Formant estimation method using inverse-filter control

This paper proposes a new method for estimating formant frequencies of speech signals, based on inverse-filter control and zero-crossing frequency distributions. In this method, which is called the inverse-filter control (IFC) method, we use 32 basic inverse filters that are mutually controlled by weighted means of zero-crossing frequency distributions. After quick convergence of the inverse filters, we can gain four to six formant frequencies as final mean-values of the zero-crossing frequencies. The proposed method (IFC) has a specific feature that it directly estimates resonant frequencies of a vocal tract, unlike analysis-by-synthesis (A-b-S) or linear predictive coding (LPC) as a spectral matching method. Therefore, spectral shapes influence indirectly alone the formant estimation in the IFC. Although the superiority of IFC to LPC was not necessarily prominent in the systematic evaluation using synthetic speech, the estimates showed satisfactorily small errors for the practical analysis. On the other hand, when observing some analysis examples of real speech, we found many fewer gross errors in IFC than in LPC. Last, we describe in brief a method for estimating a spectral envelope (or formant bandwidths) based on the obtained formant frequencies and the spectrum to be analyzed. According to the results, it is understandable that the existence of the wide-band formants also contributes to stable formant trajectories.