Amplitude Range Control 



By S. B. WRIGHT 



The art of controlling the amplitude range of telephone signals 

 involves recognition of certain characteristics in addition to those 

 used to specify the performance of ordinary transducers. Funda- 

 mentally, three kinds of characteristics are necessary to distinguish 

 different range control devices. They are (1) the steady-state 

 input-output characteristics, (2) the time actions, and (3) the range 

 over which they function. In some cases, several secondary char- 

 acteristics may be of interest, but they need not be considered in 

 determining to which class a particular device belongs. 



This paper discusses and classifies these characteristics. 



Introduction 



TN a "non-linear" transducer, the output power is not proportional 

 -*- to the input power. Consequently, the ratio of maximum to 

 minimum power at the output differs from that at the input. But the 

 ratio of maximum to minimum power is an expression of ampHtude 

 range. A device designed to alter this ratio may be called a range 

 controller. 



In telephony the term range controller includes many devices ^ having 

 specific names, such as limiters, volume control devices, range reducers, 

 compressors, vogads, expandors, etc. These devices have many prop- 

 erties in common with telephone repeaters, and a repeater may be 

 considered as a special case in which any non-linearity which may 

 exist between the output and input is unintentional. 



The purpose of one type of range controller is to reduce the range of 

 significant intensities of signals applied to a telephone circuit so as to 

 ease the requirements of the transmission medium with respect to 

 overloading and noise interference. Such a device is placed at the 

 transmitting end of the circuit. When the range is compressed at the 

 sending end of the circuit it may sometimes be desirable to expand it 

 at the receiving end to the original range. This is done with a device 

 having, in general, the same dynamic characteristic as the compressing 

 device, but a range change which is complementary. The purpose of 

 the expander is to reduce the noise heard by the listener as well as to 

 compensate for whatever characteristic signal modification occurred in 



^ For numbered references, see end of text. 



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