Beyond quasi-stationarity: Designing time-frequency representations for speech signals

This work addresses two related questions. The first is what joint time-frequency energy representations are most appropriate for speech signals, in particular, for the analysis of formant structure. Quasi-stationarity is not assumed, since it neglects dynamic regions. A set of desired properties is proposed, and a subclass of the quadratic transforms that best meets these criteria is derived, which consists of two-dimensionally smoothed Wigner distributions with gaussian kernels. The second question addressed is how to obtain suitable symbolic descriptions of the phonetically relevant features in these time-frequency surfaces. We propose time-frequency ridges in these surfaces, the 2-D analog of spectral peaks, which can be found by examining the derivatives of the time-frequency surface produced above.