110 BELL SYSTEM TECHNICAL JOURNAL 



the wave form very decidedly, but the surprising thing is that in 

 telephony the intelligibility is not seriously affected when the differ- 

 ence is made as much as fifty cycles or so. The apparent pitch of 

 the voice changes, of course, as n is varied. 



If the carrier is transmitted, either intentionally or thru incomplete 

 suppression, the situation is less favorable to asynchronous recep- 

 tion. The two carriers then beat together, giving a component of 

 frequency n which may be troublesome if the received carrier is large. 

 However, its frequency is generally below the voice range, and so it 

 can be suppressed by a filter in the detector Output. In addition the 

 received carrier beating with the side-bands gives the components 

 of the original signal. These are superposed on the displaced speech 

 from the local carrier, the corresponding components of the two 

 differing in frequency by n. As a result, the two sounds beat to- 

 gether just as two tuning forks would. For very little differences 

 in frequency a periodic rise and fall in intensity is heard. When the 

 difference is increased so that the individual beats can no longer be 

 distinguished, a sensation of roughness results. And when the differ- 

 ence is made still greater the two waves may be heard as separate 

 sounds of noticeably different pitch. The prominence of this beating 

 effect depends, of course, upon the relative magnitude of the two 

 carriers, since the two sets of speech currents are in the same ratio 

 as the two carriers. 



This effect of the received carrier may be very much reduced, and 

 in the ideal case entirely eliminated, by the use of a balanced de- 

 tector similar in structure to the balanced modulator of Fig. 3. It 

 can be shown that with such a circuit the combination frequencies 

 resulting from any two components applied at 5 are neutralized in 

 the output circuit, while the combination of each with the carrier 

 applied at C is transmitted. Thus if the side-band and received 

 carrier enter together at S, the components having, the original signal 

 frequencies are eliminated and only the displaced components remain. 



When the other side-band is added, the situation is still further 

 complicated. In the absence of received carrier, the local carrier and 

 one side-band give a set of components the frequencies of which are 

 greater than those of the signal by n, while the carrier and other side- 

 band give a set less by the same amount. These two sets combine 

 in much the same way as do the displaced and normal speech ob- 

 tained with a single side-band and received carrier. Here, however, 

 the beat frequency is 2 n. Also, as the two sets are equal in ampli- 

 tude, the beats will be much more pronounced, the intensity falling 

 to zero each time the two waves are in opposition. For slow beats the 



