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BELL SYSTEM TECHNICAL JOURNAL 



effect is discussed below. Observations of transmitted pictures have 

 shown that with the dark current of the magnitude indicated, it is 

 practically impossible to detect the impairment from the quadrature 

 component, although distortion is still evident on the computed curves. 

 Figure 14 shows the relatively greater tendency for the double sideband 

 dots to run together than the single sideband ones, for the same total 

 bandwidth. The contributions from the two dots are, of course, in 

 phase and therefore tend to add in the intervening space. It has been 

 pointed out that with single sideband transmission the corresponding 

 contributions to the quadrature component tend to cancel each other 

 under these conditions. 



Fig. 14 — Received signal for two dots, mid-band carrier. 



Discussion 



In estimating the effect of the quadrature component it is instructive 

 to compare the in-phase component and the resultant in, say. Fig. 13. 

 It will be evident that if the latter wave were used, for instance, for 

 telegraph transmission there would be a considerable bias due to the 

 quadrature component whereas the in-phase component shows prac- 

 tically no bias. Such a resultant wave would show a decided impair- 

 ment unless steps were taken to counteract this bias. 



If, however, the same figure is considered from the standpoint of 

 picture transmission it will be clear that the difference is not nearly so 

 striking. An obvious difference between a picture obtained with the 

 in-phase component and one obtained with the resultant is that there 

 is a tendency for a background of light gray to be present in the latter. 

 Secondly, there is less contrast between the blacks and the whites. 

 Both of these effects tend to be eliminated in photographic processes 

 which follow the reception. Moreover, when they are not thus elim- 

 inated, they are not readily seen on examining the picture. 



The presence of dark current increases the magnitude of the in-phase 

 component as compared to the quadrature component. Since the 

 resultant is equal to the r.m.s. value of these two components, it follows 

 that increasing one component as compared to the other causes the 



