52 BELL SYSTEM TECHNICAL JOURNAL 



Because the line crosstalk tends to be greater at the higher fre- 

 quencies, past experience has indicated the advantage of having 

 available two systems between which the crosstalk in the higher 

 frequency group will be unintelligible. Two allocations are provided 

 for this purpose, designated CS and CU. The channel bands are 

 identical in the lower frequency group (East to West) and in the 

 upper frequency group (West to East) differ only in that the carrier 

 frequencies are at opposite ends of the bands. In this group crosstalk 

 between similar bands will have the speech frequencies inverted and 

 will therefore be unintelligible. 



This arrangement does not give as high a crosstalk advantage as 

 the arrangement used previously where the bands were not only 

 inverted but also displaced with respect to each other. However, 

 better line crosstalk conditions now prevail due to the application of 

 improved transposition designs and line configurations to the more 

 recently constructed lines and to the use of new methods of mitigating 

 crosstalk on the older lines. This permits the simplification of the 

 frequency allocation, as a result of which one system may be readily 

 converted into the other with fairly simple equipment changes. It 

 will also be possible to use the voice frequency circuit on all pairs as 

 a program circuit transmitting up to 5000 cycles. The advantages of 

 the greater plant flexibility resulting from these two factors are 

 obvious. 



The new system may be used on suitably transposed lines with the 

 Type D ^ and Type H* single channel systems, whose frequency 

 bands are such that no serious near-end crosstalk problem will arise. 



Overall System 



A block diagram of a complete system, consisting of the two termi- 

 nals and a single intermediate repeater, is shown on Fig. 3. In 

 practice there might be as many as ten or more such repeaters. The 

 two terminals differ from each other only in the frequencies for which 

 their respective transmitting and receiving circuits are designed. The 

 west terminal transmits the high-frequency group of Fig. 2 and receives 

 the lower frequency group while the east terminal does the reverse. 

 The repeater is provided with means for separating the frequencies in 

 the two directions of transmission, amplifying the current to the 

 desired level, and passing them on to the next line section. 



A typical overall frequency characteristic for one of the circuits 

 derived from the new system is shown on Fig. 4. This characteristic 

 illustrates the relative freedom from distortion in the transmitted 

 frequency range. 



