THE COMPANDOR 



323 



shall have approximately 

 From (4) and (3) we have 



\{ t = s = n 



Eg = hEi'". 





E2 = KEi"\ 



(4) 



(5) 



(6) 



Now if the input voltage be increased by a factor x, the input 

 increment in db will be 20 log x. The new output will be 



£2' = KixEiY'". 



The increment in output in db will be 



20 log^ = 20 log x'l" 



-C-2 



20 



log x. 



The ratio of the output increment to the input increment in db is l/w 

 and the device is said to have a compression ratio of l/n. In other 

 words, the per cent change in relative speech voltages in passing through 

 the compressor is the same at all points in the intensity range. In the 

 general form of this circuit, t and 5 need not be equal to secure a particu- 

 lar value of l/n. 



In Fig. 4, Compressor Circuit No. 2 is shown, a backward-acting 

 type of circuit. In this circuit the control tubes can be used to per- 





■AAA/ 



■VvV 



AAA/ 



Fig. 4 — Compressor circuit No. 2. 



