RADIO TELEPHONE NOISE REDUCTION 483 



Performance 



Laboratory tests have been made in an attempt to evaluate the 

 advantages to be gained by the use of the noise reducer. It was shown 

 that, for the rather Hmited and controlled conditions which were 

 tested, definite advantage can be observed in judgment tests of the 

 effectiveness of speech transmission through noise with and without 

 the noise reducer. This advantage is of the order of magnitude of 

 3 to 5 db at the border line between commercial and uncommercial 

 conditions on the noisy circuit. 



This figure is in approximate agreement with results obtained from 

 records of performance on commercial connections. A curve is 

 available which shows the approximate relation between percentage 

 lost circuit time and transmission improvement for a long-range short- 

 wave radio telephone circuit.*^ From the records of lost circuit time as 

 affected by the noise reducer use, an improvement of 4 db is obtained 

 from this curve. 



Observations were made and records kept for twelve months of the 

 use of the device at the land terminal of the high seas ship-to-shore 

 circuit and for shorter periods on New York-London circuits. These 

 observations indicate that the noise reducer most satisfactorily reduces 

 objectionable effects where the interference consists of noise of a fairly 

 steady character. As might be expected it is somewhat less effective 

 on crashy static. If the noise is very low there is no improvement; as 

 the noise increases the benefit increases up to a certain point; when the 

 noise amplitudes begin to approach too closely the peak amplitudes of 

 the voice waves it becomes impossible to distinguish between them 

 without producing objectionable speech distortion and there is again no 

 advantage. Where volume fading is present there is a tendency to 

 accentuate the volume changes and it becomes necessary to adjust the 

 reduction control to limit this. Otherwise this effect may offset the 

 possible noise improvement. The operating practice is to adjust the 

 reducer control circuit for each noise or transmission condition so that 

 optimum reception as judged by the technical operator is obtained. 

 The general rule is to use the minimum reduction possible. 



Use of Noise Reducer with Voice Switched Circuits 



On radio telephone circuits for connection to the land telephone 

 system, control terminal equipment is used at the junction of the land 

 lines and the two one-way radio channels (one transmitting the other 

 receiving) necessary for two-way communication. In making this 

 connection a widely used method is one in which the two-wire land 

 circuit is normally connected to the receiving radio channel and is 



