400 BELL SYSTEM TECHNICAL JOURNAL 



better than the complete system. For PCM, however, with regenerative 

 repeaters the required signal-to-noise ratio in the radio part of the system 

 is independent of the number of links. Hence, we have a method of trans- 

 mission that is ideally suited to long repeated systems. 



At this point we might consider the bandwidth required to send this type 

 of signal. For a 5-digit code the required band is somewhat less than 5 

 times that required for a PAM system. It is somewhat less than 5 times be- 

 cause in a multiplex system crosstalk becomes a serious problem. In a PAM 

 system this crosstalk would add up on a long system in somewhat the same 

 manner as noise. In order to reduce the crosstalk it would probably be 

 necessary to use a wider band for the PAM repeater system than would be 

 required for a single-link system. For PCM, on the other hand, by using 

 regeneration the whole system requirement for crosstalk can be used for 

 each link. In addition, a relatively greater amount of crosstalk can be 

 tolerated since only the presence or absence of a pulse needs to be determined. 

 Both of these factors favor PCM. This is a big subject and for the present 

 we need only conclude that from considerations of the type just given the 

 bandwidth penalty of PCM is not nearly as great as might first be expected. . 



The same two factors that were mentioned in connection with crosstalk 

 also apply to noise, and a PCM signal can be transmitted over a circuit 

 which has a much lower signal-to-noise ratio than would be required to 

 transmit a PAM signal, for example. 



Hence, we conclude that PCM for a long repeated system has some 

 powerful arguments on its side because of its superior performance even 

 though it may require somewhat greater bandwidth. There are other fac- 

 tors where PCM differs from more conventional systems but a discussion of 

 these factors is beyond the scope of this paper. 



The previous discussion may be summarized as follows: One begins with 

 a pulse amplitude modulation system in which the pulse amplitude is 

 modulated above and below a mean or d-c value as indicated in Fig. 1. 

 It is assumed that it will be satisfactory to limit the amplitude range to be 

 transmitted to a definite number of amplitude levels. This enables each 

 PAM pulse to be represented by a code group of ON-OFF pulses, where the 

 number of ampUtude levels is given by 2^, n being the number of elements 

 in each code group. With this system the digit pulses can be "regenerated" 

 and the quality of the overall transmission system can be made to depend 

 upon the terminal equipment alone. 



Experimental PCM Equipment 



The experimental coder used in these studies might be designated as one 

 of the "feedback subtraction type". It functions as follows: Each PAM 

 pulse is stored as a charge on a condenser in a storage circuit. (See Fig. 4.) 



