A CARRIER TELEPHONE SYSTEM FOR TOLL CABLES 99 



In addition to the line amplifiers with their regulating equipment, 

 there are racks mounting the crosstalk balancing coils. There are also 

 sealed terminal units between the outside cable or the balancing units 

 and the office cable. These furnish access to the line or equipment 

 through jacks. 



Terminals 



The minimum distance over which a cable carrier system can be 

 operated economically is determined in large measure by the cost of 

 the terminal apparatus. Hence, the field of usefulness of the system 

 is greatly increased by keeping the terminal cost as low as is consistent 

 with satisfactory performance. Numerous developments during the 

 past few years in connection with modulation, filtering and methods 

 of carrier supply have all contributed materially toward this end. 

 At the same time, the standards of performance have not only been 

 maintained, but in many respects substantially improved. 



Channel and Group Modulation 



In the design of the terminals for the type K system, a number of 

 circuit arrangements were considered, the final choice being influenced 

 to a considerable extent by the conditions imposed upon the filters. 

 As noted above, the desirability of using the channel terminal equip- 

 ment in other broad-band systems, such as those for open-wire or 

 coaxial cable, was also an important factor. The circuit arrangements 

 selected have a first stage of modulation which raises the voice fre- 

 quencies of the 12 channels up to a range of 60 to 108 kilocycles. This 

 range is favorable to the use of crystal type band filters,^ which have 

 transmission characteristics superior to the coil and condenser type 

 and seem to be no more costly. For the type K system, a single stage 

 of group modulation shifts the frequencies to the range required on 

 .the line, 12 to 60 kilocycles, and a similar stage at the receiving end 

 returns them to the 60 to 108-kc. range. Other carrier systems will 

 also use the 60 to 108-kc. channels and by group modulation shift 

 them to the desired position in the frequency spectrum. 



The band filter occupies a space on the relay rack equal to 1/8 of 

 that required by the coil and condenser type which was used in the 

 earlier model of this system. Its attenuation characteristic in the 

 transmitting region is flat to within 1 db over a range of about 3100 

 cycles. Immediately outside of this range the attenuation rises very 

 rapidly, thus permitting very efficient use of the frequency spectrum. 



Another new device on the terminal is the copper-oxide unit used in 

 the modulating process. These units are expected to show a stability 



