EFFECT OF SIGNAL DISTORTION IN MORSE TELEGRAPHY 273 



books and were arranged in the form of sentences and paragraphs. 

 The words were unintelHgible to the operators, which prevented them 

 from supplying distorted parts of the messages from the context. As 

 an additional precaution, words which occurred frequently and might 

 have been memorized were omitted or changed arbitrarily. 



At the receiving station, which was located in a different room from 

 the sending station, the two receiving operators were provided with 

 separate sounders and typewriters. They copied each message simul- 

 taneously and wrote their own opinions of the condition of the circuit 

 at the end of each message. Care was taken to prevent the two 

 operators from exchanging opinions. 



On each day of the tests certain preliminary adjustments were made. 

 The vibroplex key was first adjusted to vibrate at 13.5 d.p.s. by "beat- 

 ing" in a local circuit with signals from a rotary interrupter. The 

 interrupter was then set to give signals at 15 d.p.s. and the transmission 

 measuring set adjusted. After these adjustments had been made, 

 standard signals from the interrupter were transmitted over the tele- 

 graph circuit and the latter adjusted for zero bias and distortion by 

 means of the measuring set. The sending operator was then asked to 

 send a few sentences over the circuit and the receiving operators 

 listened to these signals and adjusted their sounders to be unbiased. 



There was some question as to whether this adjustment of sounders 

 by the operators really produced unbiased operation of the sounders. 

 Some tests were, therefore, made in which the sounders were adjusted 

 so that they just failed to operate properly on signals containing the 

 same amount of large positive or negative bias. The results obtained 

 in this case were almost identical with those obtained when the 

 operators adjusted the sounders. 



Immediately before a test message was transmitted over the circuit, 

 distortion was introduced. Rapid measurements of distortion were 

 made on standard signals by means of the transmission measuring set 

 to determine when approximately the desired amount of distortion 

 had been introduced into the circuit. An accurate measurement was 

 then made and recorded together with the number of the test and the 

 number of the message to be transmitted. These numbers were also 

 recorded on the received messages and served to identify the trans- 

 mission measurements corresponding to the messages received by the 

 operators. 



The various received messages were analyzed both for accuracy of 

 reception and nature of errors produced. In determining the accuracy 

 of reception, the number of misinterpretations in each received message 

 was subtracted from the total number of characters in the correct 



