instantaneous Compandors 



By C. O. MALLINCKRODT 



(Manuscript Received Apr. 9, 1951) 



Instantaneous compandors have found useful application in time-division 

 systems. This paper discusses the theory of the instantaneous compandor and 

 evaluates the noise advantage when instantaneous companding is applied to 

 telephone channels. The noise advantage depends upon the noise standard of the 

 system. If the standard corresponds to that of a toll telephone system, a noise 

 advantage of about 20 db is possible. 



Introduction 



A compandor is characterized by compression followed by expansion. 

 To achieve noise reduction by compandor action,* compression is applied 

 before and expansion after the noise exposure. By compression one means 

 that the effective gain which is appHed to a signal varies as a function of the 

 magnitude of the signal, the effective gain being greater for small than for 

 large signals. In the process of expansion the effective gain also varies as a 

 function of the signal but is greater for large than for small signals. 



There are two general classes^ of compandors, "syllabic" and "instantane- 

 ous." For many years, because of theoretical and practical reasons, only the 

 syllabic type was used to any appreciable extent. Although utilized pri- 

 marily in special situations'"^, syllabic compandors have in these instances 

 served to improve telephone operation by providing a substantial noise 

 advantage. More recently* ~^^ the instantaneous type also has begun to find 

 important applications to time-division systems. Since an instantaneous 

 compandor produces effective gain variations in response to instantaneous 

 values of the signal wave, the instantaneous type is well adapted to pulse 

 systems. Moreover, in time-division pulse-modulation systems, one in- 

 stantaneous compandor usually serves a pluraUty of channels thereby afford- 

 ing additional economies. 



Theory 



Noise advantage due to compandor action arises primarily because it is 

 the weak signals that are most susceptible to degradation by noise or other 

 unwanted interference. Accordingly, weak signals are highly amplified by 

 the compressor and are carried at a relatively high level through the noise 



• For all numl>ered references, see list at end of paper. 



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