"His rather crude circuit ... consisted of an electrical buzzer to simulate the vocal cords, with a pair of inductive/capactive resonators to create resonance of the throat and mouth. The resultant speech from his electrical analog, which at best could produce two formant frequencies, consisted primarily of vowel sounds. As he varied the capacitance, resistance, and inductance of the circuit components he could continuously change from one vowel sound to another, thus also creating a few diphthongs."

John P. Cater, "Electronically Speaking: Computer Speech Generation", Howard M. Sams & Co., 1983, p. 75.

J. Q. Stewart's voice circuit