406 BELL SYSTEM TECHNICAL JOURNAL 



on long non-loaded cables operated at high speeds the presence of this 

 zero interval may be indicated only by a difference in slope of the 

 recorded curve as shown in B, Fig. 2. Interference currents, which 

 are present to some extent in all cables, are superposed upon the 



<:^y. 



m il Mi l i -lni'- '''■— '* ' * ■*■ — "-~--' '■ — 



Fig. 2 



received signals and cause troublesome distortion in the zero intervals 

 and the length of the sustained pulses. The absence of these zero 

 intervals of unit length in the two-element five-unit code, combined 

 with the fact that only the middle portion of each received signalling 

 impulse is used to operate the selecting mechanism of the printers, 

 considerably reduces the effect of interference upon the accuracy of 

 translation and makes it unnecessary to secure such refined signal 

 shape. 



The accurate evaluation, in terms of transmission speed, of the 

 relative merits of the various telegraph codes is a highly complex 

 problem which does not readily lend itself to solution through purely 

 theoretical methods since it involves consideration not only of the 

 total number of separate combinations which must be provided to 

 represent the letters of the alphabet and all other characters to be 

 transmitted, the frequency of occurrence in trafific of the various 

 characters, and the average number of unit impulses required to form 

 the combinations, but also depends upon the characteristics of the 

 line or cable and the nature and distribution of the interference 

 encountered and its effect upon the shape and definition of the received 

 signals. The application of a code to any specific case also involves 

 the more practical considerations of the type and operating charac- 

 teristics of the apparatus employed. Practical experience therefore 

 probably forms the best guide to the choice of a code. 



In consideration of the conditions referred to above and the experi- 

 ence previously gained through the extensive use of the Baudot type 

 of code on automatic telegraph circuits both in the United States and 

 Europe, it was concluded at an early stage in the development that 



