The Bell System Technical Journal 



Vol. XXIII July, 1944 ^0. 3 



Effect of Telegraph Distortion on the Margins of Operation 

 of Start-Stop Receivers 



By W. T. REA 



Recent practical and theoretical investigations of the effect of signal dis- 

 tortion on the margins of operation of start-stop telegraph receivers have led 

 to the development of improved methods of testing and adjusting receivers, 

 have enabled criteria of distortion tolerance to be set up for subscribers' and 

 monitoring receivers and regenerative repeaters, and have made possible the 

 application of more convenient and accurate standards of telegraph trans- 

 mission. This paper describes the causes of distortion occurring both externally 

 and internally to the receiver and the effects of such distortion on the operating 

 margins. Methods of determining the internal distortion of a receiver are 

 described and some of the more important considerations involved in establish- 

 ing distortion tolerance criteria are discussed. 



DURING the past decade the proportion of Bell System telegraph 

 service operated on a start-stop teletypewriter basis has shown a 

 continuous increase. Whereas in 1930 about 65% of telegraph long- 

 distance circuit mileage was manual Morse, the present proportion of 

 teletypewriter and teletypesetter service stands at 92%. The rapid growth 

 of teletypewriter switching facilities has been an important factor in this 

 development. 



Naturally, this situation has made increasingly important a thorough 

 understanding of the factors which affect the performance of start-stop 

 receivers. In the present paper, an effort will be made to show some 

 relationships between signal distortion and the operating margins of start- 

 stop receivers. 



A properly designed start-stop telegraph receiver requires only a small 

 portion of the time of each signal element to permit a selection to be made; 

 i.e. to determine whether the signal element in question is marking or 

 spacing. The remainder of the signal element gives an operating margin, 

 and serves as a reserve to take care of imperfections in the receiver or 

 distortions which the telegraph signals may suffer in their passage over Unes 

 and through repeaters. The greater the signal distortion is, the smaller 

 will be the margin which remains in the receiver to overcome the effect of 

 such factors as wear of parts, variation of adjustments, or differences in 

 speed between transmitter and receiver. 



A consideration of the effects of telegraph distortion on the margins of 



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