CARRIER NATURE OF SPEECH 



509 



the unvoiced sounds. In the vocoder, for simplicity's sake, one or the 

 other of these energy sources is used according to whether the sound is 

 voiced or unvoiced, with no provision for the mixed types of sounds 

 found in the human voice. The analyzer of the vocoder derives the 



MODULATORS I RADIATOR I SPEECH i 



MECHANICAL 





Fig. 8 — Schematic circuit of the voder. 



original speech message in terms of a modified set of parameters. This 

 analyzer suppresses the original carrier of the talker and so resembles 

 the demodulator in radio reception. The analyzer acts as an electrical 

 ear to tell the artificial vocal system of the vocoder what to say, the 

 whole vocoder acting as a synthetic mimicker. 



The basic similarity of the electrical and human speech synthesizers 

 is seen in these figures. In all three cases the message is originated 

 by the brain of the sender of the speech information. There is in each 

 case a transmission of control impulses by the talker's nervous system 

 to the appropriate muscles. The muscles produce displacements of 

 body parts formulating the speech information as a set of mechanical 

 waves. These waves appear in the vocal tract in the case of normal 

 speech; in the fingers, wrist and foot in the case of the voder, but in 

 the case of the vocoder use is made of electrical currents derived from 

 and equivalent to the vocal tract displacements in ordinary speech. 

 In each case the message contains the speech information in syllabic 

 waves. In all cases the message waves control the choice of carrier, 

 the fundamental frequency of the voiced type carrier and the spectrum 

 of power distribution in the speech output. Differences arise in the 

 details rather than in the principles. 



