528 



BELL SYSTEM TECHNICAL JOURNAL 



curve, \vc calculate that the uoise in llie entire baseband must be 43 db 

 below the power in a sine wave whicli employs the full system load capac- 

 ity. This ligure comes from reducing the 60 db full load channel ratio 

 by 30 db because of the lOOO-fold greater baseband width and increasing 

 it l)y the amount, 16 — 3 = 13 db, by which the full system load must ex- 

 ceed full load in the top channel. Thus 60 - 30 + 13 = 43 db. An FM 

 advantage of 31 db must be obtained to permit the marginal r-f signal- 



so 



75 

 70 



'!^ 65 



m 

 m 

 O 60 



5 50 

 < 



o 



^7 35 



< 



:? 30 



1 2 4 6 8 10 20 40 60 100 200 400 600 1000 



RADIO SIGNAL BANDWIDTH IN MEGACYCLES PER SECOND 



Fig. 19 — FDM-FM; performance with respect to fluctuation noise. Relations between 

 Ijandwidth and audio signal-to-noise ratio for marginal power; 4-kc channels. 



to-noise ratio of 12 db to satisfy the above requirement, (43 — 12 = 31). 

 We get this advantage in part by phase modulation gain given by 20 



log — — 6 db. This gain is referred to 100% modulation AM whose un- 



modulated carrier power is the same as the FM wave power. This means 

 that the reference is a system in which the FDM baseband appears as 

 upper and lower sidebands which, when demodulated, yield a baseband 



