectrogram into the HSV colour space so that a greater number of variables can be displayed, in this case to show variations in IF. Finally, by using these presentations it was possible to shown the points of maximal 'AM' in speech signals, along with the behaviour of IF changes around these points. These images showed that these points where found to seperate formants in the speech signal, and perhaps more importantly showed that the period of these points was equal to the pitch period of the speakers voice. Subsequent experiments found that pitch detection using these points showed regrettably high error levels, however using IF deviations on every channel produced better results. This was found to be particularly useful in female speakers, where gross errors were reduced to 10%, rising to only 15% with 0 dB white noise SNR.

What is Amplitude Modulation? - Description and examples on the effects and theory of AM

Single Channel AM Experiments - Experiments on the effects of AM on two tones in a single gamma-tone filter bank.

AM Theory for Tones with Unequal Amplitude - Theory behind the combination and extraction of dual tones to and from modulated signals.

Instantaneous Frequency Plots and AM - The use of spectrogram and IF display in HSV colour space for the investigation of first formant interaction

Analysis of IF Differences over Time and Frequency - further use IF display for location of areas of maximal AM depth.

Pitch Detection using AM Analysis Techniques - the analysis of IF changes due to AM in tracking pitch

Modified 30/5/96 by Jeremy Goslin

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