282 



BELL SYSTEM TECHNICAL JOURNAL 



speed capability is essentially the same as an AM system of equal band 

 width. 



To compare experimentally these two general types of discriminators a 

 74()-cycle band system with linear discriminator and 350-cycle shift was 



o 



20 > 



z 



to 



-20^ 

 FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 17. — Characteristics of 1920-cycle bandpass filter and associated discriminator 

 consisting of two bandpass filters. 



Q 



z 

 < 



(0 

 Q 



_i 



5UJ 



"-UJ 

 (OQ 



20 



15 



10 



DISCRIMINATOR 



10 



-5 



-10 



1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 

 FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 18. — Characteristics of 295-cycIe bandpass filter and associated discriminator 

 consisting of two bandpass filters. 



compared with a system with a bandwidth of about 1900 cycles, a discrimi- 

 nator consisting of two 740-cycle bandpass filters, and an 850-cycle shift. 

 The results are shown in V'\g. 19. The two systems are seen to reach failure 

 distortion values at the same signal-to-noise point, with the linear dis- 

 criminator becoming about vS db superior at distortions around 5%. A 

 second comparison was made using roughly equal bandwidths. A 295-cycle 



