TRANSMISSION LINES 



299 



This predistortion is accomplished by including in the circuit at 

 the input to the modulator a network having relatively high loss for 

 the lower frequencies and tapering to low loss for the higher frequencies. 

 Its maximum loss is compensated for by adding in the circuit an 

 equivalent amount of additional amplification. The characteristics 

 of such a network are illustrated in Fig. 12. To restore the normal 

 volume relationships between the different tones and overtones a 



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20 

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FREQUENCY IN KILOCYCLES PER SECOND 



14 



Fig. 12— Attenuation characteristics of "predistorting and restoring" networks. 



restoring network having complementary transmission frequency 

 characteristics is, of course, included at the output of the receiving 

 circuit. It was found with this predistortion-restoring technique that 

 a volume range increase of something like 10 db could be obtained over 

 the circuits described. 



There is available also another method which might have been 

 employed for obtaining a further increase in volume range. This 

 method, the so-called volume compression-expansion system, very 

 likely will be necessary if in the future it is desired to obtain such high 

 quality circuits on long routes where the carrier frequency range is 

 being used also for regular telephone message transmission or for other 

 purposes, and where the problem of freedom from noise and crosstalk 



