ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 185 



a more significant characteristic is the signal-to-interference ratio 

 obtained by observing the aforesaid change in receiving-circuit sensi- 

 tivity required to operate the receiving relay in a given channel, as 

 we go from a marking to a spacing condition in that channel with all 

 other channels transmitting uncoordinated signals. This is not only 

 a more practical consideration but a more severe condition, as indicated 

 by curve d' in Fig. 11. 



Carrier telegraphy, as here considered, is a marginal system of 

 operation in which the current received for a marking condition corre- 

 sponds on the average to that shown by curve c', while that for a 

 spacing condition is the one represented by d'. The difference between 

 these two characteristics is not all available for operation, however, 

 since the receiving circuit must be made sufficiently sensitive to operate 

 when the marking current has risen to half its final value, thus bringing 

 the threshold of operation 6 db closer to the spacing interference. 

 From these considerations it follows that the actual operating margin 

 is an essentially variable quantity whose approximate value is less by 

 about 6 db than the signal-to-interference ratio measured as specified 

 above, since the indicated procedure takes account of marking as well 

 as of spacing interference effects. 



The following definitions are also useful in reporting and interpreting 

 test results: 



The interference margins of a circuit are the ratios, expressed in db, 

 of the actual interference and the amount of this interference which 

 will cause failure. More specifically: 



(a) The spacing-interference margin is the increased sensitivity, ex- 



pressed in db, required in the receiving circuit of a given channel 

 adjusted to receive unbiased signals in the absence of inter- 

 ference, to just cause the receiving relay to close when inter- 

 ference alone is present (e.g., no interference = oo db; complete 

 failure = db). 



(b) The marking-interference margin is the decrease in sensitivity, 



expressed in db, required in the receiving circuit of a given 

 channel adjusted to receive unbiased signals in the absence of 

 interference, to just cause the receiving relay to open when in- 

 terference is added (e.g., no interference = 6 db, approximately; 

 complete failure = db). 



Clearly the various effects which have been defined above, being of 

 a variable and indeterminate character, contribute to the fortuitous 

 distortion of signals. This is illustrated in Fig. 12, which shows what 



