AMPLITUDE RANGE CONTROL 



531 



Referring again to Fig. 6, it will be seen that for the two smaller 

 values of RCjP the impression curves are composed of (1) the envelope 

 frequency, (2) double the fundamental frequency, and (3) a small delay 

 which can often be neglected. An approximation to envelope com- 

 pression is therefore possible by choosing RCjP to be in the proper 

 range, i.e., .0025 to .025, and making the output vary as a root or power 

 ot the impressions thus formed. 



Figure 8 shows the result of compressing the wave of Fig. Za by 

 using the impressions of Fig. 6 to determine the amplification. It was 



10 20 30 40 50 60 70 80 90 100 



PER CENT OF TIME IMPRESSION IS EQUAL TO OR LESS THAN IMPRESSION SHOWN 



Fig. 7 — The amplitude ranges of the impressions shown in Fig. 6 are bunched 

 differently, depending on the time constant. A "volume" measurement means that 

 a given impression is exceeded a small percentage of the time. In speech the peaks 

 are relatively higher than in the wave illustrated. 



assumed that the amplification varies in inverse proportion to the 

 square root of the impression. The resulting waves for RCjP = .0025 

 and .025 (medium impressions) are recognizable as something like the 

 original wave. However, for the larger values of RCjP (weak im- 

 pressions), the distortion at the beginning of the wave is quite large. 

 This is because the impressions are formed so slowly that a longer time 

 is required to drive the gain down to the desired value. 



In order to compare impression compression with instantaneous 

 compression, the ordinates of Fig. 5 and 8 were plotted in Fig. 9. 

 This shows that the greatest possibilities of bunching the waves into 

 a narrow range result from the use of instantaneous compression {b), 

 since the ratio between any value and the maximum is modified by the 



