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BELL SYSTEM TECHNICAL JOURNAL 



For AM signals, Fig. 12, varying the bandwidth has little effect when the 

 telegraph signal distortion is less than 15%. Although the wider bands 

 accepted more noise power, this added noise merely produced high-frequency 

 noise components which were removed by the low-pass filter. This added 

 noise does, however, cause the peak noise to exceed the signal amplitude at 

 a lower noise-to-carrier ratio and cause failure before that for a narrower 

 band condition. 



In the case of FS signals , Fig. 11, changing the bandwidth and the fre- 

 quency shift simultaneously, and in approximately the same proportion, 

 alters the whole distortion characteristic. At low noise levels a wider band 

 with a greater frequency shift gives an improved signal-to-noise condition. 

 However, as the noise level is increased the wider band causes the peak noise 



-18 -16 -14 -12 -10 -8 -6 -4 -2 

 RMS NOISE-TO-CARRIER RATIO IN DECIBELS 



Fig. 12. — Peak distortion versus thermal noise for AM transmission — 80-cycle cutoflF 

 low-pass filter, 



to exceed the carrier at a lower noise level than with a narrower band. Thus 

 a change to a wider band gives less distortion at low noise levels and more 

 distortion at high noise levels. This results in a much more sharply breaking 

 distortion characteristic for the wider band. This behavior is typical of 

 frequency modulation systems in general. 



Although the noise actually passed by the 740-cycle band filter was 

 approximately 8 db above that passed by the 115-cycle band filter, the dif- 

 ference in the failure points (35-40% distortion) for the two bandwidths 

 will be seen to be only about half this amount in db for both AM and FS. 

 This phenomenon is typical of carrier telegraph systems when compared 

 at the same signaling speed. This means that it is not particularly beneficial 

 to decrease band width to obtain lower distortion under high noise condi- 

 tions. The main reason for narrow bands is for more economical use of 

 frequency space. 



As to the comparison between FS and AM, the FS method has an advan- 



