672 



DISPLAY SYSTEM DESIGN PROBLEMS 



These tubes are useful for extremely high-speed teletype equipment. 

 They are capable of a writing speed of 25,000 symbols per second, provided 

 the input channels have wide enough bandwidth. A disadvantage of the 

 tubes is their large length, which is about twice that of an ordinary CRT, 

 to give room for the additional deflection and focusing system. 



The front end of these tubes may be either direct phosphor excitation as 

 in the CRT or indirect and with controlled persistence as in the direct view 

 storage tube (in which case a storage surface and flood gun is incorporated). 

 Fig. 12-47 illustrates the functioning of the latter as in the Typotron. A 



Writing Gun 



Compensation Plates 



Character Selection 

 Plates 



Character Matrix 



Character - Shaped 

 Writing Beam 



Storage Target 



Magnetic 

 Convergence Coil 



Viewing Screen 



Fig. 12-47 Schematic of the Character-Writing Typotron Storage Tube. 



first set of deflection plates directs the beam through the selected symbol of 

 the stencil, whereby it is given a cross section corresponding to the symbol. 

 A magnetic field bends the beam back to the tube axis, and compensation 

 plates align it so as to enter the second set of deflection plates, which in turn 

 direct the beam to the correct location on the storage mesh. A positive 

 charge results on this spot on the storage surface caused by secondary 

 emission in the same manner as in ordinary storage tubes. The flood gun is 

 located next to the deflection plates. The Typotron is operated as a bistable 

 storage tube; i.e., it has only two shades of gray — black and white. 



Instead of directing a narrow beam through a single symbol, a wide 

 collimated beam may illuminate the entire stencil as shown in Fig. 12-48. 

 The entire beam is focused through the first set of deflection plates (selec- 

 tion plates) which bends the beam so that only the part containing the 

 desired symbol penetrates a fixed aperture and subsequently is deflected 



