A THEORY OF SCANNING 



477 



0.2 



I 



1000 1500 2000 2500 3000 3500 4000 4500 5000 



0.05 



22,000 



17,000 18,000 19,000 20,000 21,000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 8 — Correlation between wavelength and frequency of signal components 



In telephotography the frequency of line scanning is usually low and 

 the groups of lines in the frequency spectrum are so closely spaced that 

 such fine grained details of the signal are of little practical importance 

 as far as the electrical parts of the system are concerned. In television, 

 however, these bands are widely spaced, of the order of 1000 cycles or 

 so apart, and such details of the signal are quite important. 



As a specific example, it is interesting to plot the frequency spectrum 

 of the television signal that results from scanning a circular area of 

 uniform brightness on a black background. So far as the present 

 theory extends, this may be done by converting the field components 

 of equation (9) into current components with the aid of equations (10) 

 and (11). Taking b/a = 1.28, the radius of the circle as b/3, and as- 

 suming that the field is scanned in 50 lines 20 times per second, we ob- 

 tain the amplitude-frequency spectrum shown in Fig. 9. Since it is 

 not convenient to show the individual current components — only 20 

 cycles apart — the curve shows simply the envelope of the peaks of 

 these components. At low frequencies, the energy is largely confined 

 to bands at multiples of the line scanning frequency, 1000 cycles, and 

 to an additional band extending up from zero frequency. In the re- 

 tions between the bands, the signal components are so small that they 

 do not show when plotted to the same scale. At higher frequencies the 

 signal energy as thus far computed is not confined to such bands. It 



