INSTANTANEOUS COMPANDORS 711 



AEi 



It is apparent (Fig. 4) that — is a function of the slope of the expandor 



characteristic. Represented by Ji, this ratio is an important quantity and 

 will be referred to as the "noise susceptibility of the system." Dividing (1) 

 by (2) and dropping subscripts, 



~ = - (3) 



N vs 



where S/N is the ratio of instantaneous signal to instantaneous noise at 

 the output of Fz and — the corresponding ratio without a compandor. 



Noise Susceptibility 



If it is assumed that the maximum signal is large compared to the noise, 

 then 



Because the characteristic of the expandor is nonlinear, the noise sus- 

 ceptibility, ^, varies as a function of signal input. When 5 is unity the noise 

 susceptibiUty equals that of a linear system. The object of instantaneous 

 companding is to make 5 a predetermined function of the magnitude of the 

 signal. However, the predetermined choice is not entirely arbitrary. To avoid 

 ambiguous signals at the receiver, as the input to the compressor varies con- 

 tinuously from zero to unity, the input-output characteristic must be single 

 valued. 



One notes that if ^ is averaged with respect to the expandor input voltage, 

 the average value is unity regardless of the shape of the characteristic. Simi- 



1 

 larly, if - is averaged with respect to expandor output voltage, the value 



obtained is always unity. 



Important Difference Between Syllabic and Instantaneous Types of Compandors 



At this point it seems advisable to emphasize an important difference 

 between syllabic and instantaneous types of compandors. Signals com- 

 pressed on a syllabic basis can be transmitted in a band not significantly 

 different from that occupied by the original signal. Moreover, the require- 

 ments on the phase and attentuation-frequency characteristics of the path 

 between compressor and expandor are about the same as if the signal were 

 not compressed. Accordingly, syllabic compandors can and have been applied 

 to a wide variety of existing types of systems. 



