The Bell System Technical Journal 



July, 1934 



The Compandor — An Aid Against Static in Radio Telephony * 



By R. C. MATHES and S. B. WRIGHT 



One of the important conditions which must be met by any speech 

 transmission system is that it should transmit properly a sufficient range 

 of speech intensities. In long-wave radio telephony, even after the speech 

 waves are raised to the maximum intensity before transmission, there 

 remain energy variations such that weak syllables and important parts of 

 strong syllables may be submerged under heavy static. The compandor 

 is an automatic device which compresses the range of useful signal energy 

 variations at the transmitting end and expands the range to normal at the 

 receiving end, thus improving the speech-to-noise ratio. 



This paper deals with some of the fundamental characteristics of speech 

 waves and explains how the task of changing them for transmission over 

 the circuit and restoring them at the receiving end is accomplished. It 

 is also shown that raising the strength of the weaker parts of speech gives 

 these results: 1, the successful transmission of messages for a large per- 

 centage of the time previously uncommercial; 2, a reduction of the noise 

 impairment of transmission for moderate and heavy static during time 

 classed commercial; and 3, the ability to deliver higher received volumes 

 due to the improved operation of the voice controlled switching circuits. 

 In addition to these advantages, the compandor makes it possible to 

 economize on radio transmitter power in times of light static. 



Introduction 



WHEN the original New York-London long-wave radiotelephone 

 circuit was designed, it was recognized that radio noise would 

 often limit transmission, especially for the weaker voice waves. Ac- 

 cordingly provision was made for manually adjusting the magnitude of 

 the speech waves entering the radio transmitters to such a value as to 

 load these transmitters to capacity.^ While this treatment was very 

 effective in improving the average speech-to-noise ratio and in prevent- 

 ing the strong peaks of speech from overloading the transmitter, it was, 

 of course, unsuitable for following the rapidly varying amplitudes of 

 the various speech sounds. 



The total range of significant intensities applied to the circuit is 

 in the order of 70 db, an energy ratio of 10 million to one. The manual 

 adjustments referred to above were succesful in reducing this range to 

 about 30 db. To further reduce this residual range an interesting 



* Presented at Summer Convention of A. I. E. E., June, 1934. Published in 

 Electrical Engineering, June, 1934. 



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