ACOUSTIC COUPLING IN PHONATION AND ITS EFFECT ON ...

27data points to obtain an acceptable variance, thus necessitating the use of several cyclesand affecting the accuracy of the estimates (as in the case of the autocorrelationmethod of linear prediction). In addition cepstral inverse filtering has shown difficultiesdue to the phase unwrapping constraints associated with the method. Therefore,these more complex methods tend to be used along with other schemes, such asARMA or glottal models. Even so, these methods have not shown to be more reliablethat the CPIF method previously described.Nonlinear inverse filtering was designed in an attempt to extract the uncoupledglottal airflow in a volume velocity signal [47]. This method differs from all the abovein that it attempts to remove not only the vocal tract filtering, but also its interactionwith the glottal source. A time-varying Norton equivalent airflow of the glottal sourcewas used along a feedback procedure. This inverse filter is nonlinear in that certainnonlinear parameters were used. However, the method had a number of limitationsthat have been previously discussed and resulted in a more complex implementation,and thus the scheme has not been adopted widely.All the aforementioned studies illustrate that CPIF is currently the most reliablemethod to estimate the glottal airflow from oral airflow or radiated pressure recordings,particularly for applications related to the assessment of normal and pathologicalvoice function. Its ability to describe the “true glottal flow” have not been revised,particularly under an incomplete glottal closure scenario. Furthermore, the schemecannot be used to estimate the subglottal tract impedance, as it does not fit with theall-pole requirement of the covariance method of linear prediction.2.5 Clinical assessment of vocal functionThe most common voice disorders are chronic or recurring conditions that arelikely to result from inappropriate patterns of vocal behavior referred to as vocal“hyperfunction” [25]. Clinical assessment of vocal function is pivotal to evaluate thepresence of vocal hyperfunction before and after any vocal intervention (e.g., surgery,