A stochastic mechanical model to generate jitter in the production of voiced sounds

Abstract

The quasi-periodic oscillation of the vocal folds causes perturbations in the length of the glottal cycles which are known as jitter. The observation of the glottal cycles variations suggests that jitter is a random phenomenon described by random deviations of the glottal cycle lengths in relation to a corresponding mean value and, in general, its values are expressed as a percentage of the duration of the glottal pulse. The jitter has been the subject for researchers due to its important applications such as identification of pathological voices (nodulus in the vocal folds, paralysis of the vocal folds, or even, the vocal aging, among others). Large values for jitter variation can indicate a pathological characteristic of the voice. The objective of this paper is to construct a stochastic model of jitter using a mechanical model of the vocal folds. This model assumes complete right-left symmetry of the vocal folds and considers motion of the vocal folds only in the horizontal direction. The corresponding stiffness of each vocal fold is considered as a stochastic process and its modelling is proposed. The probability density functions of the fundamental frequency related to the voices produced are constructed and compared for different levels of jitter. Some samples of synthesized voices in these cases are obtained.