DESIGN OF SYSTEM — NORTH ATLANTIC LINK 67 



channels, include: possible increase of transmission level at input of the 

 final transmitting and load limiting amplifier in the transmitting termi- 

 nal; pre-distortion ahead of this amplifier; compandors; increase of dc 

 current; and undersea re-equalization in later years. This last would be 

 very expensive, and so it is necessary to examine fully the possibilities 

 of the other measures. 



The penalty for increasing the transmission level at the input of the 

 transmitting amplifier is more peak-chopping and modulation-noise 

 peaks. The improvement that could be realized in this way is probably 

 fairly small. 



Pre-distortion is accomplished by inserting ahead of the transmitting 

 amplifier a suitable shaping network adjusted for gain in the top part 

 of the band and loss at lower frequencies. A complementary network 

 (restoring network) is placed at the receiving terminal. This measure 

 would improve signal-to-noise in the uppermost part of the band and 

 reduce it in lower channels which have less noise. Some 3-db improve- 

 ment might be thus realized in the top channel. 



Compandors would give an effective signal-to-noise improvement of 

 up to about 15 db for message telephone service, but none for services 

 such as voice-frequency telegraph. Compandors would be applied only 

 to those channels needing them. They halve the range of talker volume, 

 but also double the transmission variations between compressor and 

 expandor, and thus tend to require some increase in the overall channel 

 net loss. The program (music) channels are already equipped with com- 

 pandors which use up a part of the obtainable advantage. 



If the combination of such measures netted an effective message sig- 

 nal-to-noise improvement of 20 db in the top channel, this would counter- 

 balance aging of some 28 db in this channel, if the aging were uniformly 

 distributed along the system length. Thus considerable aging could be 

 handled without undersea modification. 



ACKNOWLEDGEMENTS 



A system of the complexity of the one described obviously results 

 from teamwork by a very large number of individuals. However, no 

 paper on this subject could be written without acknowledgement to 

 Dr. 0. E. Buckley and J. J. Gilbert and O. B. Jacobs, now retired from 

 Bell Telephone Laboratories. All of the early and fundamental Bell 

 System work on repeatered submarine cable systems, and the concept 

 of the flexible repeater, came from these sources and from their co- 

 workers. Messrs. Gilbert and Jacobs have also contributed to the present 

 project. 



