WIRE LINE SYSTEMS FOR NATIONAL BROADCASTING 145 



produce an undue amount of disturbance in neighboring circuits 

 which may be transmitting other programs or telephone messages. 

 The designer is also concerned lest when the program power is weak 

 the programs be unduly interfered with by noise or crosstalk from 

 other circuits. He must particularly consider the noise and cross- 

 talk which may be heard during pauses in programs. During such 

 pauses it is very annoying to the listeners to hear a background of 

 noises of various sorts and it is essential that the listeners be unable 

 during such pauses to pick up intelligible speech from telephone 

 message circuits crosstalking into the program circuit. 



At the present time generally satisfactory results are being obtained 

 in transmitting the volume range of about 30 decibels (3.4 nepers). 

 Considerably more must be done both in the radio and in the wire 

 systems, however, before there can be transmitted volume ranges 

 comparable with those put out by symphony orchestras, high-grade 

 artists, and the like. 



Having indicated in a general way the requirements of program 

 transmission circuits, there will next be described the wire systems 

 which are now in use in the United States. 



The present-day program transmission circuits in the United States 

 are "on a voice-frequency basis," which means that the waves trans- 

 mitted over the circuits are essentially copies of the sound waves 

 impinging on the microphones. Most of the circuits now being pro- 

 vided are carried by the familiar open wires, usually copper wires 

 0.165 inch (4 mm.) in diameter spaced about 1 foot (30 cm.) apart 

 on the crossarms. The transmission properties of an open-wire pair 

 without loading are well suited for program transmission purposes 

 since the distortion is comparatively small although it is far from 

 negligible. Spaced at intervals on these circuits, averaging roughly 

 150 miles (240 kilometers) apart, are one-way repeaters or amplify- 

 ing devices. Along with these amplifiers are other electrical devices 

 for counteracting the distortion introduced by the open-wire circuits, 

 incidental cables involved, etc. Other one-way repeaters are pro- 

 vided at the terminals of the circuit. Considerable technical refine- 

 ment is, of course, involved in the design of these amplifiers and of 

 the auxiliary apparatus associated therewith which cannot be gone 

 into here. 



In setting up the program transmission circuits, an important part 

 of the work consists in making measurements at different single 

 frequencies within the band which it is desired to transmit over the 

 circuit. Before making such overall measurements, the amplifiers 

 and auxiliary apparatus are so adjusted locally as to compensate for 



