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Abstract:
Jitter and shimmer designate small random cycle-to-cycle
perturbations of the glottal cycle length and signal magnitude
respectively. Definitions of shimmer involve different types of
signals and different kinds of signal markers. Shimmer therefore
appears to refer to a random pattern rather than to a narrowly
defined physiological property of the laryngeal oscillator. In
addition, a common observation is that numerical attributes of
jitter and shimmer are correlated. This would suggest that
shimmer is not an autonomous phenomenon but rather a consequence
of small fluctuations of the glottal cycle lengths. We therefore
discuss three simple models in the framework of which a frequency
modulation of the input is converted into an amplitude modulation
of the output. The first model is based on left-right asymmetries
of the instantaneous frequencies of vibration of the glottal
walls. The second model refers to perturbations of the
instantaneous frequencies of vibration of the upper and lower
margins of the vocal folds. The third model, finally, simulates
the amplitude modulation of the speech signal owing to the
frequency modulation of the excitation signal. In the framework
of the first two models the amplitude modulation of the glottal
volume is a consequence of frequency modulations of the glottal
walls that are spatially variable. In the framework of the third
model, the amplitude modulation arises owing to the non-flat
vocal tract transfer function.
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