Radio Telephone Noise Reduction by Voice 

 Control at Receiver * 



By C. C. TAYLOR 



In listening to speech transmitted over radio circuits, the noise 

 arriving in the intervals between the signals may be annoying. 

 There is also evidence that the intelligibility is reduced due to this 

 noise shifting the sensitivity of the ear. Reducing the noise 

 occurring in the intervals of no speech should therefore improve 

 reception. 



This paper gives the underlying requirements for a device to 

 accomplish this type of noise reduction and describes the action of 

 a typical " noise reducer." Laboratory and field tests are described 

 which show that its use is equivalent to an improvement in signal- 

 to-noise ratio which reaches a maximum value of about 5 db. It 

 also reduces false operation of the voice-operated relays used on 

 long radio telephone connections. 



Introduction 



TN transmitting speech over radio telephone circuits there are a 

 -*- number of conventional methods of increasing the signal with 

 respect to the noise. Examples of such methods are the use of higher 

 power, directive antennas, diversity reception and filters to narrow the 

 received frequency band. In addition, there are other methods of a 

 special character which reduce the effect of the noise interference with 

 the speech transmission. One example of such a device limits the 

 noise interference by eliminating the high peaks of noise of very short 

 duration and depending upon the persistence of sensation of speech in 

 the ear to bridge the gaps. Another method diminishes the noise in 

 intervals of no speech. This is the method which will be discussed 

 here. 



Speech and Noise Considerations 



Speech signals may be represented by a group or band of frequencies 

 occupying a certain interval of time. In using the conventional 

 method of narrowing the received frequency band, filters eliminate all 

 noise outside the band actually required. In fact we sometimes go 

 beyond this and remove some of the outer frequency components of 



* Presented at the Pacific Coast Convention of A. I. E. E., Spokane, Washington, 

 September 2, 1937. Published in Elec. Engg., August, 1937. 



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