TRANSMISSION LINES 287 



matter of expediency in this particular case this feature of operation 

 was not used, and three separate pairs were employed, one for each 

 channel. In the future the same technique undoubtedly would permit 

 two or possibly more of these extra-broad-band transmission paths to be 

 obtained on the same pair of conductors. The most important reason 

 for choosing the carrier method rather than transmission in the natural 

 audio-frequency range in this particular case was that, because all 

 other transmission circuits in the same cable were at a considerably 

 lower frequency and because the lead sheath of the cable acts efficiently 

 at the high frequencies to shield the pairs from induced disturbances 

 from the outside, it offered a special freedom from crosstalk and noise. 

 With these arrangements, which will be described in somewhat 

 greater detail in what follows, requirements of transmission were met 

 very satisfactorily and the reproduction of the symphony music in 

 Washington with the orchestra playing in Philadelphia suffered not 

 the least in comparison with the reproduction of the same program in 

 an auditorium in Philadelphia located but a few feet from the hall in 

 which the orchestra played. It is believed that, if necessary, by the 

 use of the same principles, line circuits may be set up and comparable 

 quality reproduction given throughout the country. However, as 

 will be evident from part of the discussion which follows, in some 

 respects the problem of meeting the requirements in transmission 

 between Philadelphia and Washington was not as difficult as might 

 be encountered in other localities. Hence even more complex arrange- 

 ments might be necessary if it were desired to establish such trans- 

 mission circuits to other points, and particularly for greater distances. 



Line Circuits 



There are several all-underground cables between Philadelphia and 

 Washington. As described in a paper ^ by Clark and Green given 

 before the A. I. E. E. in 1930, recently laid cables contain several 

 16-gauge conductors distributed throughout the cross section of the 

 cable for possible use as program circuits in chain broadcasting. 

 These pairs, however, ordinarily are loaded and equipped with re- 

 peaters at approximately 50-mile intervals so that they transmit a 

 frequency range up to about 8000 c.p.s. 



In one of the cables several pairs of this type had not yet been 

 loaded, and these pairs were used for this newer transmission. Because 

 of the higher frequencies employed and the greater attenuation en- 

 countered, it was necessary to install repeaters at more frequent 

 intervals. As may be noted in Fig. 1, the normal cable layout between 

 Philadelphia and Washington includes two intermediate repeater 



