28 



BELL SYSTEM TECHNICAL JOURNAL 



drop across the varistor unit constitutes the compressed output. Although 

 the development of this compressor has been a problem of many interesting 

 aspects, it must suffice here to point out that copper-oxide elements are 

 unsuitable at the speeds involved because of excessive capacitance, that 

 silicon is superior to at least the presently available types of germanium 



C R4 



INPUT 1( vW T 



TEMPERATURE 



CONTROL 



OVEN 



*" OUTPUT 



SILICON 

 RECTIFIERS 



Fig. 13. Instantaneous comj^rcssor. 



AMPLITUDE OF UNCOMPRESSED SAMPLES 

 IN DECIBELS BELOW MAXIMUM 



20 



30 



0-5 

 5 D 



<2 



w - 



X 

 to ^ 



^ > 



UJ S 



cr o 



Q. -J 



5U 

 O 2} 



U. _l 



O UJ 



UJ? 



00 



DU 

 l-Q 



40 L 



a 

 < 



Fig. 14. Characteristic of the instantaneous compressor. 



varistor in regard to freedom from certain memory or hysteresis effects 

 believed to be thermal in origin, and that a satisfactory yield of matched 

 silicon elements appears to be obtainable by the selection and resistance 

 padding methods employed.- Temperature control is used for the sake of 

 constancy of the compression characteristic. Aging has been found 

 negligible over a period of many months. 

 The actual compression characteristic afforded by these units is plotted 



