The same authors quantified the modulation extents after their simulation experiments reported in [17]: amplitude modulation extent between 5% and 50% and frequency modulation extent between 0.5% and 3.5%.
Furthermore, Kreiman et al. showed that modulation extent affects perception more than modulation rate [13].
As for modulation parameters, modulation extent has been reported to be much more significant for the perception of tremor than modulation rate [9,10].
Similarly to the case of respiratory tremor, its main effect is amplitude modulation of the voice signal but it also causes frequency modulation with a lower modulation extent.
Jiang et al. and Anand et al. showed that for pathological voice tremors, with rates between 4 and 7 Hz, the difference with physiological voice tremor was more in the modulation extent (or modulation amplitude) than in the modulation rate [9,10].
Regarding perception, frequency modulation seems to be the most relevant feature for the perceptual detection of vocal tremor [10,13] but tremor is only perceived if its intensity (i.e. modulation extent) is above a certain threshold [6].
Since A t) and f(t) are expected to have random values obtained from unbounded distributions, the modulation extent has been defined as the normalized standard deviation of their values along a given voice signal: (overline {sigma }_{f}) for frequency modulation and (overline {sigma }_{A}) for amplitude modulation (Equation 7). Figure 4 shows the results of simulating respiratory tremor.
Therefore, the jump in modulation extent that can be observed in Figure 4 near (overline {sigma }_{p} = 2%) is consistent with the existence of a threshold for the perception of respiratory tremor, as reported by Farinella et al. [6].
Results are presented in terms of modulation extents, both for amplitude and for frequency modulations, bearing in mind the idea that diverse anatomic sources of tremor should produce different modulation extents [24].
Normalized amplitude and frequency modulation extents for different tremor bandwidths (2.5, 5, 7.5, and 10 Hz) as a function of the normalized standard deviation of lip opening (overline {sigma }_{S}).
