Two 1/f fluctuations in sustained phonation and their roles on naturalness of synthetic voice

Two fluctuations are always observed in sustained phonation. These have been called shimmer (perturbations of glottal pulse amplitudes) and jitter (perturbations of pitch period durations). Both are known as involuntary laryngeal behaviors during the phonation. Our study on these phenomena revealed that their characteristics from nonpathological phonation are not completely random but are regulated by the power law, that is widely known as 1/f^γ fluctuation. In order to identify the roles of 1/f^γ jitter and shimmer, a series of psychoacoustic experiments was conducted. In a paired comparisons test each of twenty-five subjects individually was instructed to select more human-like synthetic voices among stimuli with four different jitter patterns such as Gaussian white noise, 1/f^γ fluctuation, summation of three sinusoids fluctuation, and no fluctuation. Another comparisons test about stimuli with three different shimmer patterns was conducted. The results of the experiments supported the idea that both 1/f^γ jitter and shimmer are crucial cues for a synthetic voice to be perceived as natural