4 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



transmitting low group to the line and receiving high group, the group 

 modulator and 304 kc oscillator are used in the transmitting side of the 

 circuit. Similarly, in the repeater, low and high group bands are interchanged 

 between input and output lines through use of group modulators with 304 

 kc carriers. 



Choice of the one group alone for prunary modulation and demodulation 

 of the speech bands stems largely from the desire to use only 12 designs of 



Table I 

 Transmission Features of N1 Carrier Telephone System 



1. Built-in compandor affording an effective signal-to-noise improvement of 20-25 db. 



2. Frequency frogging and inversion to improve crosstalk and furnish automatic 

 equalization. 



3. Built-in signaling equipment in each channel to provide supervision and dial puls- 

 ing. Tone on-tone off operation employing 3700 cycles, 



4. Message channel bandwidth 250-3100 cycles. Transmission of special services 

 (telegraph and telephoto) through standard message channel equipment. 3500 cycle 

 program channel plus 11 message channels, or 5000 cycle program channel plus 9 

 message channels provided by special program channel equipment. 



5. Automatic regulation of each channel at the receiving terminal by the individual 

 channel carrier. 



6. All alarms built in with special carrier system failure alarm operating on trans- 

 mitted carriers automatically freeing subscriber dial equipment. 



7. Use of noise generator where needed to mask inteUigible crosstalk and obtain satis- 

 factory performance in exchange type cables. 



8. Built-in resistance hybrid arrangements for 2-wire termination at non-gain switch- 

 ing points or alternative use of 4- wire termination at —16 and 4-7 levels for 

 standard interconnection to existing broadband intertoll carrier systems. As 

 much as -{-10 level is permissible for special purposes. 



9. Repeaters spaced at 8-mile intervals on 19-gauge toll cable and at shorter dis- 

 tances on high-capacity or smaller-gauge exchange cable. 



10. Power fed to pole mounted repeaters 8 miles (19-gauge toll cable) on either side 

 of an office repeater, thus requiring power supply stations about 24 miles apart. 

 Power is fed over the cable pairs by simplex connection and use of +130 volt 

 and —130 volt batteries. 



11. Automatic regulation of line repeaters by thermistor flat gain adjustment, con- 

 trolled by total output power of the 12 transmitted carriers. 



12. In service switching of repeater and terminal circuits. 



13. Small, lightweight, portable transmission measuring equipment for ofl&ce and pole 

 cabinet use. 



14. Simple order wire and alarm equipment provided to alarm power failures at un- 

 attended power offices and to permit communication with all repeater points. 



channel band filters rather than 24. Easier filter requirements, occasioned 

 by the use of double-sideband operation and the compandor, together with 

 the fact that, for the high group, all harmonics fall outside the useful band, 

 result in the elimination of the need for transmitting band filters. Thus, the 

 only filter needed in the 12-channel group of sidebands and carriers is a 

 common filter to suppress transmission of speech sidebands on harmonics of 

 the channel carriers. An important factor in the choice of the high group for 

 receiving channel band filters was the better performance obtained in the 

 simple radio type slug-tuned coils in this frequency range, and the 

 smaller size of condensers needed for tuning. 



