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BELL SYSTEM TECHNICAL JOURNAL 



over a range of frequencies. To avoid it, it is necessary that the im- 

 pedances of successive lengths of cable pair be substantially uniform 

 and also that the impedance of the equipment be closely matched to 

 the characteristic impedance of the cable pair. 



Fig. 7 — Crosstalk balancing panel. 

 Noise 



The cable pairs are fairly well protected by the lead sheath from 

 external electrical disturbances. However, high-frequency noise 

 originating in the voice-frequency repeater stations due to relay opera- 

 tion, etc., would, unless prevented, enter the cable over the voice- 

 frequency pairs and thence would be induced in the carrier pairs. 

 Such noise would be excessive at the low-level carrier inputs. It is 

 avoided by connecting, in each voice-frequency pair in the "low level" 

 cable on each side of a voice-frequency repeater station, a coil which 

 suppresses longitudinal noise currents. Similarly, it is necessary to 



