Direct View Storage Tubes (DVST)
In raster scan display we do refreshing of the screen to maintain a screen image.
DVST give an alternative method for maintaining the screen image.
Moreover, Direct View Storage Tubes (DVST) uses the storage grid which stores the picture information as a charge distribution just behind the phosphor coated screen.
DVST consists two electron guns a primary gun and a flood gun.
A primary gun stores the picture pattern and the flood gun maintains the picture display.
A primary gun emits high-speed electrons which strike on the storage grid to draw the picture pattern.
As electron beam strikes on the storage grid with high speed, it knocks out electrons from the storage grid keeping the net positive charge.
The knocked out electrons attracted towards the collector.
Also, The net positive charge on the storage grid is nothing but the picture pattern.
The continuous low-speed electrons from flood gun pass through the control grid and \attracted to the postpositive large area of the storage grid.
The low-speed electrons then penetrate the storage grid and strike the phosphor coating without affecting the positive charge pattern on the storage grid.
During this process, the collector just behind the storage grid smoothes out the flow of flood electrons.
The advantage of DVST Direct View Storage Tubes
Refreshing of CRT is not required.
Moreover, Very complex pictures can be displayed at very high resolution without flicker.
Disadvantage of DVST: Direct View Storage Tubes
They do not display color and are available with a single level of line intensity.
For erasing it is necessary to the removal of charge on the storage grid so erasing and redrawing process take several seconds.
Also, Erasing selective part of the screen cannot be possible.
Cannot use for dynamic graphics application as on erasing it produce unpleasant flash over the entire screen.
Moreover, It has poor contrast as a result of the comparatively low accelerating potential applied to the flood electrons.
The performance of DVST is somewhat inferior to the refresh CRT.