106 BELL SYSTEM TECHNICAL JOURNAL 



circuit be detuned, this symmetry is upset and we are forced to the 

 complicated relations of equations (13) and (14), from which it is 

 difficult to draw general conclusions. 



An interesting case of unsymmetrical transmission is that in which 

 one side-band is only partially suppressed owing to insufficient 

 selectivity. Let us assume that the upper side-band is transmitted 

 without distortion. Then for a given amplitude of the lower side- 

 band its effect on the amplitude curve of the reproduced signal will 

 be worst when the phase relations are such that for some frequencies 

 it aids the upper side-band and for others it opposes. The greatest 

 fractional change in the amplitude of any one signal component due 

 to the presence of the lower side-band occurs when the two oppose. 

 It is then reduced in the ratio 



B+-B- =1 _gr . (23) 



Thus, if the lower side-band component were a tenth of the upper, 

 the most it could do would be to change the amplitude of the signal 

 component by a tenth. Such a change would have little effect on 

 telephone quality, particularly as it would have this maximum value 

 at only a few frequencies. The case of telegraphy is rather different. 

 Here the two side-bands lie so close together that it is practically 

 impossible to separate them by radio selective circuits, and even 

 when, as in wire transmission, so low a carrier frequency is used that 

 the two may be separated fairly well by filters, the side-bands cor- 

 responding to the lower signal components differ so little in frequency 

 that even a sharp filter does not produce very great discrimination 

 between them. This, coupled with the fact that the phase shift 

 of a filter ceases to be linear near the edge of the transmitted band, 

 leads to very considerable amplitude distortion. 



The effect of the unsuppressed side-band on the phases of the re- 

 produced components is also rather complicated. Equation (14) 

 shows that it is a maximum when 



</>+ -0 = 



0-0_ = 9O°, 



(24) 



in which case the presence of the lower side-band changes tan ty from 



B- . 



to j5~. As in the case of amplitude, this effect is unimportant in 



telephony, but would need to be considered in telegraphy. 



