CARRIER SYSTEMS ON TELEPHONE LINES 565 



second. At the receiving terminal the various incoming ranges of 

 high-frequency currents are separated by electrical filters. Then by 

 demodulation the original voice-frequency currents are produced 

 again and are transmitted over voice-frequency circuits, the trans- 

 mission over each channel thus reaching the proper listener. In this 

 way a telephone line already carrying direct-current telegraph and 

 voice-frequency telephone services may be multiplexed so as to provide 

 additional telephone facilities. In a somewhat similar manner the 

 high-frequency range may be used instead to transmit telegraph 

 messages. In the present paper, carrier telephony alone is considered. 



The Colpitts-Blackwell paper described two carrier telephone 

 systems which had been developed up to that time, a four-channel 

 "carrier suppressed" system (type "A"), and a three-channel "carrier 

 transmitted" system (type "B"). The initial installation of these 

 systems was made about 1918 on the long lines of the Bell System. 



These earlier systems were effective in bringing about economies 

 by avoiding the stringing of additional wire on many long pole lines, 

 but there remained many opportunities for further improvement in 

 performance and simplification of equipment. New problems arose 

 to be solved in connection with the desire to operate the largest possible 

 number of systems on the same pole line. The result has been the 

 development of a substantially improved technique and a new system 

 (the type "C") which not only has provided much improved per- 

 formance over its predecessors but which has led to further economies 

 because of reduced costs. 



Carrier Telephone Growth in Bell System. Whereas the use of the 

 early types of systems was justified in competition with the alternative 

 of additional wire stringing only for distances exceeding 250 to 300 

 miles, the new system proves economical for distances considerably 

 less. This fact has naturally stimulated the application of carrier 

 telephony in the Bell System. This is shown by Figure 1, which 

 indicates the growth of these systems in terms of channel mileage 

 afforded by their use. It will be noted that the rate of growth of 

 the systems has increased greatly in the last two or three years, a 

 result of the availability of the improved system. 



At the end of 1927 there were in operation about 130,000 channel 

 miles. By the end of 1928 the figure is expected to be about 230,000. 

 This figure does not, of course, represent a very large proportion of the 

 total toll mileage of the Bell System, which includes many circuits 

 less than 100 miles in length. It is sufficient, however, to indicate that 

 the carrier telephone systems are a substantial factor in the provision 

 for the growth of the longer haul facilities, where they naturally 



