174 BELL SYSTEM TECHNICAL JOURNAL 



impressed distortion and are, therefore, not suitable for measuring 

 purposes. 



It is apparent from the above discussion that it is necessary to 

 adjust teletypewriters for minimum internal distortion before they 

 can be used for measuring purposes. Where it is desired to use 

 teletypewriters in testing, procedures have been established in the 

 Bell System to insure that they will be in proper condition and fairly 

 good results are obtained with them. 



Although the effect of distortion on the orientation margins has 

 been discussed previously^- ^' '' it may be of value to state here how 

 distortion affects the margins at the lower and upper orientation 

 limits in connection with the use of teletypewriters for testing pur- 

 poses. In general, the maximum reduction corresponds numeri- 

 cally to the total distortion as indicated by the start-stop type of 

 measuring set described above. Distortions other than bias usually 

 affect both orientation limits equally so that the amount of bias can 

 be estimated by subtracting the smaller reduction of the two from the 

 larger. In addition it is possible to obtain an idea of the characteristic 

 distortion by the indications obtained during the orientation test. 

 If the orientation limits are fairly definite, that is, if the copy changes 

 from good to bad when the range finder is moved only a small distance, 

 it is likely that the distortion is due to bias. If there is a definite 

 range over which certain characters are found to be consistently in 

 error this is due to characteristic distortion. If the limits are not 

 definite, that is, if there is a range over which errors occur but not con- 

 sistently on certain characters this is probably due to fortuitous 

 distortion. Although a qualitative analysis of the distortion may be 

 made in the manner discussed above, this indirect method is somewhat 

 laborious and may give misleading results. Moreover, it is impossible 

 to get a measurement of isolated distortions of high value. 



G. Telegraph Service Monitoring Set 

 An automatic telegraph service monitoring set has been designed 

 for the purpose of giving an alarm at repeater stations whenever the 

 distortion on circuits becomes abnormally high or whenever an 

 excessive number of large distortions or "hits" is experienced. This 

 set is still under development; however, a description of it will be 

 given because it is thought to be of general interest to those concerned 

 with telegraph transmission measuring. 



In the interest of economy and simplicity this set contains a so- 

 called shortest-pulse type of measuring circuit rather than a start-stop 

 type. Measurement on this basis will of course result in a loss in 



