1468 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



a region in which no appreciable interference is tolerated from the ad- 

 jacent band. The allowance for this is generous in the AM case. In the 

 FS case the roll-off band of Item 3 tends to use up all the space not 

 employed by the nominal effective band and the swing, and nothing is 

 indicated as left for guard band. This is illustrated in Fig. 3(b) by the 

 extremities of the roll-off band reaching the extremities of the 170-cycle 

 spacing. It is to be recognized that the illustrations are diagrammatic. 

 However, the extremities mentioned measure the bands occupied by 

 power from random signal transitions between marking and spacing 

 (as distinguished from mere mark-space reversals), analogous to the 

 bands occupied by power from AM signal transitions. Comparison of 

 Figs. 3(a) and 3(b) suggests how modulated signal components of signifi- 

 cant intensity are displaced farther from the edges of the channel with 

 FS than with AM. 



36 



32 



28 



K 24 



z 



UJ 



U 



UJ 20 



z 



1,6 



cc 

 o 



I- 



° 12 



60 80 100 120 140 160 



SPEED IN WORDS PER MINUTE 



180 



Fig. 4 — Experimental relation between telegraph distortion and speed, 

 with fixed channel filters (after Jones and Pfleger'). 



