A vowel synthesizer based on formant sinusoids modulated by fundamental frequency
Ingo Hertrich, Hermann Ackermann · The Journal of the Acoustical Society of America · 1999
A vowel synthesis algorithm was developed resembling parallel formant synthesizers. However, the formants are computed as amplitude-and phase-modulated sinusoids instead of being represented by recursive filters or resonators. The fundamental frequency is imposed on the signal in the following way: Each pitch period exhibits an initial rise of a duration of π/2 of the first formant frequency; at the end of this ramp period all formants start as sinusoids at predefined phase angles and amplitudes, and successively undergo linear attenuation toward zero amplitude at the end of each pitch period. Adjacent pitch periods, thus, do not influence each other. In principle, the signals produced by this algorithm are similar to the output of parallel formant synthesizers [e.g., D. H. Klatt, J. Acoust. Soc. Am. 67, 971–995 (1980)]. However, by incremental sample-by-sample computation of the formants’ phase angles, formant frequencies are allowed to continuously change within single pitch periods. This new algorithm creates vowels as well as formant transitions signaling stop consonant–vowel syllables such as /ba/. The output signals exhibit a homogeneous, stereotypic voice quality. Because of its additive procedure, this method might allow us to implement a variety of further acoustic aspects of the human speaking voice in a well-controled manner.