MEASUREMENT OF TELEGRAPH TRANSMISSION 149 



should be as compared to the higher-frequency components of the 

 received current waves. The required type of equaUzer in this case 

 is one which adds loss at frequencies in the vicinity of zero, without a 

 corresponding loss at the higher frequencies. If, however, the slope 

 had been in the opposite direction, an equalizer which would dis- 

 criminate against the higher frequencies would have been required. 

 As a check of the equalizer setting, the total distortion and bias are 

 usually measured using miscellaneous teletypewriter signals. 



In addition to measuring with undistorted signals applied at the 

 sending end of a circuit, the measuring technique has been expanded 

 to include measuring with distorted signals and a device has been 

 made available for field use by means of which reversals or teletype- 

 writer characters may be distorted by known amounts. This kind of 

 test furnishes additional information in that it affords an examination 

 of the effect of signal combinations which are not included in perfect 

 telegraph signals. It is of value because in actual operation a given 

 telegraph section may not have perfect signals impressed at the 

 sending end due to distortion occurring in previous sections or at the 

 transmitter. Although such a test furnishes valuable information 

 for line testing, it has been used in the field up to the present mainly 

 in testing the distortion-tolerance of subscriber-station teletypewriters 

 with signals from the adjacent central office and in maintaining re- 

 generative repeaters. 



It is necessary, of course, to make transmission measurements on 

 the manual Morse circuits which still constitute a considerable part of 

 the total mileage. Testing such circuits by the same methods as used 

 for teletypewriter circuits has been found to give good results. How- 

 ever, due to improvement of telegraph circuits, the transmission- 

 maintenance problem in this case consists mainly of keeping the bias 

 within reasonable limits for which purpose simple tests with reversals 

 can be used. 



New Measuring Devices 



In the following is given a description of a number of testing methods 

 and arrangements which have been found useful in recent years both 

 in the field and in development work. 



A. Start- Stop Distortion- Measuring Set for Central-Office Use 

 A start-stop type of measuring set for testing teletypewriter circuits 

 has been developed and is now used generally in maintenance work 

 and special testing in the field and in laboratory work. This repre- 

 sents an outstanding advance in that it provides a quick and convenient 

 means for reading, directly from conventional-type milliameters as 



