Devices for Controlling Amplitude Characteristics of 

 Telephonic Signals* 



By A. C. NORWINE 



This paper describes a family of devices which automatically 

 respond to signals and control the circuit amplification in such a 

 way as to improve transmission. Their general characteristics are 

 outlined, their differences explained, and some of their applications 

 are listed. 



Introduction 



THE transmission of speech energy over electrical circuits is 

 attended by the interesting and sometimes difficult problem of 

 preserving the original signal in spite of limitations in the transmission 

 medium. These limitations include load carrying capacity, inter- 

 ference with other service, noise, change in attenuation with time and 

 many others. Because of special limitations it is sometimes desirable 

 to alter the amplitude characteristics of the speech or other signal 

 energy without, of course, materially lowering its intelligibility. In 

 high quality systems the peak voltage from some speech sounds of a 

 given talker may be over 30 db (some 30 times) higher than from his 

 weakest sounds when there is very little inflection in the speech. 

 Loudness changes for emphasis will increase this range of intensities. 

 Ordinary message systems do not have to contend with quite so wide a 

 range of instantaneous voltages from a single talker, but different 

 talkers under extreme terminal conditions produce about a 45 db range 

 of average voltage, which is additive to that for a single talker. Conse- 

 quently, a voltage range of about 70 db (over 3000 to 1) must be 

 considered for message circuits. 



In order to accommodate such ranges of intensity to certain trans- 

 mission media such as radio links a new family of automatic devices has 

 been developed. In general all of these contain amplifiers or attenu- 

 ating networks whose loss or gain is changed according to some 

 function of the applied input and which may have a variety of time 

 sequences in their control circuits. It is hoped that by the classi- 

 fication and description of some of these devices their distinguishing 

 characteristics and fields of usefulness will be made somewhat clearer. 



* Presented at the Pacific Coast Convention of A. I. E. E. and I. R. E. in Port- 

 land, Oregon, August 9-12, 1938. 



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