722 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



INTRODUCTION 



Pulse transmission is a basic concept in communication theory and 

 certain methods of modulating pulses to carry infoiTaation approach in 

 their characteristics the ideal performance allowed by nature. In certain 

 applications, such as telegraphy, pulse signalling and data transmission, 

 it has the advantage of great accuracy, since the information is trans- 

 mitted in digital form by "on-off" pulses. This at the same time facili- 

 tates regeneration of pulses to avoid accumulation of distortion from 

 noise and other system imperfections, together with the storing, auto- 

 matic checking and ciphering of messages, as well as their translation 

 into different digital systems or transmission at different speeds, as may 

 be required in extensive communication systems. Another characteristic 

 of pulse systems is that improved signal-to-noise ratio can be secured in 

 exchange for increased bandwidth, as in pulse code, pulse position and 

 certain other methods of pulse modulation. Finally, pulse modulation 

 systems permit multiplexing of communication channels on a time divi- 

 sion basis, which under appropriate conditions may have appreciable 

 advantages over freciuency division in the design of multiplex terminals. 



In pulse modulation systems, pulses are applied at the transmitting 

 end in various combinations, or in varying amplitude, duration or posi- 

 tion, depending on the type of system. Pulses thus modulated to carry 

 information may be transmitted in various ways, or undergo a second 

 modulation process suitable to the transmission medium. The received 

 pulses will differ in shape from the transmitted pulses because of band- 

 width limitations, noise and other system imperfections. The performance 

 of the system in the absence of noise can be predicted if the "pulse trans- 

 mission characteristic" is known, that is, the shape of a received pulse 

 for a given applied pulse. 



Although the pulse-transmission characteristic suffices for detennina- 

 tion of system performance it is customary for various reasons to relate 

 it to the "transmission-frequency characteristic," that is, the steady- 

 state transmission response expressed as a function of fiequency. For 

 one thing the transmission-frequency characteristics of various existing 

 facilities and their components are known, and for new facilities can be 

 determined more readily by calculation or measurements than the pulse- 

 transmission characteristic. But the more fundamental reason is that the 

 transmission-frequency characteristics of various system components 

 connected in tandem or parallel can readil.y be combined to obtain the 

 over-all transmission characteristic, while this is not the case for pulse 

 transmisssion characteristics. It is thus possible to analyze complicated 

 systems with the transmission-frequency characteristic as a basic 



