INSTANTANEOUS COMPANDORS 



709 



after they have been compressed than before. Clearly, if the samples are 

 compressed in accordance with an arbitrary but known law, and if the re- 

 ceiver expands them by an exactly inverse operation, the wanted informa- 

 tion can be recovered. 



Pulses from the sending end are fed to the transmitting medium (Fig. 1) 

 and conveyed to the receiving terminal. This might be done by any of a 

 number of different ways and the details of this portion of the system are not 

 important to this discussion. What is important is that the analysis will 

 assume that the signal at the input to the receiving end, except for noise 

 accumulated along the way, is an exact but delayed copy of the signal leav- 

 ing the transmitting end. The low-pass filter ¥{ (Fig. 1) is similar to Fi 

 and has been inserted to reject unwanted high-frequency noise. 



INPUT VOLTAGE 

 COMPRESSOR 



OUTPUT VOLTAGE 

 EXPANDOR 



Fig. 3 — Instantaneous compandor. 



The filtered PAM pulses go to the input of an instantaneous wide-band 

 expandor whose characteristic is the inverse of that of the compressor. 

 By interchanging the designations "input" and "output" on the compressor 

 characteristic (Fig. 3) the characteristic becomes that of an expandor. The 

 combination of compressor and expandor makes the over-all system linear as 

 illustrated by the dotted lines in Fig. 3. The pulses from the expandor go to 

 the sampler which is accurately synchronized® . 7 , lo . n^ Channel 1 pulses 

 from the sampler go to F3, a low-pass filter similar to Fi, and produce in the 

 output of Fz a copy of the original signaP together with noise accumulated 

 for the most part in the transmission medium. 



Signal-to-Noise Ratio 



To understand how the compandor afTects the signal-to-noise ratio of 

 the system, consider a single operation at the receiver. The magnitude of a 



