THEORETICAL FUXDAMEXTALS OP PULSE TRAXSMLSSIOX 723 



parameter, and to specify reciuirements that must be imposed on the 

 transmission -frequency characteristic of the system and its compoiuMits 

 for a given transmission performance. 



A fundamental problem in pulse modulation systems is transmission 

 distortion of pulses by system imperfections in the form of phase and 

 gain deviations over the transmission band or a low-frequency cut-off, 

 usually referred to as "characteristic distoi'tion," which may give rise 

 to excessi\-e interference between pulses and resultant crosstalk noise or 

 errors in reception, depending on the type of system. Because of such 

 interference, characteristic distortion limits the number of pulse ampli- 

 tudes peiTuissible in the transmission of information or messages over a 

 given channel, and may reduce the rate at which pulses can be trans- 

 mitted in sj'stems emploj'ing only two pulse amplitudes, the minimiun 

 number. It thus places a limitation on channel capacity which, unlike 

 signal distortion by noise, cannot be overcome by increasing the signal 

 power. 



Characteristic distortion is an important consideration particularly in 

 wire systems where there is a low-frequency cut-off caused by trans- 

 formers, and where the transmission band may extend over several 

 octaves with substantial variation in attenuation and phase shift, or 

 may be sharply confined by filters. In wire systems there are also fine 

 structure deviations from a smooth attenuation and phase characteristic 

 of a more or less random nature, resulting from small random impedance 

 variations and mismatches along the lines. Gain and phase deviations 

 remaining even after fairly elaborate equalization may be appreciable 

 and difficult to overcome, especially in S3'stems comprising a large num- 

 ber of repeater sections. 



The purpose of this paper is to present a compendium of theoretical 

 fundamentals on pulse transmission in a form suitable for engineering 

 applications, both from the standpoint of design of new pulse transmis- 

 sion systems and pulse transmission over existing facilities. Emphasis is 

 placed on considerations of various system imperfections, because of 

 their importance from the standpoint of transmission performance, and 

 since literature on this question is rather limited. Certain fundamental 

 properties of transmission-frequency characteristics are discussed, to- 

 gether with general relations between frequency and pulse transmission 

 characteristics and special transmission characteristics of importance in 

 pulse systems. This is followed by a presentation of methods of evaluat- 

 ing pulse distortion from various types of gain and phase deviations, to- 

 gether with resultant transmission impairments or limitations on pulse 

 transmission rates in low-pass, symmetrical and asymmetrical sideband 



