Instantaneous Compandors on Narrow Band Speech Channels 



By J. C. LOZIER 



(Manuscript Received Aug. 15, 1951) 



If speech is passed through an instantaneous compressor, the original speech 

 frequency spectrum is substantially widened. It is known that instantaneously 

 compressed speech can be transmitted over a medium with a passband no wider 

 than that occupied by the uncompressed speech, and the original signals re- 

 covered without distortion. The conditions required for such distortionless 

 transmission are examined. The analysis indicates that more severe requirements 

 must be imposed on the attenuation and phase characteristics of the system when 

 this reduced bandwidth mode of operation is used. The practical value of this ex- 

 change of transmission requirements is a matter for experimental determination. 



Introduction 



WHEN a signal such as speech is instantaneously compressed in ampli- 

 tude, harmonics and cross modulation products are generated which 

 extend the frequency spectrum of the original signal by many octaves. It 

 is proposed to demonstrate that the additional products thus generated are 

 necessary for the distortionless recovery of the original signal. Then the 

 conditions will be examined under which this broadband signal can be 

 transmitted without distortion through a bandwidth no wider than that 

 occupied by the spectrum of the uncompressed speech. Finally, some of the 

 practical aspects of using intantaneous compandors on narrow band 

 speech channels will be considered, with emphasis on the nature of the trans- 

 mission requirements placed on the medium. The advantages to be obtained 

 from the use of instantaneous compandors have already been presented by 

 Mallinckrodt.^ 



Bandwidth vs Distortion 



If a single frequency tone is compressed by a 2 to 1 compressor,^ and then 

 the fundamental alone is expanded, it can be shown that the resultant 3rd 

 harmonic distortion is only 13 db below the fundamental. Expansion of 

 both the fundamental and the 3rd harmonic output of the compressor will 

 reduce this 3rd harmonic distortion to 29 db below the fundamental. Ex- 

 pansion of the fundamental plus the 3rd and 5th harmonics will reduce the 

 3rd harmonic distortion in the recovered signal to 45 db below the funda- 



^C. O. Mallinckrodt "Instantaneous Compandors," B.S.TJ., Vol. XXX, No. 3, 

 July 1951. 



* In a 2 to 1 com[)ressor, the outi)Ut amplitude is the sciuare root of the input amplitude. 

 The name comes from the fact tnat, in such a compressor, the output amplitude will 

 increase 1 db for each 2 db increase in input amplitude. 



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