AMPLITUDE CHARACTERISTICS OF TELEPHONIC SIGNALS 551 



inputs, and it would also reinsert its gain much more quickly; it would 

 affect the naturalness of the sound of the signal more than the slower 

 peak limiter but it would also cause the signal to over-ride noise some- 

 what better. In a third variety of service the harmonic distortion 

 introduced by a limiter is a secondary matter, the prime consideration 

 being that the peak amplitude of the signal shall not exceed a specified 

 value. This may be because higher amplitude signals would produce a 

 tremendous increase in distortion or crosstalk into other channels or 

 would damage expensive equipment farther along in the circuit. For 

 these cases we may use the fastest possible type of limiter, the peak 

 chopper, which simply cuts off any peak exceeding a certain value. 



The crosstalk suppressor, Fig. 10, is a splendid example of the fine 

 distinction between volume controlled and voice operated switching 

 devices. This device has been described, but in the present state of 

 the art its time functions have not been definitely fixed. If the 

 characteristic of loss versus input is made steep enough and the speed 

 of operation fast enough it will sound like a switching circuit and may in 

 fact be replaced by a relay-switched attenuating network. If made 

 somewhat slower and given a smaller slope of loss versus input it 

 approaches the limited range expandor or noise reducer. 



Applications and Expected Advantages 



It may be of interest to give some approximate figures on the 

 magnitudes of the advantages to be obtained by the use of some of 

 these devices. It will be understood that the values to be given are 

 simply illustrative, some having been obtained from field service on 

 particular models and some from tests on laboratory equipment under 

 special conditions. 



Vogads appear to be most useful in such circuits as transoceanic 

 radio connections, where it is important to properly operate the 

 terminal switching equipment and to transmit over the radio circuit 

 speech energy from loud and weak talkers equally well. It is essential 

 in such cases that noise should not be increased in amplitude during 

 speech pauses, hence the gain retaining feature of the vogad. On such 

 a circuit a vogad will reduce a 45 db volume range to about 2 to 4 db. 

 This is equivalent to expert manual volume control. 



Volume limiters are in use at the present time to prevent peaks of 

 speech energy in carrier circuits from "splashing" into telegraph 

 channels. '^ Some 5 to 10 db limiting is allowed on loudest talkers, 

 which causes little degradation of the speech channels but makes 

 possible the use of telegraph on the same carrier system. There is no 



