208 BULL SYSTEM TECHNICAL JOURNAL 



factor but the second term does not. Thus, if selective fading erases 

 the carrier at any time, reducing its ampHtude to zero or a small value, 

 the signal, represented by the fundamental tone, practically dis- 

 appears, even though the side-bands have not faded out, and there remains 

 only the harmonic. This is the residual distortion shown in Fig. 41 

 and which can often be heard during a fading out period. It is 

 caused by the two side-bands beating together in the detector. We 

 have here exposed a fundamental defect in the usual form of modu- 

 lated signal transmission. The amplitude of the received signal is 

 subject to all the whims of the carrier and to paraphrase freely an 

 old saying we might remark that a signal is no stronger than its 

 carrier. We may at once conclude that one way to reduce fading 

 is to suppress the carrier and resupply a constant amplitude carrier 

 at the receiving station. 



Analyzing further the first term of the expression representing the 

 detected signal, the first part of the bracketed portion results from 

 beating together in the detector of the carrier and upper side-band 

 and the second part from the carrier and lower side-band. It is clear 

 that one of the side-bands may fade out completely and the other 

 will still bring in the signal, provided the carrier is not also lost, with 

 a phase shift to be sure but nevertheless not seriously reduced in 

 amplitude. In telephony this kind of phase shift is relatively un- 

 important. Here we have an evident advantage in transmitting both 

 side-bands since they support each other's frailties. But if the two 

 side-bands suffer phase shifts in transmission, as we have earlier 

 shown may be produced by wave interference, such that <^i and cj)? 

 differ by t radians or 180 degrees, the two components will cancel 

 each other provided their amplitudes Ai and A2 remain equal. In 

 other words all three components — carrier and both side-bands — may 

 arrive at the receiver with full amplitude and yet no signal will be 

 detected from them except a second harmonic component. This is 

 obviously a disadvantage of transmitting both side-bands since, at 

 such an instant, if one of them were eliminated the signal would 

 reappear. 



We conclude that there is, on the basis of such a brief analysis, 

 not much to choose between single side-band and double side-band 

 transmission when the carrier is transmitted also. 



But if we wish to realize the advantages of carrier sujjpression a 

 choice is not difficult. A carrier suppression system in which both 

 side-bands are transmitted requires that the replacement of the 

 carrier at the receiving station be done with almost absolute accuracy 

 as to frequency and phase, a thing which involves very serious prac- 



