502 BELL SYSTEM TECHNICAL JOURNAL 



a long period of time the question arises whether to use a long- or short- 

 term value of the carrier. The answer may well be the same as in 

 the analogous radio problem. If weather causes a carrier frequency 

 to be slightly high all day, this higher value is taken as the normal 

 carrier in studying short-term effects such as the degree of modulation. 

 But in long-term studies of carrier stability the deviations from the 

 mean represent a frequency modulation which is observed as a "mes- 

 sage" effect. 



Due to the inseparability of the message wave motion and its asso- 

 ciated wave of impedance change in the modulating mechanism there 

 may be confusion in distinguishing between the modulating elements 

 and the source of the message waves. The rule followed here is simple. 

 From the standpoint of the human flesh lining the vocal tract, the 

 message source is internal, the modulating elements, external. The 

 message consists of those muscular motions (or pressures or displace- 

 ments) in the vocal tract which are present in the "silent talker" 

 and are volitional in nature. This definition excludes the oscillatory 

 motions which make up the carrier. The modulating elements are 

 acoustic in nature since the carrier starts as a sound stream and ends 

 as a modulated sound stream. 



There are three important variations of the voice carrier and so 

 three types of message and of associated modulation. These varia- 

 tions are: first, selecting the carrier; second, setting the fundamental 

 frequency of the voiced carrier; and third, controlling the selective 

 transmission of the vocal tract.'' The message waves in the three 

 cases will be discussed with the corresponding modulation reserved for 

 consideration under the next heading. 



Selecting the carrier appears as a simple start-stop message, com- 

 plicated somewhat by the presence of two types of carrier and by lo- 

 cating the constriction for the unvoiced type at several places in the 

 vocal tract. We may think of a start-stop type of message for each 

 point where constrictions are formed, including the vocal cords for the 

 voiced type of carrier. A constriction message may be plotted as the 

 opening between vocal parts at the constriction with critical values 

 for the onset of audible carrier. The constrictions are to a certain 

 extent independent. Thus with the vocal cords vibrating, a constric- 

 tion from the tongue tip to the upper teeth may also be formed, as in 

 making the "z" sound. Again, in whispering, there may be simul- 



^ A fourth message characteristic prescribes the intensity of the speech. This 

 message may be included in the carrier selection if the carrier is selected for intensity 

 as well as type. The matter of intensity is passed over rather lightly here because a 

 comparison is being developed between the human and electrical speech synthesizers 

 with the final intensity in the latter under control of an amplifier setting. 



