544 BELL SYSTEM TECHNICAL JOURNAL 



curved line. The accuracy with which he is able to accomplish this 

 will depend upon how far he goes between adjustments of the steering 

 mechanism and upon the number of positions in which he is able to 

 set it. 



By this more or less artificial device the continuous magnitude- 

 time function as used in telephony is made subject to the same type 

 of treatment as the succession of discrete selections involved in 

 telegraphy. 



Rate of Communication 



So far then we have derived an expression for the information 

 content of the symbols at the sending end and have shown that we 

 may evaluate a transmission system in terms of how well the wave as 

 received over it permits distinguishing between the various possible 

 symbols which are available for each selection. Let us consider next 

 how the distortion of the system limits the rate of selection for which 

 these distinctions between symbols may be made with certainty. 



Limitation by Intersymbol Interference 

 We shall assume the system to be free from external interference 

 and to be such that its current-voltage relations are linear. In such 

 a system the form of the transmitted wave may be altered due to 

 the storage of energy in reactive elements such as inductances and 

 capacities, and its subsequent release. To evaluate the effect of such 

 distortion in making it impossible to determine correctly which one 

 of the available symbols had been selected, we may think of this 

 distortion in terms of "intersymbol interference." In order to 

 determine the result of any one selection an observation is made at 

 such time that the disturbance resulting from that selection has its 

 maximum effect at the receiving end. Superposed on this effect 

 there will be a disturbance which is the resultant of the effects of all 

 the other symbols as prolonged by the storage of energy in the system. 

 This resultant superposed disturbance is what is meant by intersymbol 

 interference. Obviously if this disturbance is greater than half the 

 difference between the effects produced by two of the values available 

 for selection at the sending end, the wave resulting from one of those 

 values will be taken as representing the other. Thus a criterion for 

 successful transmission is that in no case shall the intersymbol inter- 

 ference exceed half the difference between the values of the wave at 

 the receiving end which correspond to the selection of different values 

 at the sending end. 



Obviously the magnitude of the intersymbol interference which 

 affects any one symbol depends on the particular sequence of symbols 



