504 BELL SYSTEM TECHNICAL JOURNAL 



air in the vocal system, resulting in a set of air waves that likewise 

 contain all the information of speech. These airborne message waves, 

 however, are at syllabic rates and so below the frequency range of 

 audibility. 



The Voice Modulators 



The three voice modulators associated with the three speech messages 

 are the mechanisms of (a) selecting the carrier, {h) setting the funda- 

 mental frequency and (c) controlling the selective transmission. The 

 mechanism for starting and stopping a voice carrier is simple. Assume 

 a sustained carrier of either the voiced or unvoiced type. It can be 

 stopped by opening the constriction at which it is formed. This alters 

 the acoustic impedance of the opening which is then the modulating 

 element in this case. 



The modulating mechanism for controlling the fundamental fre- 

 quency appears in the vibrating portions of air at the glottis. The 

 exact mechanism is of no importance here so long as the message wave 

 at the vocal cords finds means for altering the fundamental frequency 

 under the control of the will.^ This is a case of frequency modulation 

 of multiple carriers harmonically related. 



The modulating mechanism for controlling the transmission through 

 the vocal tract as a function of frequency consists of the masses and 

 stiffnesses of air chambers and openings in the vocal tract. These are 

 varied under control of the message in the form of muscular displace- 

 ments of vocal tract parts. There is a more complicated modulation 

 in the vocal tract than in the usual electrical circuit for amplitude 

 modulation because the varying impedances are reactive in the voice 

 mechanism but resistive in the electrical circuit and also because 

 several independent modulator elements are used in the voice mecha- 

 nism as against either a single one or a group functioning as a unit in 

 the simple electrical modulator. The reactive nature of the vocal 

 impedances leads to the selective control of the amplitudes of the 

 various harmonics of the voice carrier. The amplitude modulation of 

 each carrier component by the combined message waves produces an 

 output containing the carrier and sideband frequencies. 



Comparison of Speech Synthesizing Circuits 



The fundamental processes in human speech production are thus 

 analogous to those of electrical carrier circuits. There is a switching 

 of voice carrier energy comparable to that in voice frequency telegraph ; 



8 For a simplified theory of the larynx vibration see R. L. Wegel, Bell Sys. Tech. 

 Jour., Vol. 9, p. 207 (1930) and Jour. Acous. Soc. Amer., Vol. 1, Supp. p. 1, April 1930. 

 The analogy of the larynx to a vacuum tube oscillator is described in an abstract, 

 Jour. Acous. Soc. Amer., Vol. 1, p. 33 (1929). 



