856 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



The disk scanner was a linear transmitter, since the ampUtude of the 

 signal was directly proportional to the film brightness. The fundamental 

 requirement for a receiving tube was therefore as stated by Dr. Davisson 

 in an early memorandum: 



"If screen brightness is proportional to beam current, as for most screens 

 it is, then beam current in the receiving tube should be proportional to sig- 

 nal voltage; the modulations of beam current by signal voltage should be 

 linear. Failure to meet this requirement results in falsification of tone values 

 in reproduction, and when departures from linearity are marked [it leads] 

 to unsatisfactory pictures." 



s' V 



M, 



(a) 



(b) 



(c) 



s' 



M? M? 



Ml' 



Fig. I — Principle of deflection modulation. 



It was this fundamental concept of a beam current directly proportional 

 to input voltage, which led to the development of a tube employing deflec- 

 tion modulation. This is a type of modulation in which the modulating 

 voltage causes the electron beam to be deflected across a defining aperture 

 in such a manner that increasing modulating voltage will cause a larger 

 area of the beam to pass through the aperture and thus increase the bright- 

 ness of the screen proportionally to the modulating voltage. The principle 

 of this type of modulation is illustrated in Fig. 1. 



Figure 1(a) shows a narrow beam of electrons passing through the slit S 

 (perpendicular to the plane of the paper) and arranged to form a sharp 

 image of the ^lit in the plane of the square aperture S\ The relation of the 

 slit image to the square aperture is shown in Fig. 1(b). A bias voltage is 

 applied across the modulator plates Mi and Mi so that the slit image falls 

 just off the square aperture for no signal voltage. As signal voltage is ap- 

 plied across the modulator plates the slit image will move across the aper- 

 ture in such a manner that the cross-sectional area of the beam beyond the 

 aperture is proportional to signal voltage (for small angles of deflection, 

 such as used here). 



