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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 13 — Outlying-station test set. 



measurement of operating and interfering currents, and the portability 

 of the set, it is expected to be of considerable benefit in telegraph 

 transmission maintenance work at outlying subscriber stations. 



D. Distortion-Distribution Recorder 



A short test to determine the peak value of the distortion existing 

 on a circuit at any particular time is of great value but in development 

 work it is sometimes desired to know the frequency of occurrence of 

 different values of distortion ; in other words, to obtain data from which 

 a distribution curve of distortion may be plotted. Such distribution 

 curves were referred to in the earlier paper and it was stated there that, 

 in general, the distribution of distortion is in accordance with the normal 

 law. To check this on different types of circuits in connection with 

 work on transmission ratings of telegraph circuits and for other labora- 

 tory uses a "distortion-distribution recorder" has been devised. 



The distortion-distribution recorder operates on a synchronous basis 

 and is suitable only for testing over looped-back circuits. It may be 

 used at any speed up to about 75 words per minute (28.5 dots per 

 second, 57 bauds). It indicates the distortion of the transition at 

 either end of a signal pulse, the indication being in terms of the dis- 

 placement of the transition from its correct time of occurrence in the 



