TRANSMISSION OF INFORMATION 



553 



tude-frequency curve of the system. Similarly we add to the phase - 

 frequency curve of the impressed wave, which is zero at all frequencies, 

 the phase-frequency curve of the system and obtain for the received 

 wave a phase- frequency curve identical with that of the system. 

 The corresponding magnitude-time function gives the instantaneous 

 value of the received current resulting from the impressed impulse. 

 Thus we see that the steady state transfer admittance of a system is 

 identical with the steady state description of the wave which is 

 received over the system when it is subjected to an impulsive driving 

 force. Once the form of this received wave is known the received 

 wave resulting from any applied wave may be deduced by assuming 

 the applied wave to consist of an infinite succession of impulses 

 infinitesimally close together whose magnitudes vary with time in 

 accordance with the given magnitude-time function. Methods for 

 integrating the effect of this infinite succession of responses to impulses 

 so as to obtain the transmitted wave have been developed. 



From this review it is evident that the so-called frequency distortion 

 and transient distortion are merely two methods of describing the 

 same changes in wave form which result from the storage of energy 

 in parts of the transmission system. 



Significance of Product of Frequency-Range by Time 



Distortion of this sort with its accompanying intersymbol inter- 

 ference may be unavoidable in the design of the system, or it may be 

 deliberately introduced. The use of electrical filters to obtain multi- 



C/2 



>4ii 



Fig. 5 



plex operation, as in carrier systems, is an example of its deliberate use. 

 Consider the effect of introducing a low pass filter, as shown in Fig. 5, 

 into an otherwise distortionless transmission system. If the imped- 

 ances of the circuits to which the filter is connected are approximately 

 pure resistances of the values indicated in the figure, steady state 

 frequencies above a critical value known as the cut-off frequency 

 are so reduced as to be made practically negligible, while frequencies 

 below this value are transmitted with very little distortion. The 



