PROGRAM TRANSMISSION CIRCUITS 247 



demonstration of stereophonic transmission and reproduction of music' 

 Studio-transmitter loops have been provided to transmit wider frequency 

 bands than the 5,000 cycles currently provided on the nation-wide net- 

 works. At the present time, many of the studio-transmitter loops are being 

 set up to transmit bands up to 15,000 cycles. A demonstration will be 

 given at the close of the paper of the transmission of programs over cable 

 circuits about 1,200 miles in length with frequency bands extending to 

 15,000, 8,000, and 5,000 cycles. The 5,000-cycle circuit is of the type in 

 present commercial use. The 8,000-cycle circuit is of a type to which 

 much of the present program plant can readily be modified. The 15,000- 

 cycle circuit consists of a standard carrier system to which has been added 

 program terminal equipment now under development. 



In the consideration of transmission requirements for program circuits 

 other than nominal frequency band, the variation in performance with length 

 and type of circuit is important, since the factors tending to impair trans- 

 mission are in the nature of small amounts of distortion or noise which 

 accumulate over the length of the circuit. If these effects varied in some 

 definite manner with length, transmission requirements could be fixed on 

 that basis. However, good engineering practice frequently requires choos- 

 ing for the various sections of a long circuit, different types of facilities 

 whose contributions to the total effects are not in proportion to their length. 

 Even the determination of the maximum permissible distortion and noise 

 on a circuit is influenced by outside factors such as are involved in the broad- 

 casters weighing operating flexibility and cost against the frequency of 

 occurrence of unfavorable network routings and the number of stations 

 affected. For example, in order to secure operating flexibility with a mini- 

 mum of total network mileage, most of the networks employ the so-called 

 "round robin" principle for a part of the network. In this arrangement the 

 circuit follows a route from station to station forming a continuous loop 

 which returns to its starting point. This naturally results in increased 

 circuit mileage between the program source and the more distant listeners 

 with an attendant increase in undesired transmission effects. For these 

 reasons no exact or specific transmission requirements can be stated for 

 even the over-all performance of program transmission service. 



Volume Range 



'The permissible volume range for a program circuit is determined by 

 the maximum volume which can be transmitted as limited by nonlinear 

 distortion or crosstalk, and the minimum volume which can be transmitted 

 without impairment from the noise present on the circuit. 



In connection with their design the various types of program circuit are 

 subjected to listening tests in which the transmission of program over a 



