TOLL TRANSMISSION IN THE UNITED STATES 125 



certain line structures. For other structures, the frequency range is 

 narrower, but for all these systems the frequency range is transmitted 

 as a single band, and split into communication channels for telephone 

 or telegraph only at the terminals. If the transmission of television 

 signals should become necessary, a very broad band — one or more 

 million cycles — would, of course, be required. Although I have 

 referred to television, our primary interest in broad-band wire trans- 

 mission is for telephone transmission purposes, where the wide trans- 

 mission band can be used to give a large number of talking channels. 



You will recall that the idea of deriving more than one communica- 

 tion channel from a single pair of conductors, by what we now call 

 carrier methods, is old — in fact, as old as the telephone itself. Until 

 quite recently, however, physical devices and methods have not been 

 available to make the carrier method utilizable in practice. Beginning 

 about fifteen years ago, the Bell System began to install carrier 

 systems, and since that time this method has had continued growth 

 on open-wire lines, with the result that a substantial amount of toll 

 traffic is now carried over carrier systems on open-wire lines. A 

 relatively simple form of carrier equipment provides one two-way 

 telephone circuit in addition to the usual voice frequency circuit, 

 while more elaborate and refined equipment adds three two-way 

 telephone circuits. 



In addition to the economic urge to obtain the largest possible 

 number of telephone channels over a given pair of wires, there is an 

 additional factor that has influenced the development of broad-band 

 systems, and that is, the speed of transmission. Even in the lowest- 

 speed telephone circuits, the speed of transmission of voice waves, as 

 judged by ordinary standards, is high, being from ten to twenty 

 thousand miles per second (32,000 km. per sec.) in the loaded cable 

 circuits that are now used for many of the long toll lines. For ordinary 

 distances, moreover, the speed of transmission is not of any particular 

 moment, but when the voice must be transmitted over distances of 

 thousands of miles, it becomes important. Echo effects become 

 exaggerated, and in a long connection, the actual time for speech to 

 reach from the first subscriber to the second subscriber, added to the 

 time required for the second subscriber to answer the first subscriber, 

 may become an annoying factor. The broad-band transmission 

 method furnishes circuits, however, in which the speed of transmission 

 is raised from about 20,000 miles per second, as on loaded cable circuits, 

 to a speed approaching that of light. 



In developing a new toll system, there are many other factors, of 

 course, that must be considered. In our case, just as in yours, there 



