500 BELL SYSTEM TECHNICAL JOURNAL 



after the generation of the carrier. Modification of the carrier wave 

 characteristics by other than the ampHtude of the message need not 

 be considered here. In the voice mechanism significant ampHtude and 

 frequency modulations of the carrier occur. Phase modulation takes 

 place also but will not be discussed because the listener's ear is not 

 very sensitive to these phase changes in the carrier. 



In attempting to segregate the carrier elements of speech we run 

 into one serious difficulty. In an idealized carrier circuit as shown in 

 Fig. 3 connections can be cut between the two energy sources and the 

 modulator so that each boxed element can be studied independently. 

 With the human flesh of the voice mechanism this is no longer feasible; 

 the use of cadavers would help very little because normal energizing 

 is then impossible. The same difficulty often appears in electrical 

 modulators as, for example, within a modulating vacuum tube where 

 a grid voltage modulates a plate current. In such a case of common 

 parts it is necessary to discuss the action of each of the three elements 

 in the presence of the other two. 



With this carrier theory review as a background we are in a position 

 to analyze the three elements making up the carrier transmitting 

 system of the human voice. While the picture presented is over- 

 simplified in details the principles hold and aid in applying carrier 

 methodology to explain the mechanism of speech. 



The Voice Carrier 



In electrical circuits the carrier is obtained from an oscillatory energy 

 source. The same holds for speech. In the electrical circuit the os- 

 cillatory waves (a-c.) are ordinarily generated from a supply of d-c. 

 energy.^ The same is true in speech with the compressed air in the 

 lungs furnishing the steady supply. Confusion must be avoided, for 

 in speech the conversion of steady to oscillatory energy is often de- 

 scribed as modulation. Here this conversion of energy form will be 

 considered as an oscillatory action so that the term modulation can be 

 reserved for the low-frequency syllabic control of this oscillatory energy 

 to produce the desired speech. Oscillatory then will refer to automatic 

 natural responses while modulatory will refer to forced responses which 

 are controlled volitionally. This distinction is consistent with carrier 

 terminology. 



In the simplest electrical modulating circuits the carrier is a sine 



^ In the usual electrical circuit the carrier is cut off by turning off the output but 

 leaving the carrier oscillator energized as, for example, in voice frequency telegraphy. 

 In the voice mechanism, however, the oscillator is stopped at the source. The 

 difference between the electrical on-off switching and the start-stop switching of 

 speech is not fundamental but results from the use of the most suitable action in 

 each case in view of the conditions prevailing. 



