MEASUREMENT OF TELEGRAPH TRANSMISSION 183 



tudes of bias, the Bias In-Bias Out characteristic for a circuit having 

 an admittance as computed above would have the general character- 

 istics indicated in Fig. 28. 



Considered in terms of the transient behavior, if the circuit had the 

 transfer admittance defined by the above equation, or any of the infinite 

 number of other prescribed admittances using higher frequencies, the 

 transient resulting from a single transition would be such as to have 

 zero value at each of all possible future transition points. Also if 

 the circuit were adjusted so that the four simple characters were trans- 

 mitted with negligible distortion, the transients would fulfill the condi- 

 tions for the satisfactory transmission of the other 1020 possible 

 characters. 



From Fig. 28, it is obvious that a circuit which had a perfect transfer 

 admittance and a frequency band width large compared to the char- 

 acter repetition frequency, would have a Bias In-Bias Out character- 

 istic which crossed the 45 -degree line at many points and approached 

 it as a limit. It is interesting to note the relation between the devia- 

 tions of the Bias In-Bias Out characteristic from the 45-degree line 

 and the frequency band width. In the example under discussion there 

 are five waves in the characteristic which correspond to the frequency 

 band width divided by the number of characters per second. The 

 band width required is numerically equal to the product of the number 

 of waves in the Bias In-Bias Out characteristic and the number of 

 characters transmitted per second. 



Start-Stop Teletypewriter Signal 

 Start-stop teletypewriter systems may employ varying speeds and 

 signal arrangements. The 60-word-per-minute (60-speed) system is 

 the one most generally used in the Bell System and is taken as an ex- 

 ample in this appendix. Similar methods of analyses and tests could 

 be applied to other systems. 



The 60-speed teletypewriter signal consists of a starting unit which 

 is always spacing, five selecting units, and a stop signal which is 1.42 

 units in length and is always marking. The duration of a unit signal 

 pulse is 22 milliseconds and the total length of each character is, 

 therefore, 1 -f 5 + 1.42 = 7.42 times units or 163 milliseconds. With 

 no pause between succeeding characters there are 368 operations per 

 minute or 



368 1 . , ^ ,. , 



-ryr- — -7-^ =6.13 opcratious per second. 

 60 .163 



The problem of transmitting these signals without distortion is 



