AMPLITUDE CHARACTERISTICS OF TELEPHONIC SIGNALS 549 



the receiving terminal of the medium, but the intelligibility of speech 

 over such a system has been shown in the laboratory to be good. 



Special compandors for high quality service may require compression 

 and expansion which vary with frequency. The exact characteristics 

 will depend upon band width, program material and transmission 

 medium. For transmission media in which the noise reproduced at the 

 receiving end is principally at the higher frequencies an unusual effect 

 is obtained if the usual variety of compandor is used. Low frequencies 

 unaccompanied by high frequencies will cause a gain change in com- 

 pressor and expandor, thus changing the background of high-frequency 

 noise which is not masked by the low-frequency signal energy. The 

 resulting swishing noise has been given the onomatopoeic name of 

 "hush-hush effect." To avoid this, recourse may be had to split band 

 compandors in which the compression and expansion is done only at 

 high frequencies or separately for low and high frequencies. The 

 successful application of the latter method is, however, more difficult 

 than it appears from its simple description. 



Distinguishing Characteristics 



It is important to distinguish between the half vogad. Fig. 2, and 

 the compressor. Fig. 4. As shown in Table I the latter operates on 

 syllabic variations and the former on the average volume of the input. 

 Thus the half vogad reduces the range of output volumes to one-half 

 that at the input while the compressor reduces the range of syllabic 

 power at its output to one-half that at the input. In other words, the 

 compressor reduces the ratio of peak to average power on constant 

 volume speech, while the half- vogad simply adjusts for that volume and 

 does not alter the peak ratio. There is, of course, the additional 

 important difference that the half-vogad retains its gain setting during 

 silent periods while the compressor, by virtue of having followed the 

 syllabic power, has its maximum gain during silent periods. 



Volume limiters. Fig. 3, may be mistaken for vogads. Fig. 1, because 

 during speech input above a certain value the two may produce the 

 same output volume. They both employ something like a measure- 

 ment of average power over periods longer than a syllable to determine 

 their gain settings. The important difference is that a vogad retains 

 its gain setting when speech currents are not present, while a volume 

 limiter approaches its maximum gain during such periods. In terms of 

 the output resulting from a range of input volumes there is another 

 important difference if the volume limiter operates over only part of the 

 input range: the vogad reduces the width of the distribution curve of 

 volumes to a very small value, while the volume limiter moves all the 



