frequencies of from about 50 to about 5,000 cycles compared with a 

 band of frequencies of 250 to 2,750 cycles commonly transmitted by 

 message circuits, and means by which a broader band of frequencies 

 can be transmitted over program transmission circuits have been de- 

 veloped. Another important requirement of program transmission 

 circuits is that they shall be able to handle a wide range of input power. 

 Generally speaking, the power may be varied over a range of 10,000 to 

 1, without the overloading of the amplifiers or other apparatus on the 

 circuit at the highest levels or interference with the program by 

 extraneous noises at the lowest levels. 



Because of these and other special requirements a large part of the 

 telephone plant devoted to program transmission is designed specifi- 

 cally for that service. In the toll cables special 16-gauge pairs have 

 been placed and these pairs are equipped with loading and with 

 amplifiers designed to produce satisfactory transmission circuits. The 

 equalization for variations in attenuation, the regulating arrangements 

 to assure constant efficiency, and the compensators for the difference 

 in the velocity of transmission of currents of different frequencies 

 present special problems. 



On open-wire lines the conductors used are generally of the same 

 type as those provided for telephone message circuits. On the other 

 hand, it is necessary to give up the use of direct current telegraph and 

 generally necessary to give up the use of phantoms on circuits used for 

 program transmission. Also, in some cases the number of carrier 

 channels which can be superposed upon the conductors is reduced. 

 The amplifiers and other equipment used in connection with these 

 conductors for program transmission are of special design. 



Not only is the plant for program transmission of special design but 

 even to a greater extent the operating features are special to this type 

 of service. For many conditions continuous monitoring is necessary 

 during the transmission of the program. Special switching arrange- 

 ments are required to make possible rapid changes in the connection 

 of program transmission networks at the moment of a change in 

 program. 



At the present time there are about 60,000 miles of program trans- 

 mission circuit maintained for full-time and recurring program service, 

 of which about 40,000 miles are in daily service in the Bell System on 

 full-time networks. The extent of the network devoted regularly to 

 this purpose is indicated in Fig. 19. 



44 



