144 BELL SYSTEM TECHNICAL JOURNAL 



fall into the last-mentioned classification and the increased use in the 

 Bell System of stop-start teletypewriter transmission has afforded 

 the opportunity of making telegraph transmission tests on working 

 circuits without the need of transmitting information regarding 

 the character of the sent signals. Of course, when sectionalized 

 transmission measurements are desired it is necessary for proper 

 interpretation to transmit the results of the measurements to a single 

 point for analysis, but the communication of this information is not 

 at all burdensome. 



Aside from the ability to measure on a straightaway basis, which is 

 primarily a field-maintenance requirement, testing equipment should 

 preferably be direct-reading without the need of measuring adjust- 

 ments on the part of the tester. Direct-reading devices in general effect 

 considerable reduction in the time required for measurement. This 

 feature is especially important in the field where a rapid test of possible 

 trouble conditions is desirable and in the laboratory where the large 

 numbers of tests which are necessary for thoroughly checking a 

 telegraph transmission system under all of the likely operating con- 

 ditions become even at best tedious and time-consuming. 



The major development in telegraph transmission testing within 

 the past few years has been the provision of instrumentalities which 

 possess the desirable properties indicated above. They permit the 

 rapid and direct reading of signal distortion on working teletypewriter 

 circuits. The same instrumentalities when used with selected or 

 miscellaneous teletypewriter test signals also permit the rapid de- 

 termination of the capabilities of a telegraph circuit in the field or in 

 the laboratory. 



A paper published in 1927 ^ discussed fundamental concepts relating 

 to signal distortion and described a number of measuring devices 

 which had been employed in the Bell System. Another paper ^ 

 treated the design of telegraph circuits for distortionless transmission 

 from the standpoint of the steady-state characteristics. The funda- 

 mental ideas set forth in these papers have continued to form the 

 basis for development of the technique of measuring telegraph trans- 

 mission. However, it has been necessary better to adapt these ideas 

 to start-stop teletypewriter operation and changing field requirements. 



Operation of telegraph circuits by means of start-stop teletype- 

 writers *• ^ using 7.4-unit code has become of much greater importance 

 in the Bell System in the past dozen years. The majority of private- 

 line service is now furnished on a teletypewriter basis; also teletype- 

 writer exchange (TWX) service,* inaugurated in 1931, has become an 



1 References are listed at end of paper. 



