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BELL SYSTEM TECHNICAL JOURNAL 



may happen to a single dot. The extreme conditions of operation on 

 the particular channel under consideration are those where all of the 

 channels are marking or spacing. If they are all spacing, there is only 

 noise and unsuppressed carrier, and the change in received current 



Fig. 12 — Effect of interference on signal distortion. 



when the one operating channel changes from space to mark is the 

 vertical distance between points 5 and s' , while when all the other 

 channels are marking the change is from m to m' . The ratio of the 

 currents corresponding to either of these pairs of points, when expressed 

 in db, may be conveniently called the marking-to-spacing ratio. Prac- 

 tically, of course, the power over the line will be constantly and 

 fortuitously varying, so that the actual arrival curves will lie somewhere 

 between certain limiting values indicated by the dotted lines ms' and 

 sm' , giving rise to a range of fortuitous distortions which, if the re- 

 ceiving equipment was adjusted with all the other channels marking, 

 will depend on the length of the extreme intervals xx' and yy'. 



The results obtained in the course of experimental observations of 

 some of the above quantities are given below. They were secured on 

 a 700-mile H44 cable circuit of the type described at the beginning of 

 this paper. In order to obtain uniform results, the output of each of 

 the 17 repeaters in tandem in this circuit was adjusted to the same 

 level, so that each output tube contributed about equally to the total 

 modulation effects and a similar uniformity existed relative to the 

 most heavily energized loading coils. In practice, the saturation 

 effects would not be likely to exceed this, and generally would be 

 somewhat less. 



Figure 13^4 shows typically the effect on the received current of 

 increasing the total power transmitted over the line when all channels 

 are marking simultaneously. This phenomenon (which may be called 

 crowding) is a measure of the intermodulation which is caused by the 



