284 BELL SYSTEM TECHNICAL JOURNAL 



ate and low distortions. For a system occupying a given bandwidth the 

 two-bandpass-filter discriminator provides some improvement at the failure 

 point but is still somewhat inferior to the linear discriminator at moderate 

 distortions. More importantly the two-bandpass-filter discriminator im- 

 pairs the signaling speed capabilities to an extent which depends upon the 

 shape of the cutoff of the filters used. 



Bandwidth After Demodulation {Low-Pass Filtering) 



In an ^w system the low-pass filtering after demodulation can, to a large 

 degree, make up for a greater than necessary bandwidth before demodula- 

 tion. During marking intervals the added noise admitted by a wide band 

 causes noise in the demodulated output at frequencies higher than the signal- 

 ing frequency and this can be filtered out, unless the noise is so great as to 

 over-modulate the carrier. During spacing intervals there is no carrier and 

 hence only the noise is rectified. Added noise admitted by a wide band 

 causes not only higher-frequency components in this rectified noise, which 

 may be filtered out, but also an increase in the d-c. component. This tends 

 to cause marking bias of the received signals as the noise level increases. 



In an FS system, where the carrier is present continuously, the added 

 noise from a wider band produces high-frequency noise components in the 

 demodulated output which can be filtered out by the low-pass filter if the 

 noise level is low. As the noise level increases there are short intervals 

 when the noise envelope exceeds the carrier. The action of the limiter is 

 to give preference to the greater signal, in this case the noise, and since the 

 noise will appear to the discriminator as a carrier fluctuating around mid- 

 band as a center, the demodulated output momentarily dips toward zero. 

 As the noise increases, the duration and frequency of these holes in the signal 

 increase. The low-pass filter, by excluding frequencies considerably in 

 excess of the maximum signaling speed, prevents these holes in the signal 

 from producing false or extra transitions in the telegraph signal output. 

 The low-pass filtering, however, cannot prevent the true transitions from 

 being displaced by this type of noise component since the signal is obliterated 

 momentarily. Its most important function is to prevent a breakup in the 

 signal output until a fairly high distortion is reached. For noise peaks 

 exceeding the carrier the low-pass filter of an AM system also serves much 

 the same purposes. 



In Fig. 21 is shown the effect of changing the bandwidth of the low-pass 

 filter in an FS and in an AM system in the presence of thermal noise. The 

 effect of a narrower low-pass filter is seen to consist mainly in shifting the 

 breaking point toward a higher noise level. Similar characteristics for the 

 case of impulse noise are shown in Fig. 22. 



