MEASUREMENT OF TELEGRAPH TRANSMISSION 145 



important factor, having already grown to the point where the trunk- 

 circuit mileage employed is a large part of the total telegraph mileage. 

 Incidentally, there has been at the same time a general increase in 

 operating speeds, so that the majority of the circuits now operate at a 

 nominal speed of 60 words per minute (23 dots per second or 46 bauds). 

 As regards the requirements for measuring apparatus for field 

 transmission maintenance, the desired precision and convenience have 

 increased considerably in the last few years. This is due to several 

 causes, chief of which are the continuing desire to give better service 

 with greater freedom from interruptions and isolated errors, increase 

 in speeds of operation, and the use of more complicated circuit layouts 

 with more sections in tandem, particularly in Press and TWX service. 

 For complicated circuits it is very advantageous to employ mainte- 

 nance procedures in which each section is measured and adjusted 

 separately to close limits, to avoid the more costly and otherwise less 

 desirable overall line-up. Furthermore, a need has arisen for accurate 

 transmission measuring devices for other uses such as checking the 

 condition of receiving teletypewriters, transmitting keyboards and 

 regenerative repeaters,^ and use in "equalizing" of telegraph circuits, 

 that is, the application of wave-shaping arrangements for reducing 

 distortion. Finally, in line with improvements in main-line circuits 

 greater emphasis has been placed on maintaining loops and circuits 

 to outlying points so that they introduce but little distortion. 



Adaptation of Measuring Technique to Teletypewriter Basis 

 The earlier types of measuring sets were arranged to measure the 

 total change in the duration of signal pulses, that is, the combination of 

 the displacements at the beginning and end of any given pulse. This 

 method of measuring gives results which are directly indicative of the 

 impairment for Morse operation since the interpretation of the signals 

 depends on the total duration of pulses. This method also gives 

 a moderately good indication of the effect of distortion for teletype- 

 writer operation. 



In start-stop teletypewriter operation there are two ways in which 

 circuit imperfections may cause the transmission to be impaired. In 

 the first place, imperfections other than constant delay (known as line 

 lag), which may be neglected, may cause the start transition of any 

 character to be displaced with respect to the time at which it should 

 occur. This causes the starting of the receiving mechanism to be 

 advanced or retarded and effectively displaces the succeeding transi- 

 tions of the character. Secondly, other imperfections may also cause 

 any of the succeeding transitions to be displaced in either direction. 

 The combination of these two effects determines the effective distortion. 



