CARRIER NATURE OF SPEECH 513 



the bottom. The trace below shows the defining current for the funda- 

 mental frequency, while the ten traces above show currents indicating 

 the rectified power in ten frequency bands of 300 cycles width except 

 that the lowest one extends from to 250 cycles. The slow rates of 

 change are noted in the message currents when compared to the original 

 speech wave. 



Demonstrations of the vocoder indicate to the ear the carrier nature 

 of speech. Thus the carrier used for remaking speech, whether a 

 monotone or a hiss sound, is observed to have no intelligibility when 

 heard alone. The message currents derived from spoken speech are 

 not audible. However, intelligible "speech" is produced by the modu- 

 lation of either type of carrier by the message currents of selective 

 transmission. Similarly, there can be used for the carrier a wide 

 variety of sound from the puffs of a locomotive to instrumental music. 

 Upon imprint of the transmission message currents from spoken speech, 

 new forms of odd sounding but nevertheless intelligible "speech" are 

 produced. 



The carrier conception of speech reveals what is important and not 

 important in evaluating speech characteristics. An example of in- 

 terest is the matter of phase. It has long been known that phase 

 was unimportant to the ear at reasonably low listening levels. From 

 the carrier point of view this is natural, for the phase changes referred 

 to are those in the carrier and so, unimportant. When the phases of 

 the message components are altered, there is a very noticeable effect on 

 the ear, for phonetic units are now being shifted. 



The great advance in recent years in the application of carrier 

 circuits has been guided by mathematical theory. Since in electrical 

 speech synthesizers the carrier and message currents are separated 

 physically, it is possible to use carrier equations expressing the modu- 

 lation phenomenon. Similar equations may be written for the voice 

 mechanism as represented by Fig. 6. This has been done in the at- 

 tached appendix, thus separating speech into syllabic and carrier 

 factors. 



APPENDIX 

 Mathematical Relations 



The speech concepts developed in the body of the paper may be 

 expressed in mathematical terms which not only give the fundamental 

 relations in simplest form but also aid in the application of the well- 

 established carrier technique to speech. For voiced sounds, periodic 



