LONG DISTANCE TELEPHONE CIRCUITS 541 



amount of noise on the circuit at any given time, the less noise the 

 more sensiti\'e the device. It has been found possible to adjust this 

 device to sufficient sensitivity to permit working a 3600-mile (6000- 

 kilometer) circuit with 9 db net loss. When noise is added to the 

 circuit the sensitivity of the device, of course, diminishes but the 

 added echo is largely masked by the increased noise. 



Delay Distortion 



For a 3600-mile (6000-kilometer) H-44-25 circuit the difiference in 

 the delay at 1000 cycles and at 3000 cycles amounts to about 0.025 

 second. Experiments which have been made on circuits introducing 

 very little non-linear distortion indicate that this amount of delay 

 distortion by itself is not particularly serious. However, on the long 

 four-wire circuits where non-linear distortion is also present, the 

 effect of delay distortion becomes more pronounced so that it becomes 

 quite objectionable. Delay distortion correctors would therefore be 

 required for H-44-25 circuits of this length, although for circuits of 

 1800 miles (3000 kilometers) they do not appear necessary. 



Time of Propagation 



When a long connection is built up using cable circuits, the delay 

 proper, quite apart from delay distortion, becomes important. For a 

 3600-mile (6000-kilometer) length of H-44-25 circuit equipped with 

 delay equalizers the time of propagation is about one-quarter of a 

 second in each direction. This time of propagation is generally 

 considered about all that should be tolerated and is the C. C. I, 

 tentative limit for a complete connection. 



Adverse Interaction of Two Echo Suppressors 



When voice-operated devices are introduced on very long cable 

 circuits another complication results. Assume, for example, that the 

 3600-mile (6000-kilometer) connection is made up of two links, each 

 equipped with an echo suppressor of the usual type, either mechanical 

 relay or vacuum tube operated. Assume that the echo suppressors 

 are 1800 miles (3000 kilometers) apart, the delay between these 

 devices being one-eighth of a second for each direction of transmission. 

 When conversations are carried on over this circuit it is found that 

 occasionally when the speakers at the two ends utter words at nearly 

 the same instant both echo suppressors respond, each echo suppressor 

 blocking one direction of the circuit. Consequently, certain words or 

 parts of words are lost. In telephone conversations it is found that 

 with such a circuit arrangement if the time of propagation in each 

 direction between echo suppressors does not exceed about 0.1 second, 



