480 



BELL SYSTEM TECHNICAL JOURNAL 



The varid-Iosser makes use of the resistance variation with current of 

 copper-oxide rectifier disks. This variable resistance shunts a fixed 

 resistance in series with the windings of a repeating coil as shown in 

 Fig. 2. The maximum loss is determined by the fixed resistance when 

 small current is flowing through the disks while the varying loss is 

 determined by the shunting copper-oxide resistance which decreases 

 rapidly with increasing current above a threshold value until a low 

 value is reached. The minimum loss is limited by the output of the 

 control tube approaching a maximum and the shunting resistance 

 becoming so small that additional decrease affects the loss inappreciably. 



The variable resistor setting in the reduction control circuit de- 

 termines the input amplitude at which reduction begins and therefore 

 the point above which the loss remains substantially constant. If there 

 is a difference in amplitude between speech and noise, the reduction 



Fig. 3 — noise reducer panel. 



control may be so adjusted that the noise on the circuit, when no speech 

 is present, is appreciably reduced. The action then is as follows : In the 

 absence of speech, noise is reduced usually the maximum value of 

 20 db; during intervals of lower speech amplitudes the loss decreases in 

 proportion to the increase in amplitude, and during speech of high 

 amplitude both noise and speech are transmitted without loss. As the 

 noise encroaches upon the range of speech amplitude, it becomes 

 necessary to reduce greater amplitudes, thereby also further reducing 

 the weaker parts of speech. 



The noise reducer is contained on a 1\ inch panel for relay rack 

 mounting. Figure 3 gives a front view. The panel contains the 

 reduction control resistor and an In-Out key which, in the Out 

 position, gives the device a fixed loss. Both resistor and key may be 

 duplicated external to the panel with the wiring arranged to give 

 remote control. 



