ADVANCES IN CARRIER TELEGRAPH TRANSMISSION 181 



A second method for securing a stable grid-bias voltage is shown in 

 Fig. lOB. Here, advantage is taken of the fact that the voltage re- 

 quired to maintain discharge in a cold-cathode neon-tube is constant, 

 by bridging such a tube across the negative 130-volt telegraph battery 

 in series with a resistance. The desired voltages for the even and odd 

 numbered detectors are then derived by tapping off at suitable points 

 on a second resistance which is connected across the neon tube. 



Interference 



Interference in a particular channel may manifest itself either by 

 the presence of current when none is intended or by a diminution of 

 the signal current during a marking condition. The former is called 

 spacing interference, and tends to change spacing units to marking 

 units; the latter is termed marking interference, since it is observed 

 during marking units, tending to change them to spaces. 



The principal sources of spacing interference are: 



1. Unsuppressed carrier. 



2. Noise, lightning, etc. 



3. Infiltration from adjacent channels. 



4. Modulation products. 



For marking interference, these are: 



1. Crowding (Saturation effects). 



2. Out-of -phase parasitic currents. 



Parasitic currents (noise, modulation, etc.) are usually not an im- 

 portant source of marking interference, as their phase relative to that 

 of the carrier forming the signal must fall within a rather narrow range 

 to be effective. 



If there exists some unsuppressed carrier during spacing intervals 

 which is due to the design of the sending circuit, it will be fixed in value 

 and may therefore be taken care of in the initial adjustments of the 

 receiving circuit. All the other effects are of a chance character, being 

 for the most part dependent on the transmission circumstances on 

 associated channels or circuits. These effects, therefore, lead to 

 fortuitous distortion. 



The effectiveness of all forms of interference is dependent upon the 

 ratio of their magnitudes to that of the signals with which they inter- 

 fere. On the other hand, the absolute magnitude of the greater part 

 of this interference depends upon the signal level. To establish a 

 balance between these two tendencies, the telegraph power per channel 

 which is transmitted over the circuit is selected so as to minimize the 



