RADIO TELEPHONE NOISE REDUCTION 



479 



waves are not permitted to control for any noise condition within the 

 range of usefulness of this device. Thus the noise in the absence of 

 speech is always reduced and the portions of the initial and decay 

 periods of the speech sounds which are also reduced vary with this 

 adjustment for noise intensity. Of course, if the speech-to-noise ratio 

 becomes too small or if other transmission conditions interfere, an 

 improvement becomes impossible. 



Circuit Arrangement 



Figure 2 shows the circuit of the noise reducer in simplified schematic 

 form.^ Incoming waves pass from left to right through the fixed pad, 



INPUT 



REDUCTION 



'-W\^^W-l VsA 4iN 



MAX. MIN. 9 — *^^ 



4 OUT 



OUTPUT 



RECTIFIER 



0-P 6+P 



Fig. 2 — Simplified schematic of noise reducer. 



the vario-losser and the amplifier to the output. At the input, part of 

 these waves pass through the reduction control branch circuit which 

 includes a variable resistor, an amplifier and a rectifier. The direct 

 current produced by the rectifier is applied through the condenser and 

 resistance filter to the copper-oxide losser circuit. For current below a 

 threshold value, no appreciable change occurs in the losser and the loss 

 introduced is about 20 db. As input increases, rectified current reaches 

 a value where the loss begins to change rapidly. It becomes db at an 

 input about 20 db above the point at which the loss starts to change. 

 The design is such that the loss remains substantially constant for 

 higher inputs. 



