THE COMPANDOR 



321 



or db scale, the energy range is then cut in half. Such a device may be 

 called an instantaneous compressor. At the distant end a circuit 

 which is simply the inverse of that at the transmitting end is used. 

 The output voltage is taken off of a low resistance in series with a 

 parabolic element, thus restoring the signal substantially to its original 

 form. This circuit may be called an instantaneous expandor. This 

 scheme was successfully tested in the laboratory but unfortunately 

 possesses a very serious limitation for practical application in the 

 telephone plant. This is due to the fact that, to properly maintain 

 the characteristics of the compressed signals, a transmission band 

 width without appreciable amplitude or phase distortion of about 

 twice the normal proved necessary. 



The Compandor 



The principle of the present device is the use of a rate of amplitude 

 control for the compressing and expanding devices intermediate be- 

 tween manual and instantaneous control which may be considered ap- 

 proximately as a control varying as a function of the signal envelope.^- ^ 

 Such a modulation of the original signal in terms of itself does not 

 appreciably widen the frequency band width of the modified signal as 

 compared with the original signal. The transmitting device is called 

 the compressor; the receiving device, the expandor; and the complete 

 system, the compandor. 



The functional behavior of a typical compressor may be considered 

 with reference to the simplified schematic circuit No. 1 of Fig. 3. 



LINEAR I ] 3> t 



ECTIFIER zf= =ir > Eo 



T Tf t 



Fig. 3 — Compressor circuit No. 1. 



