496 BELL SYSTEM TECHNICAL JOURNAL 



copying the human mechanism in principle, it was soon apparent that 

 carrier circuits were being set up. Tracing the carrier idea back to 

 the voice mechanism there was unfolded, a little at a time, the carrier 

 nature of speech. Ultimately the speech mechanism was revealed in 

 its simplest terms as a mechanical sender of acoustic waves analogous 

 to the electrical sender of electromagnetic waves in the form of the 

 radio transmitter. Each of these senders embodies a modulating de- 

 vice for molding message information on a carrier wave suitable for 

 propagation of energy through a transmission medium between the 

 sending and receiving points. 



The Carrier Elements of Speech 



This carrier basis of speech will be illustrated by simple speech 

 examples selected to show separately the three carrier elements of 

 speech, namely, the carrier wave, the message wave, and their com- 

 bining by a modulating mechanism. These illustrations serve the 

 purpose of broad definitions of the carrier elements in speech. 



The illustration chosen for the carrier wave of speech is a talker's 

 sustained tone such as the sound "ah." In the idealized case there is 

 no variation of intensity, spectrum or frequency. This carrier then is 

 audible but contains no information, for information is dynamic,^ ever 

 changing. The carrier provides the connecting link to the listener's 

 ear over which information can be carried. Thus the talker may pass 

 information over this link by starting and stopping in a prearranged 

 code the vocal tone as in imitating a telegraph buzzer. For trans- 

 mitting information it is necessary to modulate this carrier with the 

 message to be transmitted. 



For the second illustration, message waves are produced as muscular 

 motions in the vocal tract of a "silent talker" as he goes through all 

 the vocal effort of talking except that he holds his breath. The mes- 

 sage is inaudible because the motions are at slow syllabic rates limited 

 by the relatively sluggish muscular actions in the vocal tract. Never- 

 theless these motions contain the dynamic speech information as is 

 proved by their interpretation by lip readers to the extent visibility 

 permits. Another method of demonstrating the information content 

 of certain of these motions is the artificial injection of a sound stream 

 into the back of the mouth for a "carrier" whereby intelligible speech 



2 The information referred to is that in the communication of intelligence. There 

 is, however, static information in the carrier itself. This serves for "station identi- 

 fication" in radio and may similarly help in telling whether it was Uncle Bill or 

 Aunt Sue who said "ah." 



