Speech processing using group delay functions

In this paper we demonstrate the feasibility of processing the Fourier transform (FT) phase of a speech signal to derive the smooth log magnitude spectrum corresponding to the vocal tract system. We exploit the additive property of the group delay function (negative derivative of the FT phase) to process the FT phase. We show that the rapid fluctuations in the log magnitude spectrum and the group delay function are caused by the zeroes of the z-transform of the excitation components of the speech signal. Zeroes close to the unit circle in the z-plane produce large amplitude spikes in the group delay function and mask the group delay information corresponding to the vocal tract system. We propose a technique to extract the vocal tract system component of the group delay function by using the spectral properties of the excitation signal