246 BELL SYSTEM TECHNICAL JOURNAL 



seen to be somewhere between 45 and 65 decibels. There is, therefore, no 

 advantage to the listener in providing a permissible volume range materially- 

 wider than this in the transmission system. 



The above discussion applies primarily to the transmission of symphonic 

 and similar high-grade program material. Much program material is less 

 exacting in its requirements, but on the other hand some sound effects 

 such as the tearing of paper require the reproduction of higher frequencies 

 for complete naturalness. 



With these broad considerations in mind, the requirements of high-quality 

 program circuits may now be taken up. As has been noted, the program 

 network is but one part of the over-all broadcasting system which, in ad- 

 dition, includes microphones and studio equipment, radio transmitters, 

 and the home receivers with their loud-speakers. It may be taken as a 

 goal for the program networks that their transmission be nearly enough 

 distortionless so that the over-all performance in regard to naturalness of 

 reproduction will not be limited by them. 



To meet such a requirement for short program circuits having only one 

 or two sections is not difficult technically and does not in general require 

 costly types of plant. However, the vast country- wide program networks 

 are made up of many sections of circuits in tandem, which as mentioned be- 

 fore may total in some cases as much as 7,000 miles. This makes it necessary 

 to design and operate the individual circuits to very close limits so that the 

 cumulative discrepancies in the whole network will not exceed tolerable 

 values; and to consider carefully the types of plant employed lest by virtue 

 of sheer numbers of units involved, the total cost be out of line with the 

 over-all grade of service being given the listner. These two conflicting 

 factors are important ones in the consideration of transmission requirements 

 for networks. The determination of the practical working characteristics 

 of program networks involves a consideration not only of the physical and 

 cost factors discussed above but also of such other factors as cost of studios, 

 broadcast transmitters and receivers, and the limitations of the frequency 

 allocations of broadcast stations. 



From the standpoint of frequency band the consideration of all factors has 

 resulted in the major present-day program networks being set up to transmit 

 a frequency band with an upper limit of about 5,000 cycles. All program 

 facilities installed in the last ten years or so, however, have been designed 

 to be adaptable to the future transmission of frequencies up to 8,000 cycles. 

 Operation on an 8,000-cycle basis, however, requires the release of additional 

 frequency space now occupied by other services in much of the plant and a 

 general readjustment of the program-circuit characteristics. In 1933, 

 experimental wire circuits were set up between Philadelphia and Washington 

 to transmit frequency bands up to 15,000 cycles. These were employed in a 



