534 BELL SYSTEM TECHNICAL JOURNAL 



control ratio. In the case of envelope compression (c), the lag causes 

 a reduction in the amount of compression of the instantaneous voltages 

 and the result is seen to be about half way between curves a and h of 

 Fig. 9. The remaining curves of Fig. 9 are representative of the 

 device ^ used on the long-wave transatlantic radiotelephone circuit. 



Volume Control 



To avoid both a large range and also the necessit}^ for a continuous 

 record, practical speech amplitude measuring instruments are not 

 directly concerned with either instantaneous, envelope, or impression 

 voltages. Instead a value is determined, corresponding to an im- 

 pression which is exceeded only a small percentage of the time. This 

 is the principle underlying speech measurements with "volume in- 

 dicator" type of instruments. In the case of speech, which is much 

 more complex than the simple wave we have discussed, curves like 

 Fig. 9 are steeper, i.e., there are relatively more peaks and a larger range 

 to complicate the problem. 



A particular device capable of compressing according to the re- 

 quirement that the dynamic "volume" range should be reduced, is 

 attained by a combination of several separate range controllers. One 

 is provided to reduce the gain very rapidly when the output volume is 

 too high. A second increases the gain, at a much slower rate, when the 

 impressions formed on the condenser are consistently too low. A 

 third disconnects the condenser from the input when the applied 

 voltage is very small, so that the distortions inherent in change of gain 

 by weak impressions will occur only at times of large and sudden 

 decreases of volume. In the device ^ employed to control volumes 

 applied to a radio transmitter at Norfolk, Virginia, a fourth control 

 provides for rapid partial compression of high peaks, thus improving 

 the modulation. It is unnecessary to re-expand for the purpose of 

 restoring the intelligibility, since the distortion is virtually limited to a 

 change in loudness. 



Part 3 — Range 



Range controllers, like repeaters and attenuators, are limited as to 

 the input range they can accept and the output range they can provide. 

 These limits may be due to thermal noise at the low end and output 

 carrying capacity at the high end. Heretofore, in this paper, the 

 terms "input" and "output" have been purposely left somewhat 

 vague so as to be as general as possible. However, the limits of input 

 and output of a range controller take on particular significance when 

 it is considered that the signal input range may difTer both from the 



