298 



BELL SYSTEM TECHNICAL JOURNAL 



Over-All Performance 

 While the system was set up specifically to provide transmission for 

 the demonstration into Washington on April 27, 1933, it was operated 

 over a period of sev^eral weeks and complete tests and measurements 

 were carried out for the purpose of gathering information on cable 

 carrier systems. The complete layout of apparatus and lines provided 

 between Philadelphia and Washington is shown in Fig. 10. 



V) -2 



Z 



o 



a. -6 



100 



500 



1000 



5000 10,000 20,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 1 1 (right) — Freqiicnc}' characteristics over carrier channels used between Phila- 

 delphia and Washington, D. C. 



The over-all frequency transmission characteristics of the three chan- 

 nels that were set up are shown in Fig. 11. These curves differ from 

 those shown in Fig. 7, and include the complete high frequency line 

 circuit with its 150 miles of cable, repeaters, equalizers, and other 

 equipment. It may be seen that between the desired frequency 

 limits the circuit is substantially flat in transmission performance to 

 within ± 1 db. Various noise measurements made on the over-all 

 circuit indicated that the circuits fully met the requirements that had 

 been set up, and that the line and apparatus noise was inaudible in the 

 auditorium at Washington even during the weakest music passages. 

 The circuit also was found to be free from nonlinear distortion to a 

 satisfactory degree. Harmonic components generated when single- 

 frequency tones were applied to the channels at high volumes were 

 found with one unimportant exception to be more than 40 db below 

 the fundamental. 



As a means of obtaining a further increase in volume range, which 

 was not actually required for this demonstration, tests were made 

 with a so-called predistortion-restoring technique. In this the higher 

 frequency components of the music were transmitted over the carrier 

 channels at a volume much higher than normal in relation to the 

 volume of the lower frequencies. By this means any noise entering 

 the carrier channels at frequencies equivalent to the higher music 

 frequencies is greatly minimized in effect. 



