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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



FREQUENCY *- fm 



Fig. 39 — Sub-channel with nearly linear delay distortion. 



Peak intersymbol interference may exceed the above rms values by a 

 factor of about 3. In a system employing two pulse amplitude (1 and 

 — 1), the maximum tolerable intersymbol interference is 1. This value 

 would thus be attained in the above case for dmax/max = '^ • Hence, in a 

 system employing two pulse amplitudes, and in the absence of noise 

 and intersymbol interference from other sources, there would be no 

 limitation on phase chstortion for co > wi , provided the phase charac- 

 teristic is linear between co = and wi . 



11. BAND-PASS CHARACTERISTICS WITH LINEAR DELAY DISTORTION 



In Fig. 39 is shown a transmission frequency characteristic together 

 with an assumed delay distortion d\}//doo. When a portion of the trans- 

 mission band is employed for pulse transmission, as for example in pulse 

 signalling, data or telegraph transmission over portion of a voice channel, 

 there may be an appreciable component of substantially linear delay 

 distortion, as indicated in the above figure. The departure from a linear 

 variation can usually be approximated by a cosine variation in delay, 

 and the system can then be regarded as made up of two components in 

 tandem, one with linear the other with cosine variation in delay. The 

 effect of the latter can be evaluated by the methods outlined in Section 6, 

 and the effect of a linear variation by methods established in this section. 



In Fig. 40 is shown a symmetrical amplitude characteristic with linear 

 delay distortion over the transmission band. Phase distortion with 

 respect to the midband frequency is in this case 



^(m) = ^u and ^(-w) = ^u, (11.01) 



