484 BELL SYSTEM TECHNICAL JOURNAL 



form thus. In order to investigate this experimentally, as well as other 

 factors that concern the choice of method, the effects on telegraph transmis- 

 sion of interchannel interference and of varying the signaling speed, trans- 

 mission level, mean carrier frequency, and line noise, were determined for 

 several different methods, using the same channel filters. In order to test 

 the two-band methods* using the same frequency range occupied by the 

 one-band arrangements, narrow-band filters would be required to divide the 

 frequency range into two parts. Since such filters were not available, it 

 was necessary to use two adjacent frequency bands each similar to that used 

 with the on-o£f method. However, some tests were made of a two-band 

 arrangement using somewhat narrower filter pass bands. 



A special wide-band frequency-shift arrangement using filters of about 

 twice the band width of the other frequency-shift arrangement, was tested 

 mainly in order to observe the effect of band width on sensitivity to noise 

 and interference. 



In all of the noise tests, thermal or resistance noise was used. With noise 

 of the impulse type it is possible that somewhat different results would have 

 been obtained, but it is believed that the difference would not have been 

 great. 



Conclusions 



A study of the test results leads to the following conclusions which apply 

 for the conditions assumed, and which are thought to be of general applica- 

 tion, except for modifications which may be made necessary by future tech- 

 nical advances : 



1. There is no important advantage in frequency-shift carrier telegraph 

 over the on-off method as used in the Bell System for stable, quiet cir- 

 cuits, either wire or radio. However, the frequency-shift method shows 

 some improvement in operating through noise. The frequency-shift 

 method has disadvantages as regards complication and cost. Further- 

 more, it may be seriously affected by carrier frequency drift and 

 interchannel interference, although the effects of these can be mitigated 

 to some extent by special devices. 



2. For high-frequency radio transmission over long distances, which is 

 subject to comparatively severe non-selective fading, a great advantage 

 is realized from the use of frequency-shift telegraphy with a fast receiv- 

 ing limiter instead of the conventional "continuous wave" or on-off 

 method. For satisfactory operation it is still necessary that the signal 

 level be kept sufficiently higher than the noise level in the transmission 

 band. 



* See the section entitled "Explanation of Terms". 



