864 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



"focal spot" of non-uniform density. This in turn results in rounded corners 

 of the modulation curve. Even so, the linear portion of the modulation curve 

 corresponded to a beam current ratio of about 10: 1. 



As will be seen from the curve, the tube may be used equally well for 

 either positive or negative modulation, but in the demonstrations positive 

 modulation was employed. 



An actual modulation curve for the tube later used for 441 line pictures 

 is shown in Fig. 8. The dotted line indicates the modulation characteristic 

 without any modification, while the solid curve shows the improvement 



100 



80 



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 g60 



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30 



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Fig. 8 — Measured modulation curve. 



near cut-off obtained by incorporating a non-linear circuit in the output 

 stage of the video amplifier. With this circuit, linear modulation was ob- 

 tained over a brightness range of nearly 100 to 1. 



Due to the variable width of the scanning spot it is obvious that con- 

 ventional aperture equalization is not applicable. The width of the rectan- 

 gular spot changes from maximum at full brightness to zero or nearly zero 

 in the deep shadows. In other words, the correct aperture equalization 

 would be a function of brightness. Some theoretical computations indicated 

 that the effective horizontal resolution, without any aperture equalization, 

 might be well above the vertical resolution determined by 240 lines. That 

 this was actually so was indicated by the fact that, by unbalancing the 



