326 



BELL SYSTEM TECHNICAL JOURNAL 



sirability of using it depends on other reasons. First, it restores the 

 naturalness of the speech sounds. Second, the apparent magnitude 

 of the noise is greatly reduced since noise comes in at full strength only 

 when speech is loudest and is reduced by the loss introduced by the 

 expandor at times when the energy is low between syllables. When 

 no speech is being transmitted, noises up to a certain limit, which 



20 



5 -10 



-20 



-25 



-30 



-20 -15 -10 -5 5 



INPUT IN DECIBELS REFERRED TO 1 MILLIWATT 



Fig. 6 — Experimental input vs. output characteristics (1000 cycles steady state). 



corresponds to the maximum energy in received speech, are reduced 

 in varying amounts from about 20 db to zero depending on their value. 

 When speech is present the effect of the expandor is determined by 

 the sum of the instantaneous speech and noise voltages, so that the 

 effect on the noise, whether it is large or small, is determined largely 

 by the existing speech intensity. For a circuit having somewhere 

 near the limit of static, the use of the compandor allows on the average 

 5 db more noise than when it is not used. When the noise is less than 



