26 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



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10 



100 



150 200 



300 400 500 600 800 1000 1500 2000 

 FREQUENCY IN CYCLES PER SECOND 



3000 40C0 



Fig. 6 — Frequencj^ characteristic of typical telephone circuit. 



Circuit Noise 



The circuit noise, referred to a zero level point is as follows: 



London-New York 

 New York-London 

 London-Montreal 

 Montreal-London 



Best 30 dba 

 Best 29 dba 

 Best 30 dba 

 Best 30 dba 



worst 36 dba 

 worst 41 dba 

 worst 33 dba 

 worst 31 dba 



Two circuits at present exceed the objective of 38 dba in the New York- 

 London direction only ; the higher noise levels refer to the high frequency 

 channels in the Oban-Claren\'ille cable. After additional data on the 

 effect of cable temperature variations are accumulated, refinements will 

 be made in the equalization and adjustment of levels on the Oban-Claren- 

 ville link. It is expected that the two worst channels can then be made 

 to meet the objectives — still without the use of compandors. 



Frequency Characteristics of Program Channels 



Fig. 7 shows the measured frequency characteristic of a London-New 

 York program channel; this is typical. 



Telegraph Channels London-Montreal 



In the Agreement it was envisaged that at least six 50-baud telegraph 

 channels could be provided in each direction in the Canadian half circuit. 

 In fact, eleven such channels have been provided using carriers spaced 



