CARRIER TELEGRAPH METHODS 531 



imum operating speed. From a noise standpoint the two-source arrange- 

 ment without limiter has a slight advantage over the Unear on-ofif method. 

 When a limiter is used, the two-source arrangement is inferior to the linear 

 on-off method due to the distortion inherent in this arrangement. 



In the one-source two-band arrangement the limiter does not cause dis- 

 tortion, and a wider range of frequency components is transmitted than by 

 the two-source method. For these reasons, the one-source two-band 

 arrangement has a maximum working speed considerably higher than the 

 two-source arrangement. The one-source two-band arrangement ranks 

 close to the two-source method with limiter in its ability to withstand non- 

 selective level changes, but its susceptibility to carrier frequency changes is 

 greater. 



A frequency-shift arrangement utilizing approximately the same band 

 width as the two-source arrangement is found to have appreciably higher 

 speed and less sensitivity to noise, but unless compensation is provided this 

 frequency-shift arrangement is considerably more susceptible to carrier 

 frequency changes. A further reduction in noise sensitivity may be ob- 

 tained by the use of a low-pass filter at the detector output with a cut-off 

 frequency low enough to limit the speed to that attainable with the normal- 

 band frequency-shift arrangement. 



Economic considerations should also be given due weight in selecting an 

 optimum arrangement for a specific application. For example, the on-off 

 method has been widely used on certain wire lines of the Bell System, because 

 appreciable level changes are gradual and the lines are relatively quiet. 

 For this application, the level compensated on-off method therefore gives 

 satisfactor\- service with a minimum amount of apparatus, and the terminal 

 arrangements are fairly simple and easy to maintain. On radio links subject 

 to noise and fading, the more expensive frequency-shift and two-source 

 methods have frequently been selected because of their greater reliability 

 under such adverse conditions. The on-ofif and two-source arrangements 

 have the advantage that common carrier generators (or oscillators) may 

 be used for a number of channels. 



