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BELL SYSTEM TECHNICAL JOURNAL 



The automatic volume indicator is connected across the output of 

 the intermediate amplifier of the caller's control circuit. This is a 

 high level point in the circuit at which the form of the electrical 

 wave is essentially a duplicate of that of the acoustic speech wave 

 which produces it. 



Fig. 7 — Interior of sidetone booth — shuuuii; cUUiKia] mxiith and transmitter changer 



and agitator. 



The device works as follows: The speech voltage applied across the 

 input is amplified, rectified, and applied to an integrating circuit, the 

 output of which, after further amplification, is applied to a voltage 

 measuring circuit. The maximum value of this voltage for each sen- 

 tence is measured in terms of the number of 2 db steps by which it 

 exceeds an arbitrary minimum, a relay corresponding to each step 

 being operated through contacts made by the master timing commu- 

 tator on the automatic system for recording and analyzing the data. 

 The relay system is arranged so that if more than a predetermined 



