Reliable glottal-closure-instant (GCI) estimation from short analysis frames

It is well known that the first formant is maximally excited at the instant of glottal closure. Therefore, it is natural to utilize the energy in a band containing the first formant as a cue to the GCI. In practice, however, the actual GCI lies a few samples prior to where this energy signal attains a local maximum. Moreover, such an estimate makes no use of any period information regarding the GCI's. Consequently, secondary excitations within a period can lead to spurious GCI's. It is therefore proposed to augment the information contained in the first formant with the linear prediction error. Although, prediction error has been widely used for pitch determination, it is not sufficient to locate the GCI reliably because of ambiguities arising from multiple peaks, especially for vowels like /u/ (as in foot). Interestingly, these experiments have shown that secondary excitations tend to result in peaks in the residual error signal at locations different from those in the formant energy signal. Furthermore, in the absence of spurious excitation, the residual error can contain valuable independent period information. Therefore, the product of these two signals yields accurate GCI estimates. Such an algorithm has been tested on all vowels in a variety of environments and has been found to be very robust. Analysis frames as short as 5–10 ms have been used.