A Formant Tracker for Speech Sounds

Abstract

The central frequencies of the principal resonance regions, or formants, have been found to contain sufficient information to resynthesize intelligible speech, as far as vowels and other nonturbulent sounds are concerned. When a single formant is isolated by filtering, one-half the average zero-crossing density, ρ0, is shown to be a convenient approximation to the formant's central frequency. Filters which are controlled electronically by a preliminary approximation to the first-formant frequency, in such a way as to isolate the first two formant regions, have been built and tested. LP and HP filters for the first and second formants, respectively, are based on a circuit by Linvill which uses a transistor negative-impedance converter. An alternate filter of the tuned-circuit type has proved more satisfactory for the first formant. The variable elements in the HP and tuned-circuit filters are Increductors, while the LP filter uses a circuit based on the Miller effect to represent an electronically controllable capacitor. [The research in this paper has been made possible through support and sponsorship extended by the Electronics Research Directorate of the Air Force Cambridge Research Center, under Contract No. AF19(604)-1039, Item I.]