PROGRAM TRANSjMISSIOX CIKCL/TS 237 



originate, a supplementary transmission system must be provided, inter- 

 connecting the many studios and broadcasting stations. The wire networks 

 that perform this function comprise the subject matter of this paper. 



The present extent of the wire-line facilities which are associated with the 

 major portion of these networks is indicated in Fig. 1. The width of the 

 lines on this chart has been made proportional to the numbers of circuits in 

 the various sections. The total length of these circuits is in excess of 1 10,000 

 miles, and it is not unusual for a program originating at some point on a 

 network to traverse more than 7,000 miles of circuit before being broadcast 

 by the most remote station. 



The requirements which the program networks must meet are in the final 

 analysis determined largely by the needs of the broadcasters. The objective 

 of a program network service is to meet these needs in as complete and 

 prompt a manner as possible consistent with reasonable cost. With this 

 objective in mind, it is necessary in planning the plant to consider not only 

 the day-to-day needs, but the possible future needs as well. The importance 

 of this may be appreciated when it is considered that plant provided today 

 for program transmission service will need to be adaptable to the service 

 requirements ten or twenty years hence. As a result of such planning, 

 cables and equipment installed five, ten, and fifteen years ago meet present- 

 day requirements, and, with some rearrangements, will take care of those 

 likely to develop tomorrow. 



The detailed planning of program transmission circuits requires considera- 

 tion of: 



1. The numbers of circuits likely to be required, section by section, over 

 each route; 



2. The provisions for reliability, flexibility, operation, and supervision 

 essential to a high-grade network service; 



3. The transmission requirements, or electrical characteristics, necessary 

 to achieve a natural reproduction of the program. 



These three general classes of requirements will be considered in order. 



Number of Circuits Required 



The circuits which have been established on a full-time basis for continu- 

 ing use form the backbone of the program networks. Even for these circuits, 

 however, permanence is relative since frequent extensions and rearrange- 

 ments are made to meet changing requirements of the broadcasters. Aside 

 from these fuUtime circuits there are intermittent requirements occasioned 

 by special events and other short-period needs of the broadcasters, some of 

 which involve networks almost as extensive as the full-time networks. 

 In addition reliability of service requires provision for rerouting the net- 

 works in the event of trouble. Figure 2 shows the year-by-year growth in 



