THEORETICAL FUNDAMENTALS OF PULSE TRANSMISSION 



993 



point on the ami)litude chanicteristic and fmax = 2/i . Under this con- 

 dition there is no intersymbol interference in the absence of phase dis- 

 tortion. 



The al)(-)ve eciuations ai)i)ly to jieak intersymbol interference, (>])tained 

 b^' taking the maximum positixe and negative values of the summation 

 term in (13.01). As discussed in prcxious sections, certain types of trans- 

 mission system imperfections gi\e rise to pulse distortion extending over 

 long time intervals, such as fine structure (le\-iations over the transmis- 

 sion band, a low-frequency cut-off and pi'onounced band-edge phase 

 deviations. Evaluation of peak intersymbol interference is then rather 

 difficult, and a more convenient approximate method is to evaluate 

 rms inters3'ml)()l interference, which can be related to vms deviation in 

 the transmission fre(iuency characteristic by methods discussed previ- 

 ously. Peak inters3'mbol interference may then be estimated by applying 

 a peak factor between 3 and 4, depending on the type of transmission 

 distortion. 



If Poinr) designates an ideal impulse characteristic, which is zero for 

 71 = ±1, ±2 etc., the deviation from the ideal envelope of a pulse 



5 40 



1.0 1.5 2.0 2.5 3.0 



dMAX'MAX = 2d^lAyT| 



•"MAX I WAX 



Fig. 43 — Margin against exoessivo jjcak interference in systems emploj'ing 

 two ))ulse amplitudes with intervals between i)ulses r = ti = l/2/i = 1/fmax. for 

 impulse transmission characteristic as shown in Fig. 23. 



