552 BELL SYSTEM TECHNICAL JOURNAL 



wide-spread use of volume limiters in point-to-point radio service so 

 far, but in cases in which there is no disadvantage in raising noise in 

 silent periods in speech, such as in push-to-talk installations, proper 

 transmitter loading can be obtained with volume limiters fairly 

 cheaply. 



One commercial model peak limiter, used as part of a program 

 amplifier ^°' " is capable of introducing a considerable amount of 

 compression without overloading on peaks, but for the preservation of 

 adequate program volume range it is being recommended that only 

 3 db peak limiting be allowed. This, of course, reduces the range of 

 intensity of the program, but from the standpoint of the listeners it is 

 equivalent to doubling the transmitted power or obtaining the same 

 signal-to-noise ratio with half the transmitted power. 



Limited range compressors might be used either on land lines to 

 insure full loading or on radio links whose fading is too severe to permit 

 the use of normal compandors. There is no commercial application of 

 either sort at the present time. Peak choppers are, however, used on 

 some high power radio transmitters which might otherwise be tempo- 

 rarily disabled by high peaks in the signal being transmitted. 



The chief usefulness of compandors is on radio links in which the 

 transmission of a compressed signal with subsequent expansion permits 

 operation through higher noise or with lower transmitter power. On a 

 long-wave transatlantic radio telephone circuit a compandor with 

 40 db range has been shown to allow an increase in noise of some 5 db 

 before reaching the commercial limit. ^ With smaller amounts of noise 

 the noise advantage of the compandor approaches half its range in 

 decibels. This benefit is sometimes applied to a reduction of trans- 

 mitter power. 



Radio noise reducers have been used to advantage in connection 

 with short-wave ship-to-shore and transoceanic radio telephone 

 service. In the former, routine transmission rating is given on a 

 judgment basis using a merit scale from 1 to 5, 5 being practically 

 perfect transmission and 1 so poor that intelligibility is very close to 

 zero. It will then be seen that the observed improvement of >^ to 1 

 point in transmission rating due to the noise reducer is of considerable 

 importance. Perhaps more graphic figures are those for transoceanic 

 service, where the reduction of noise in the receiving path not only 

 reduces the noise heard by the listener but also improves the voice 

 operated switching with the indirect result that at times receiving 

 volume increases of 5 to 15 db are realized.^ 



As has been noted, the radio noise reducer is a special use of an 

 expander alone. There are also two interesting applications for a 



