Glottal inverse filtering with the closed-phase covariance analysis utilizing mathematical constraints in modelling of the vocal tract.

Closed-phase (CP) covariance analysis is a glottal inverse filtering method based on the estimation of the vocal tract with linear prediction (LP) during the closed phase of the vocal fold vibration cycle. Since the closed phase is typically short, the analysis is vulnerable with respect to the extraction of the covariance frame position. The present study proposes a modified CP algorithm based on imposing certain predefined values on the gains of the vocal tract inverse filter at angular frequencies of 0 and pi in optimizing filter coefficients. With these constraints, vocal tract models are less prone to show false low-frequency roots. Experiments show that the algorithm improves the robustness of the CP analysis on the covariance frame position.