Vowel Enhancement in Early Stage Spanish Esophageal Speech Using Natural Glottal Flow Pulse and Vocal Tract Frequency Warping

This paper presents an enhancement system for early stage Spanish Esophageal Speech (ES) vowels. The system decomposes the input ES into neoglottal waveform and vocal tract filter components using Iterative Adaptive Inverse Filtering (IAIF). The neoglottal waveform is further decomposed into fundamental frequency F0, Harmonic to Noise Ratio (HNR), and neoglottal source spectrum. The enhanced neoglottal source signal is constructed using a natural glottal flow pulse computed from real speech. The F0 and HNR are replaced with natural speech F0 and HNR. The vocal tract formant frequencies (spectral peaks) and bandwidths are smoothed, the formants are shifted downward using second order frequency warping polynomial and the bandwidth is increased to make it close to the natural speech. The system is evaluated using subjective listening tests on the Spanish ES vowels /a/, /e/, /i/, /o/, /u/. The Mean Opinion Score (MOS) shows significant improvement in the overall quality (naturalness and intelligibility) of the vowels.