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BELL SYSTEM TECHNICAL JOURNAL 



In other words, the bands of components in the frequency spectrum 

 really overlap. Consequently the components of one band may coin- 

 cide in frequency with the components of adjacent bands. Such coin- 

 ciding components are illustrated schematically in Fig. 10. 



m BAND FREQUENCY m + 2BAND 



Fig. 10 — Coinciding lines of confused bands. 



It is obvious that a single a-c. component cannot transmit the sep- 

 arate amplitudes and phases of two or more image components. Con- 

 sequently the receiving apparatus has no information to judge how the 

 components in the original image are supposed to be distributed in the 

 reproduction. 



The situation is most serious where the intensities of coinciding com- 

 ponents have the same order of magnitude, that is, at the centers of the 

 frequency regions intermediate to the strong bands. The confusion 

 in these regions is the most important factor that renders them in- 

 capable of transmitting any appreciably useful image detail. 



On first consideration it would appear that the overlapping of bands 

 in the signal might result in a hopeless confusion. The situation is 

 saved, however, by the fact that components with large n numbers will 

 tend to be weak due to the convergence of the Fourier series, and are 

 further reduced, as will be shown later, by the effects of a scanning 

 aperture of finite size. They therefore do not usually seriously inter- 

 fere with the stronger components. The interference usually manifests 

 itself in the form of serrations on diagonal lines and occasional moire 

 effects in the received picture. 



Confusion in the signal may be practically eliminated by using an 

 aperture of such a nature that it cuts off all components with n numbers 

 greater than N/2, that is, cuts off each band before it reaches the center 

 of the intervening frequency regions so that adjacent bands do not 

 overlap. The practical possibilities of this arrangement will be dis- 

 cussed further below. 



The mere elimination of confusion in the signal itself does not neces- 

 sarily prevent the appearance of extraneous components in the repro- 

 duced image. The receiving apparatus itself must be so designed that 



