286 BELL SYSTEM TECHNICAL JOURNAL 



channels, also adds the requirement that these channels must be sub- 

 stantially identical in their transmission characteristics. 



With the exception of the last, all these aspects of the problem are, 

 of course, not peculiar to symphony music transmission. They exist 

 as part of the problem of satisfactorily transmitting any telephone 

 message. However, the requirements of this new high quality trans- 

 mission have set a new high standard of refinement, even as compared 

 with that required for ordinary radio chain broadcasting. For ex- 

 ample, ordinary telephone message transmission commonly is carried 

 out by circuits having a frequency range not exceeding 200 to 3000 

 cycles per second. Much present-day radio broadcasting involves a 

 transmission band only from about 100 to 5000 c.p.s. This new high 

 quality transmission, however, requires a range from approximately 

 40 to 15,000 c.p.s. Further, with reference to the required freedom 

 from interference, ordinary radio broadcasting seldom exceeds a volume 

 range greater than 30 decibels. The new high-quality system, how- 

 ever, requires a volume range of at least 65 db, which is more than 

 3,000,000 to 1 expressed as a power ratio. 



In considering the specific problem of transmitting from Philadelphia 

 to Washington for the demonstration given on April 27, 1933, several 

 alternative methods of providing the required transmission paths 

 presented themselves. The arrangement chosen consisted in bridging 

 the distance between the two cities by means of carrier channels over 

 cable conductors. From the telephone toll ofhce in Philadelphia to the 

 toll ofifice in Washington, three carrier transmission paths were provided 

 in which the music frequencies were stepped up from their normal 

 position in the audible range to considerably higher frequencies. The 

 frequency range from 40 to 15,000 c.p.s. picked up by the microphones 

 was transmitted over line circuits in a range from 25,000 to 40,000 

 c.p.s. After being thus stepped up in frequency, the high frequency 

 currents were applied to three non-loaded pairs in an all-underground 

 cable which was equipped with repeaters at approximately 25-mile in- 

 tervals. At Washington, step-down or demodulation apparatus restored 

 the frequencies to their normal position in the spectrum. 



For transmission between the auditorium in Philadelphia and the 

 toll office there, a distance of approximately three miles, and for trans- 

 mission in Washington between the telephone toll office and the 

 auditorium, about half this distance, normal frequency transmission 

 over small-gauge pairs in ordinary exchange cables was employed. 



The use of the carrier method for the long distance transmission has 

 several advantages. In general, it permits multiplex operation; i.e., 

 more than one message or program on the same pair of wires. As a 



