INSTANTANEOUS COMPANDORS 



713 



It is apparent that the characteristic cannot follow the exponential law 

 at low values of input voltage, because, if the relationship is exponential, E 

 is not zero when V is zero. This difficulty is avoided by using a characteristic 

 which is linear for input voltages below a given value and exponential for 

 input voltages above this value. A characteristic of this type is illustrated 

 in Fig. 5. The point at which the characteristic changes from a linear to an 





DO 

 Q.-I 



DID 

 O 



0.1 



0.4 0.5 0.6 



0.7 



0.8 0.9 1.0 



0.020 





0.012 



mO 



< 0.004 



0.05 



0.10 0.15 



INPUT VOLTAGE 



0.20 



0.25 



Fig. 5 — Expander characteristic. 



exponential relationship is referred to as the '^ transition point." The transi- 

 tion from one function to the other occurs smoothly and the first derivative 

 of the output with respect to the input is continuous at the transition point. 



Logarithmic Compandor 



Since the characteristic of the compandor is an odd function, all formulas 

 will be limited to the positive portion. The exponential portion of the ex- 

 pandor characteristic is given by 



(V-l)IVt 



(5) 



