708 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



determine the samples; if we are given the samples we can determine the 

 signal wave. 



In Fig. 1, if we ignore the compressor, the samples are immediately fil- 

 tered by another low-pass filter designated F2 which temporarily will be 

 assumed to be similar to Fx. If F2 attenuates all frequencies higher than 

 B and if each filter includes accurate in-band equalization including cor- 

 rection for phase distortion, then the wave at the output of F2 except for 

 delay will be an attenuated replica^^ of the wave at the output of Fx. 



TRANSMITTING TERMINAL TRANSMITTING RECEIVING TERMINAL 



TRANSMITTING 

 MEDIUM 



INPUT SIGNAL 



CHANNEL 1 



\ 



COMPRESSOR 



LOW- 

 PASS 

 FILTER 



TO 



OTHER 



CHANNELS 



i 



LOW- 

 PASS 

 FILTER 



EIHIKl^ 



LOW- 

 PASS 

 FILTER 



EX PAND OR CHA NN EL 1 QujpuT 



'EH 



LOW- 

 PASS 

 FILTER 



\=:z\ TO 



y OTHER 

 CHANNELS 



SEQUENTIAL 

 SAMPLER 



SAMPLER & 

 SEQUENTIAL 

 DISTRIBUTOR 



Fig. 1 — Block schematic of multi-channel PAM system. 



Fig. 2— PAM pulses. 



If the cut-off frequency of Fi is raised sufficiently, then in the output 

 of this filter one finds PAM pulses clearly separated in time. These pulses 

 are samples, on an enlarged scale, of the signal that would have existed had 

 the cut-off frequency been in the neighborhood of B. 



If now the compressor (Fig. 1) is taken into account, then the PAM pulses 

 at the output of the sequential sampler will be impressed upon the input 

 of the compressor. The general form of a compressor characteristic is indi- 

 cated in Fig. 3. The compressor is essentially instantaneous if its bandwidth 

 is wide enough so that it can effect the required change in the magnitude of 

 each pulse without increasing its duration. It is also significant to note 

 that, theoretically, no more bandwidth^^ js needed to transmit the samples 



