MEASUREMENT OF TELEGRAPH TRANSMISSION 179 



A previous paper ^ developed methods of determining the correct 

 transfer admittance for distortionless transmission under certain 

 assumed conditions. One general and simplifying assumption was 

 that the time interval between transitions in the telegraph signals would 

 be an integral number of time units. If telegraph circuits were to be 

 designed for the transmission of telegraph signals of this nature, dis- 

 tortionless transmission would be expected when the overall transfer 

 admittance of the circuit was one of the many possible admittances 

 discussed in the previous paper. Although a knowledge of the admit- 

 tance requirements is a helpful guide in the design of circuits and per- 

 mits the establishment of certain boundaries, the exact adjustment of 

 transfer admittance to the proper value for satisfactory transmission 

 on the basis of admittance measurements presents many practical 

 difficulties and up to the present has not been generally used. 



Another approach to the problem is the actual transmission of 

 miscellaneous signals of the type required and the adjustment of the 

 transfer admittance on a cut-and-try basis until the overall results are 

 satisfactory. For relatively simple circuits satisfactory results can be 

 obtained in this manner. However, for circuits which are electrically 

 long and contain complex networks, a more orderly approach is 

 desirable. 



The problem may also be approached from the standpoint of ad- 

 justing the transfer admittance of the circuit so as to minimize the 

 transient associated with each transition at the times at which succeed- 

 ing transitions may occur. This may, of course, be done by means of 

 oscillograph observations, but this procedure has serious practical 

 disadvantages. An advantageous method, however, is to measure the 

 characteristic distortion of simple signal combinations while making 

 the adjustments. For this purpose, signals, each composed of two 

 transitions, repeated at intervals long compared to the duration of the 

 appreciable transient, are used. If the circuit is adjusted so as to 

 transmit without distortion signals having respectively separations of 

 one, two, three, etc. signal elements, between transitions, the require- 

 ments for distortionless transmission of miscellaneous signals of the 

 nature under consideration are met, as will be shown below. 



Repeated two-transition signals with varying integral time-unit 

 intervals between transitions may be considered as telegraph re- 

 versals having a frequency determined by the period of repetition 

 and bias determined by the interval between transitions. There- 

 fore, telegraph reversals of varying bias may be used as a source of 

 test signals, with a simple bias-measuring set at the receiving end and 

 the input bias-output bias characteristic of a circuit determined for 



