A THEORY OF SCANNING 479 



will be shown farther on, however, that the effect of the use of a finite 

 aperture for scanning is to confine the signal energy more rigorously 

 to such bands throughout the frequency range. 



The theoretical energy distribution for the circular area is in excel- 

 lent agreement with actual frequency analyses of television currents, 

 which show the energy confined to bands at multiples of the line scan- 

 ning frequency with apparently empty regions between. It is evident 

 from the theory so far, however, that these regions are not really empty 

 but are filled with weak signal components representing fine details of 

 the subjects; and subjects of greater pictorial complexity than a simple 

 circular area may be devised to give large signal components in such 

 regions. We must therefore look for other factors to explain why these 

 frequency regions do not transmit any appreciable details of an image. 



Confusion in the Signal 



With the usual method of scanning, one such factor is the confusion 

 of components in the signal. This confusion arises from the fact that 

 two or more image components sloping across the field in different 

 directions may intercept the line of scanning with their crests spaced 

 exactly the same distance apart along this line of scanning. As the 

 scanning point passes over them they thus give rise to signal current 

 components of exactly the same frequency. Consequently the two 

 image components are represented by a single, confused, signal current 

 component that can transmit no information whatever in regard to 

 their relative amplitudes and phases. This confusion evidently de- 

 pends on the scanning path. 



If the image field is scanned in A'^ lines, the velocity v of the scanning 

 point parallel to the y axis is 



and the signal frequencies from equation (11) are 



/ = ^(»>+-«)- (13) 



Field components with indices m, n and ni', n' such that 



m -f ^ = m' -I- ^ (14) 



give rise to current components of the same frequency. 



