CARRIER NATURE OF SPEECH 503 



taneous constrictions, both of the unvoiced type, one at the vocal 

 cords and one in the mouth. As the voice has two distinct types of 

 carrier, the vocal cord tone and the breath tone, the selection sets up 

 one of four carrier conditions at any instant: no carrier, vocal cord 

 tone only, breath tone only, or a combination of vocal cord tone and 

 breath tone. This start-stop message resembles the on-off type of 

 telegraph where switching controlled by other muscular motions sets 

 up speech information in another code, that of telegraph. As men- 

 tioned earlier a communication system can be made with the vocal 

 system by starting and stopping a voice carrier in a vocal imitation of 

 a telegraph buzzer. While this would be a clumsy way of communi- 

 cating information it marks the start-stop control of the voice carrier 

 as a speech message and not part of the voice carrier. Another check 

 is that the "silent talker" does form such constrictions. 



The second type of message wave specifies the fundamental fre- 

 quency with any related voice changes for the voiced type of carrier. 

 This message, in a mechanical form, may be the time variation of the 

 tension of the vocal cords. As the frequency of each upper harmonic 

 is changed in the same ratio as the fundamental frequency, a single 

 parameter suffices for all of the carrier components. The unvoiced 

 carrier has no message of this type impressed since the unvoiced sounds 

 are not characterized by pitch. 



The third and final type of message wave controls the selective 

 transmission in the vocal tract. By comparison, the first two types 

 of message are simple, with the selecting of carriers ideally changing 

 all components of the carrier by the same amplitude factor and the 

 fundamental frequency control changing them by a uniform frequency 

 factor. The vocal transmission, however, results from a multi-reso- 

 nance condition with more than one degree of freedom. There follows 

 a selective amplitude modulation with some carrier components de- 

 creasing in amplitude at the same instant that others are increasing. 

 Maximum transmission occurs when a component coincides with an 

 overall resonance, minimum transmission when it coincides with an 

 anti-resonance and intermediate transmission for other cases. The 

 voice message for transmission appears in mechanical form as the dis- 

 placements of lips, teeth, tongue, etc., with as many such displacements 

 considered as are needed for adequately expressing the speech content. 

 This infers finding the simplest lumped impedance structure equivalent 

 to the distributed impedance structure of the vocal tract to the neces- 

 sary degree of approximation. 



All these mechanical displacements of vocal parts that together 

 constitute the voice message lead to corresponding displacements of 



