120 BELL SYSTEM TECHNICAL JOURNAL 



this same company provides a portion of the faciHties which make it 

 possible for a person speaking at one place almost anywhere in the 

 civilized world to have his voice heard at almost any other. I refer 

 to broadcasting. 



Figure 1 shows a wire map of a few of the principal toll lines in the 

 United States. This toll plant affords facilities that, in connection 

 with the local plant, enable any telephone subscriber at any point to 

 communicate promptly with a subscriber at any other point in the 

 United States, Canada, or Mexico. 



With the growth of radio broadcasting, a service with which you 

 are all familiar, it became necessary to provide circuits to interconnect 

 radio broadcasting stations. Figure 2 shows a wire map of such 

 interconnecting circuits commonly spoken of as "program circuits." 

 Figure 3 shows schematically the radio-telephone circuits that connect 

 the United States to foreign countries. Another extension of the 

 service rendered by the Bell System is indicated by Fig. 4, which shows 

 a wire map of circuits devoted to the telephotograph service. 



It is not my purpose to take your time to discuss these past develop- 

 ments, since they have already been described quite fully in technical 

 literature, which I know is available to you. I propose rather to 

 discuss some of the more recent trends in toll circuit development in 

 the United States, but the subject is so large that I can touch only 

 upon the more salient factors, indicating to you the direction in which 

 the art is moving. 



This new art, or perhaps more accurately this extension of an older 

 art, utilizes the results of continuing researches on vacuum tubes and 

 their uses as amplifiers, modulators, and oscillators, on filters as a 

 means of splitting up broad bands of frequencies into the relatively 

 narrow bands required for telephony or the still narrower bands 

 required for telegraphy, and on methods of electrically isolating a 

 particular circuit so as to avoid crosstalk and noise. These are not 

 all the factors requiring research, but are merely some of the more 

 important ones. 



In this connection, it should be emphasized that these new systems 

 or methods are still under development, and that their development 

 for commercial application will require continued effort over a con- 

 siderable period. We have come to group these new systems or 

 methods under the term "high-frequency broad-band wire trans- 

 mission." Instead of confining ourselves to a frequency range ex- 

 tending to about 30,000 cycles, as used for our present carrier systems, 

 we are setting for our objective the transmission and utilization of 

 bands of frequencies a million or more cycles wide in the case of 



