320 BELL SYSTEM TECHNICAL JOURNAL 



just operates on the crests of loud syllables and one which operates 

 sufficiently well not to clip speech is also about 30 db. 



Considering on Fig. 2 that the coordinates are in terms of the aver- 

 age r.m.s. value over a period of time small compared with the time of 

 a syllable, there is a spread of at least 30 db in signal intensity extend- 

 ing down from the maximum for each talker. Thus for the weakest 

 talker this spread is indicated by the bracket Y and for the strongest, 

 by X. Any other talker, as Z, falls somewhere in between. After 

 manual control of volume this spread of intensities is represented by 

 the bracket X', Y', Z' for all talkers. This residual spread makes 

 desirable a means for further compressing the range of intensities in 

 the speech signals so that the weaker parts of sound are transmitted 

 at a higher level without at the same time raising the peak values of 

 speech and so overloading the transmitter. 



Types of Compression Systems 



This problem can be approached in several ways. One, for in- 

 stance, is from the frequency distortion standpoint. As many of the 

 weaker consonants have their chief energy contribution in the upper 

 part of the speech band, a simple equalizer which relatively increased 

 the energy of the higher frequency consonants before transmission 

 and another which restored the frequency energy relations after trans- 

 mission should be found of value. Tests have confirmed this expecta- 

 tion to some degree. Unfortunately, the best type of equalizer de- 

 pends upon the type of subscriber station transmitter, so that in general 

 only a compromise improvement can be obtained. 



Another general method of approach is that of amplitude distortion 

 in which the weaker portions of the syllable are automatically increased 

 in intensity in some inverse proportion to their original strength. The 

 manual control of volume described above may be considered the 

 genesis of this method. Early suggestions * included the use of an 

 auxiliary channel such as a telegraph channel for duplicating the con- 

 trol operations in the reverse sense at the receiving end, thus restoring 

 the original energy distribution. Another early suggestion along this 

 line was made by George Crisson of the American Telephone and 

 Telegraph Company. '^ If a voltage be applied to a circuit consisting 

 of a two-element vacuum tube (with a parabolic characteristic) in 

 series with a large resistance, the instantaneous voltages across the 

 tube are approximately the square root of corresponding voltages ap- 

 plied. ■ Thus a voltage originally 1/100 of the peak voltage can be 

 transmitted at an intensity of 1/10 of the peak or ten times its original 

 intensity. If the instantaneous energy is expressed on the logarithmic 



