MEASUREMENT OF TELEGRAPH TRANSMISSION 



187 



Computations have also been made for the case of a repeated signal 

 combination consisting of any two teletypewriter characters. The 

 results indicate that this may also be transmitted without distortion 

 if the requirements have been met for the six simple characters sent 

 repeatedly. 



When distortion is present, there is deviation of the received current 

 from the desired value at the time it should be 1/2 and the deviation to 

 be expected at any transition for any character may be computed from 

 equations similar to those given above. This, of course, differs from 

 the amount of distortion which would be measured in per cent. It is 

 of interest that the characteristic distortion on the more complicated 

 characters may be materially greater than that measured on the simple 

 two-transition characters. 



The transient behavior of a circuit having the transfer admittance 

 specified by the smooth curve of Fig. 30 may be determined by methods 

 utilizing the Fourier integral. However, certain characteristics of this 

 transient, namely, the points at which it must be zero, are known in 

 advance from the conditions entering into the determination of the 

 admittances. Although these conditions do not directly prescribe 

 the magnitude of the transient at other points, additional information 

 may|beJobtained|from an inspection of Fig. 31. 



1.0- 

 .5 

 0- 



DISTRIBUTION OF — 

 I FUTURE TRANSITION 

 (POINTS 



Fig. 31 — Received current for "letters" character assuming the 

 transfer admittance of Fig. 30. 



This figure shows the computed received current over a telegraph 

 circuit having the admittance shown in Fig. 30 when a repeated tele- 

 typewriter "Letters" signal is transmitted. It may be noted that the 

 transient decays to inappreciable amplitudes at times greater than the 

 shortest time unit of .022 second. This is significant, since it means 

 that a particular arrangement of transitions is not of practical im- 

 portance as long as transitions do not come at intervals closer than 

 .022 second. With this admittance, therefore, no difficulty would be 



