70 BELL SYSTEM TECHNICAL JOURNAL 



the oscillator, it being very difficult to match sounds for intensity 

 which differ in quality. 



A very satisfactory type of audio-frequency generator is shown in 

 Fig. 36; it is a buzzer which operates, not by making and breaking 

 current, but by varying it periodically with a microphonic button. 

 The vibrating parts of this generator may be tuned to any audio- 

 frequency, e.g. 800 cycles, and it gives quite accurately a sinusoidal 

 variation of current, although it is customary to insert a filter (Sec. 6) 

 to insure the input energy being accurately of one frequency. 



VII. Thermionic Modulators 



In discussing modulation the terminology which has been de- 

 veloped in connection with radio and carrier-current signaling will 

 be used. 



By the term "modulation" is meant the varying of the amplitude of 

 a relatively high frequency wave, so that its envelope represents a 

 particular low frequency wave or combination of such waves. (See 

 Curves A, B and C, Fig. 37). The combination of low frequency 



l'H;i :■■'■•.•:. ii-m. ..,,.- ,,..,... .-[,, ,,ni,| 



u.sJ!iiii,!iJii,il::l; l :,i:i.!lJiiillli:lmjli;Mjji 



If). ilTin^fTf, -f)""' 



'uiliH |lk ' ''''tilliii"'" '''iii.iii 



Fig. 37 



modulating waves may be very complicated, as in the case of speech, 

 but the principle involved is common to all cases of modulation and 

 can be clearly brought out by the consideration of a single low fre- 

 quency. 



