Speech and Audio Signal Processing: Processing and Perception of Speech and Music, Second Edition

CHAPTER 20

THE CEPSTRUM AS A SPECTRAL ANALYZER

20.1 INTRODUCTION

In Chapters 11 and 12, models of speech and music production were introduced. The basic structure of these models could be identified as an excitation that was input to a system of resonators; the convolution of the former with the impulse response of the latter component produced the approximation to the modeled speech or music signal. It is therefore natural to contemplate an analysis of the signal as a separation of two components corresponding to source (or excitation) and filter (or resonator) respectively. This is an example of a process that is often called deconvolution, or the separation out of a signal from an impulse response that has been convolved with it. In the channel vocoder, for example, the excitation is modeled as either a quasi-periodic pulse train (caused by vocal cord vibration) or a noise signal caused by turbulence. In Chapter 16 we studied how the auditory system perceives the pulse train component of the excitation; in Chapter 31, methods of detecting both the periodic and noisy components will be reviewed. In Chapter 19 there was a brief discussion of how a vocoder analyzer models the spectral envelope, which is a function of the vocal tract articulator positions. In summary, a channel vocoder separates excitation from the filter, and therefore goes some way towards performing deconvolution. ...