The Bell System Technical Journal 



Vol. XIX October, 1940 No. 4 



The Carrier Nature of Speech 



By HOMER DUDLEY 



Speech synthesizing is here discussed in the terminology of 

 carrier circuits. The speaker is pictured as a sort of radio broad- 

 cast transmitter with the message to be sent out originating in the 

 studio of the talker's brain and manifesting itself in muscular wave 

 motions in the vocal tract. Although these motions contain the 

 message, they are inaudible because they occur at syllabic rates. 

 An audible sound is needed to pass the message into the listener's 

 ear. This is provided by the carrier in the form of a group of 

 higher frequency waves in the audible range set up by oscillatory 

 action at the vocal cords or elsewhere in the vocal tract. These 

 carrier waves either in their generation or their transmission are 

 modulated by the message waves to form the speech waves. As 

 the speech waves contain the message information on an audible 

 carrier they are adapted to broadcast reception by receiving sets 

 in the form of listeners' ears. The message is then recovered by 

 the listeners' minds. 



SPEECH is like a radio wave in that information is transmitted over 

 a suitably chosen carrier. In fact the modern radio broadcast 

 system is but an electrical analogue of man 's acoustic broadcast sys- 

 tem supplied by nature. Communication by speech consists in a 

 sending by one mind and the receiving by another of a succession of 

 phonetic symbols with some emotional content added. Such material 

 of itself changes gradually at syllabic rates and so is inaudible. Ac- 

 cordingly, an audible sound stream is interposed between the talker's 

 brain and the listener. On this sound stream there is molded an im- 

 print of the message. The listener receives the molded sound stream 

 and unravels the imprinted message. 



In the past this carrier nature has been obscured by the complexity 

 of speech.^ However, in developing electrical speech synthesizers 



^ Speech-making processes are here explained in the terms of the carrier engineer 

 to give a clearer insight into the physical nature of speech. The point of view is essen- 

 tially that of the philologist who associates a message of tongue and lip positions 

 with each sound he hears. This aspect also underlies the gesture theory of speech 

 by Paget and others and the visible speech ideas of Alexander Melville Bell. The 

 author has been assisted in expressing speech fundamentals in carrier engineering 

 terms by numerous associates in the Bell Telephone Laboratories experienced in 

 carrier circuit theory. Acknowledgment is made in particular of the contributions 

 of Mr. Lloyd Espenschied. 



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