THE N-1 CARRIER SYSTEM 13 



Because of frogging the maximum repeater gain is that required at the 

 mean frequency instead of maximum Hne frequency. Furthermore, average 

 repeater spacings required in opposite directions of transmission are identical. 

 In a system without frequency frogging repeaters and using different fre- 

 quency bands in the two directions, the average repeater spacings required 

 in the two directions would be quite different. In the latter system, for 

 various controlling reasons repeaters are installed at the closer spacing of the 

 high-frequency group for both directions of transmission. 



Frequency frogging and inversion also benefit regulation. With tempera- 

 ture change, a first order change in flat line loss occurs, a second order 

 change occurs in slope and a considerably smaller bulge change occurs. 

 In so far as the two successive line sections are at the same temperature, 

 slope changes are nearly compensated. The bulge changes are so slight that 

 they impose but small regulation requirements in the channel equipment 

 even in a long system. 



Signaling 



Each voice frequency channel of the N system contains in addition to 

 the compressor and expandor circuit, a signaling circuit operating at 3700 

 cycles for both dialing and supervision. Physically both the signal sending 

 and receiving circuits are included in the expandor plug-in subassembly. 

 The compressor and the carrier subassemblies, together with the expandor- 

 signaling subassembly, comprise the channel terminal unit. Compressor 

 and expandor-signaling subassemblies are alike and interchangeable among 

 the 12 N channels. Carrier subassemblies differ only in respect to the 

 oscillator and filters. 



Signaling over the N system may assume a variety of forms. In ringdown 

 operation 1000-cycle ringing signals may pass over the voice channel without 

 need for the N signaling circuit, or, on the other hand, ringing may be 

 converted to d-c. and passed over the N system by turning its 3700 cycle 

 signal tone on and off. In dial operation digits are carried as in the national 

 toll-line dialing plan either by multifrequency key pulsing or by dial make 

 and break connections of a single-frequency tone. With multifrequency 

 key pulsing the two frequency combinations of 700, 900, 1100, 1300, 1500 

 and 1700 cycles pass directly through the voice channel. Supervisory on- 

 hook and off-hook signals, as well as dial pulse signals, are carried by turn- 

 ing 3700 cycles on and off. In addition, called numbers may be transmitted 

 over the N system either by the operator verbally or by recorded methods 

 of the panel call announcer system. Revertive pulsing and panel call indi- 

 cator signals are not provided for in the N system, there being little need for 

 N systems in panel ofl&ces where this equipment is used. 



The ^ carrier signaling system uses 3700 cycle tone just out of the voice 



