638 



BELL SYSTEM TECHNICAL JOURNAL 



vides more separate channels (six) than has ever before been attempted, 

 while the particular arrangement employed is different in many other 

 important respects from anything which has been used in the past. 



In order to better appreciate the reasons for adopting the system 

 finalh' agreed upon, it may be of interest to review briefly the essen- 

 tial characteristics of carrier systems and the different types which 

 are available.^ 



Carrier systems may be divided into two general classes, namely, 

 balanced or grouped, depending upon the manner in which the currents 

 in the two directions are prevented from interfering with each other 



TO OTHER CHANNELS TO OTHER CHANNELS 



BALANCED I 



SYSTEM ^_J^ 



w vflOO i - sOCO/ — i I 



TO OTHER CHANNELS 



TO OTHER CHANNELS 



TO OTHER CHANNELS 

 TO OTHER CHANNELS 



TO OTHER CHANNELS 



TO OTHER CHANNELS 



Fig. 1 



at the terminals. In the balanced system this separation is accom- 

 plished by means of a three-winding transformer or hybrid coil to- 

 gether with a balancing artificial line such as is used with a voice- 

 frequency repeater. In the grouped system, different carrier fre- 

 quencies are used for transmission in the two directions and their 

 separation at the terminals is effected by means of suitable band- 

 pass filters. These two systems are shown diagrammatically in 

 Fig. 1. The balanced system has the advantage that for each channel 



' The general principles of carrier-current telephony are described at considerable 

 length in a paper by Messrs. Colpitis and Blackwell which was published in Volume 

 XL of the Journal of the Institute. 



