10 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



Use of small and cheap filter coils and repeater transformers with per- 

 malloy cores is possible without harmful modulation. 

 Less precise transformer impedance and balance requirements, in con- 

 junction with reduced size, eliminates the need of electrostatic shields 

 between windings. 



Terminal 



Aids in elimination of transmitting channel band filter, and in large 

 reduction of receiving band filter requirements. 



Permits higher levels of carrier, speech and signaling tone without in- 

 tolerable noise, crosstalk or interchannel modulation effects. 



Equipment 



Much more freedom is allowed in equipment layout and wiring, per- 

 mitting more compacting, miniaturizing and less use of shield plates, 

 shielded cans, and shielded wiring, without harmful noise pickup and 

 crosstalk couplings. 



Operation is feasible from common office battery with large reduction in 

 individual circuit filtering. Signaling and speech circuits can be used on 

 the same ofiice battery without need for separate office wiring, fusing and 

 alarms. 



Frequency Frogging 



Like the compandor, frequency frogging is vital to the N system, and 

 numerous benefits result from its use. Primarily the purpose was to eliminate 

 interaction crosstalk, i.e., crosstalk from the output of one repeater into a 

 paralleling voice pair and thence back into the input of other repeaters. 

 In K carrier cables this crosstalk path was eliminated by using two cables 

 and at a repeater point connecting one cable to repeater inputs and the 

 second cable to repeater outputs. The voice pair passing by the repeater 

 point and remaining in the one cable thus was not exposed to both repeater 

 inputs and outputs. In the N system in a single cable a modulator in each 

 repeater frogs the frequency band from low group to high group, and in the 

 following repeater back again from high group to low group. Thus, repeater 

 outputs are always in one frequency band, and repeater inputs in the other, 

 so that the crosstalk through the paralleling voice path can always be 

 blocked by a filter at the repeater input. This approach is invaluable in N 

 carrier where the alternative to frequency frogging is to use a second cable 

 or to add suppression filters in all the paralleling voice pairs. In Fig. 4, 

 cable frogging in K carrier and frequency frogging in N carrier are illustrated 

 diagrammatically. 



In addition to frequency frogging, the two frequency bands are inverted 



