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BELL SYSTEM TECHNICAL JOURNAL 



Figure 3b indicates values for the same wave in which the negative 

 ordinates have their signs reversed by means of an ideal full-wave 

 rectifier. The resulting wave contains frequencies which were not 

 present in the original, prominent among them being second and higher 

 harmonics of the original. The range of instantaneous values shown 



5 



RECTIFIED ENVELOPE-^ 

 RECTIFIED 



TIME 



Fig. 3 — A wave's amplitude varies from positive maximum to negative maximum. 

 If symmetrical, the amplitude may be expressed as varying in only one direction from 

 zero to maximum by rectification. 



on Curve b of Fig. 4 is only half that of the instantaneous voltages. 

 About 99 per cent of the values lie in a 60 db range. 



The instantaneous values of the envelope of the rectified wave 

 follow curves 3c and 4c. In speech the envelope is composed of 

 many rather low frequencies which are determined by the rates of 

 enunciation of syllables. For this reason they are sometimes called 

 the syllabic frequencies. If it were possible to make the control vary 

 as a function of the envelope, the result of using a control ratio of 1/2 

 on the wave of Fig. 3a would be as shown in Fig. 5c. This was 



