590 



BELL SYSTEM TECHNICAL JOURNAL 



Frequency Directions. The importance of the use of a separate 

 frequency for each direction of transmission may be considered by 

 reference to Figure 14. If there are two paralleHng telephone circuits 



Level ot Input to Carrier Aooarstus 



Leve^at Input to Carrier Apparatus 



Figure 14 — Diagram illustrating occurrence of near-end crosstalk between carrier 

 systems employing the same frequency for opposite directions of transmission 



employing frequencies (/i) in the same range, and if there exists 

 between the two circuits a certain amount of crosstalk, when there is 

 a talker at the terminal of one system (No. 1) and a listener at the 

 same terminal of the other system (No. 2), then the speech from 

 the talker at the high level will enter directly into the sensitive receiving 

 circuit of the listener. This is commonly called "near-end" crosstalk. 

 In the case of a carrier circuit, the transmitting terminal would 

 involve a certain amount of amplification. The receiving circuit 

 would likewise, so that the net effect would be that the crosstalk 

 between the two circuits would be amplified by the combined amount 

 of gain or amplification present in the sending and receiving circuits. 

 In telephone parlance it would be stated that this is a situation in 

 which substantial level differences exist between the two circuits. 



On the other hand, in the case of two adjacent carrier systems 

 employing the same frequencies for the same direction of transmission, 

 a crosstalk situation involving only "far-end" crosstalk would exist, 

 as illustrated in Figure 15. This assumes that near-end crosstalk 

 by reflection as discussed later has been eliminated. In this case 

 the talker and the listener would be situated at opposite terminals of 

 the paralleling circuits and the crosstalk, while being amplified like 

 the near-end crosstalk by the total gain in the transmitting and 

 receiving circuits, suffers the attenuation of the line circuit which more 

 than offsets the amplification. This is, therefore, a very substantial 

 factor in favor of the two-frequency method of operation. 



