522 



BELL SYSTEM TECHNICAL JOURNAL 



may accompany the transformation. Such effects depend upon the 

 slope of the output-input characteristic. 



The control ratio of a range controller might be defined as the output 

 range in db divided by the input range in db within the non-linear 

 region of interest. The ratio is obtained in such a way as to eliminate 

 transient effects, i.e., using steady-state sine waves. 



Typical Control Ratios 

 Figure 2 shows some typical output-input characteristics for various 

 transducers having control ratios between zero and infinity. While 



INPUT IN DECIBELS BELOW ARBITRARY REFERENCE 

 60 50 40 30 20 10 0, 



COMPRESSED 

 (OUTPUT) WAVE 



Fig. 1 — ^The signal modification caused by a non-linear transducer depends upon 

 the slope of the output-input characteristic. 



these typical characteristics are straight lines there is nothing to pre- 

 vent a range controller having a control ratio which varies with input. 

 However, when complementary action is required at the receiving end 

 it is more readily obtained when the control ratio is constant. Also, 

 some physical elements used in the design of range controllers are 

 most readily adapted to a straight line characteristic. 



Compressors (that is, devices having control ratios less than 1) may 

 be divided into two classes: (1) Complete * and (2) Incomplete. In a 

 complete compressor (control ratio = 0) the output is held constant 

 within the range of the device. This control ratio gives a maximum 



* This is not usually of practical interest but is useful as an ideal limit of operation. 



