184 BELL SYSTEM TECHNICAL JOURNAL 



filter action does not depend, to any large extent, upon the amount of 

 power per channel, since the ratio of the disturbing currents to the 

 signal remains approximately constant. Finally, such residual carrier 

 as exists during spacing intervals may be variable in amount due to 

 changes in the resistance of the sending relay contacts. If the power 

 levels are increased sufficiently the total spacing interference will 

 usually become due preponderantly to the modulation effects and 

 hence a function of total power on the line, as indicated in curves 

 d and d' . 



In order to estimate the quality of transmission to be expected from 

 circuits in view of these various interfering factors, it is desirable to 

 establish the following two definitions: 



The signal-to-interference ratio * of a circuit is the ratio, expressed 

 in db, of the normal marking current plus the interference, to the 

 interference alone. 



The marking interference is the ratio, expressed in db, of the 

 normal marking current alone, to the marking current plus the 

 interference. 



A variety of results may be obtained under these definitions depend- 

 ing upon the methods of observation: it is customary, therefore, to 

 adopt the following practical specifications: 



Signal-to-interference Ratio: The change in sensitivity, expressed 

 in db, required in the receiving circuit of a given channel, all other 

 channels being in a marking condition, to just cause the armature of 

 the receiving relay to go to its marking contact; first, with steady 

 marking current transmitted over the channel under test, and 

 second, with the channel under test opened at the sending end. It 

 is understood that the currents are turned on and off" by operating 

 the sending relays and moreover that the receiving relay operates 

 on one half the steady marking current. (E.g., no interference 

 = CO db; complete failure = 6 db, approximately.) 



Marking Interference: The change in sensitivity, expressed in db, 

 required in the receiving circuit of a given channel to just cause the 

 armature of the receiving relay to go to its spacing contact; first, 

 with steady marking current transmitted over the channel under 

 test only, and second, with the interference added thereto. (E.g., 

 no interference = db; complete failure — 6 db, approximately.) 



While the above method for measuring spacing interference is the 

 one used in practice owing to the ease with which it can be applied, 



* More precisely the signal to spacing-interference ratio. 



