RADIO TELEPHONE NOISE REDUCTION 477 



In connected speech, silent intervals occupy about one-fifth to one- 

 third of the total time. Also there are frequent intervals when the 

 sounds are rather weak. However, if we attempt to suppress noise 

 during all these intervals, experience shows that the suppression be- 

 comes too obvious, and the speech is apt to sound mutilated. For this 

 reason the function of any device to be used for reduction of noise in 

 the intervals between speech is to operate rather quickly to remove 

 suppression and pass the speech and approximately to sustain this 

 condition for sufficient periods to override weaker intervals so that 

 obvious speech distortion does not occur. 



To reduce the noise in the intervals between speech it is necessary to 

 depend for control upon either the speech itself or upon some auxiliary 

 signal usually under the control of the speech at some point in the 

 circuit where the signal-to-noise ratio is better. This latter condition 

 is illustrated on a circuit where the carrier is transmitted only during 

 speech intervals. The carrier then acts as an auxiliary signal which 

 operates a device at the receiver to remove loss.*- ^ The device to be 

 discussed below utilizes the speech itself at the receiver to perform this 

 function. 



In using the speech in this way it is obvious that control can be 

 accomplished only when the speech energy sufficiently exceeds the 

 noise energy so that the presence of the speech is distinguishable. The 

 device could operate abruptly as, for example, a relay which removes a 

 fixed loss in the operated position and restores it when non-operated. 

 Experience indicates that the use of such a device makes the suppression 

 too obvious if it is to follow the speech sounds closely. It is desirable, 

 then, to perform this reduction by more or less gradually removing loss 

 as the speech increases to accentuate the difi^erence between levels of 

 speech sounds and levels of noise which occur in the gaps between 

 speech. 



Noise Reducer 



This kind of performance has been secured in a device known as a 

 noise reducer. A comparison of the action of the noise reducer and a 

 relay having similar maximum loss is shown in Fig. 1. This figure 

 shows the input-output characteristics of these devices over the voice 

 amplitude range to which they are subjected on a radio circuit. The 

 noise reducer may be likened to a relay with a variable loss, the loss not 

 varying instantaneously but over a short period of time. The loss, for 

 any short period, may be any value within the loss range and the device 

 has, therefore, been likened to an elastic or shock absorbing relay. 



The noise reducer has no loss for strong inputs, considerable loss for 

 weak inputs and changes this loss gradually over a short interval of 



