PRIVATE LIXE DATA TKAXSMISSIOX 



14G7 



^- 



170 'Xy 



2 X NOMINAL 

 EFFECTIVE 

 BAND 

 74 -Aj 



ROLL- 

 OFF 

 18.5 '~0 

 \ 



170'\' 



■*\ 



NOMINAL 

 EFFECTIVE 

 BAND X 

 37 '\j 



FM SWING _ ♦ 

 70Oj 



ROLL- I 

 OFF 

 130) 



FREQUENCY ^-*> 



(a) AM (b)Fs 



Fig. 3 — Utilization of telegraph channels. 



on 170 cycles between carriers, was conservatively developed for the 

 60 word per minute speed of the time. The 100 word per minute speed 

 has used up some of this conservatism. The use of frequency shift in the 

 same channels has, however, used up the spectrum space even more. 



An outline of the band allowances is given in Table IV, and illustrated 

 in Fig. 3. Item 2 of the table is based on the 100 word per minute speed, 

 using double sideband. On this basis, the number is equal to the num- 

 ber of bits per second. This is the minimum double sideband over which 

 that number of bits can be transmitted, according to the Xycjuist 

 theory. Each such sideband is sometimes called a "nominal effective 

 band." In practice various allowances are necessary over this minimum. 



In the first place a roll-off is necessary because filters are not infinitely 

 sharp, and in addition the nature of the modulation itself forms a roll- 

 off. Roll-off also leads to a signal which is more free of overshoots and 

 generally "cleaner" than when a sharp cutoff is used. Item 3 and Fig. 

 3(a) and (b) show an allowance for roll-off. For the AM case this amounts 

 to half the nominal effective band. For the FS case there is not quite 

 that much space available. 



For the FS signal it is necessar}' to allow for the frecjuency swing 

 as Item 4. For the 43A1 system this amounts to 70 cycles. In Fig. 3(b) 

 the spectrum includes the region comprised by the FM swing, and up- 

 per and lower sidebands. The upper and lower sidebands as formed by 

 the modulation of a random signal are shown extending respectively 

 above and below the extremities of the swing (instead of only above and 

 below a central carrier, as they would with AM). 



A final allowance in Item 5 is a "guard band." This is taken to mean 

 a region in which the signal energy is negligible, but at the same time 



