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BELL SYSTEM TECHNICAL JOURNAL 



Both show rising characteristics in the region of 2000 cycles per second. 

 While it is feasible to design a transmitter having a substantially flat 

 field characteristic, experience has shown that the transmission 

 obtained with a transmitter having a rising response in this frequency 

 region more nearly approaches direct speech, ^ when used with a 

 representative line and the new receiver, than that obtained with one 

 having a more uniform field response. 



Non-Linear Distortion. — A substantial reduction in non-linearity 

 has been effected in the new transmitter unit. 



It is a well-known fact that the slope of the input-output curve of 

 most transmitters is not unity even for sound intensities other than 



< 10 



4 5 6 7 8 9 10 20 30 40 50 70 100 



PRESSURE IN BARS 



Fig. 7 — Sum and difference components as a function of the intensity 

 of the fundamental frequencies. 



those at which overloading occurs. However, this departure from non- 

 linearity does not entirely account for the modulation products de- 

 veloped when two frequencies are impressed on the transmitter. 

 Measurements of the sum and difference components in the output of 

 the new transmitter and the earlier transmitter for frequencies of 1500 

 and 1700 cycles per second are shown on Fig. 7. It will be noted that 

 whereas the difference component produced by the earlier instrument 

 is equal to the fundamental within the speech range, the sum and 

 difference components are both 10 db or more below the fundamental 

 at all the intensities measured in the case of the new unit. 



A tendency for the difference component to be considerably more 

 pronounced than the sum component in the case of the earlier trans- 

 mitter is characteristic of all of the commercial instruments measured 



