HIGH-SPEED OCEAN CABLE TELEGRAPHY 243 



Principles Involved in Operation 



To realize practically the full benefit of the high speeds of operation 

 of which loaded cables are capable, required the development of new 

 types of terminal apparatus. Although many of the functions per- 

 formed by the apparatus on loaded cables are similar to those involved 

 in the operation of ordinary cables, many new problems were intro- 

 duced by the higher speed of the loaded cable and by its peculiar 

 electrical characteristics. Also new means were required to secure 

 efficient two-way working. 



With both loaded and non-loaded cables the following steps are 

 involved in operation: translation of messages into signal impulses 

 and the application of these impulses to the cable; correction of 

 distortion or, as it is commonly called, signal shaping; amplification 

 of the feeble received impulses; and reconversion of the restored 

 received impulses into messages. The requirements to be met in 

 accomplishing the first and last of these steps with the loaded cable 

 are different from those in the case of the non-loaded cable, principally 

 on account of the higher speed of the former. The requirements to 

 be met in signal shaping and amplification are different for the loaded 

 cable both because of its peculiar distortion and because of its high 

 speed of operation. 



The means commonly employed on non-loaded cables for sendmg 

 messages involves translation of a message, usually by machine 

 methods, into electrical impulses of the standard cable code in which 

 a dot of the continental Morse code is represented by a positive 

 impulse of definite duration and a dash is represented by a negative 

 impulse of the same duration. A train of impulses of equal length 

 but of varying polarity is thus applied to the cable at the sending end. 

 This train of impulses is distorted and greatly attenuated by the cable 

 but is partially restored in shape and size by terminal apparatus and 

 is finally received on a siphon recorder which makes a record in the 

 form of a wavy line on a paper strip. In the form and spacing of the 

 humps and depressions of this wavy line an expert operator recognizes 

 the positive and negative impulses which were applied at the sending 

 end and which he is able to translate into the original message. 



The necessary correction of distortion of signals on ordinary cables 

 is accomplished by simple electrical networks at the termmals. Ad- 

 vantage is also taken of the mechanical characteristics of moving 

 coil instruments. The fundamental principles ^ involved may be 



^ For a more detailed discussion of the principles of correction of distortion as 

 applied to non-loaded cables see J. W. Milnor, Jour. A. I. E. E., Vol. XLI, pp. 

 118-136, 1922. 



