DVST terminals also use
the random scan approach to generate the image on the CRT screen. The term
"storage tube" refers to the ability of the screen to retain the
image which has been projected against it, thus avoiding the need to rewrite
the image constantly.
An alternative method for
maintaining a screen image is to store the picture information inside the CRT
instead of refreshing the screen. A direct-view storage tube (DVST) stores the
picture information as a charge distribution just behind the phosphor-coated
screen. Two electron guns are used in a DVST. One, the primary gun, is used to
store the picture pattern; the second, the flood gun, maintains the picture
display as shown in the figure below.
Figure: Direct View Storage Tube
Various components of DVST:
1.
Electron guns -
Two electron guns are used in DVST: Primary Gun and Flood Gun. Primary gun is
used to store picture pattern. Flood gun is used to maintain picture display on
phosphor coated screen.
2.
Phosphor Coated Screen -
In DVST the inner surface of CRT is coated with phosphor crystals is of high
persistence that emit light when beam of electrons strikes them.
3.
Storage Mesh -
It is thin and high-quality wire that is coated with dielectric and is located
just behind phosphor coated screen. Primary gun deposits pattern of positive
charge on this grid and it is transferred to phosphor coated screen by
continuous flood of electrons produced by flood gun. Thus, Storage Mesh stores
picture to be displayed in form of positive charge distribution.
4.
Collector -
This grid is located just behind storage mesh and purpose of this negatively
charged grid is to smooth out flow of flood electrons.
Working Principal Step by
Step
1.
Writing Phase (Image Creation)
o The
writing electron gun (primary gun) is used to draw vectors/lines (just
like in a normal random-scan vector CRT).
o This
gun operates at relatively high energy and moves slowly.
o When
it strikes the dielectric-coated storage mesh, it causes secondary
emission of electrons.
o The
areas hit by the writing beam lose electrons → become positively charged
(a pattern of positive charge corresponding to the drawn lines is stored on the
mesh).
o The
unwritten areas remain at a more negative or neutral potential.
2.
Display / Viewing Phase (Continuous
Display without Refresh)
o The
flood gun continuously emits low-velocity electrons toward the entire
storage mesh.
o These
flood electrons pass through the collector grid (which helps make their
flow uniform).
o The
positively charged portions of the storage mesh attract these low-energy
flood electrons → they pass through the mesh holes.
o The
negatively charged (or neutral) areas repel the flood electrons → they
do not pass through.
o The
electrons that pass through hit the phosphor-coated screen (which is at
high positive potential).
o This
causes the phosphor to glow brightly in exactly the pattern where the writing
beam had previously deposited positive charge.
o Result
→ steady, flicker-free image that persists as long as the charge pattern
remains on the mesh.
3.
Erasing the Image
o DVST
cannot selectively erase parts of the image.
o To
erase → a high-voltage pulse or special erase gun floods the entire storage
mesh with electrons → neutralizes / removes the positive charge pattern.
o The
whole screen goes dark → new image must be redrawn from scratch.
Advantages of DVST:
- For
picture display it does not require refreshing.
- Display
complex pictures at high resolution without any flicker.
- No
use of frame buffer or refresh buffer.
Disadvantages of DVST:
- Not
used for dynamic graphic such as animation.
- These
systems do not display colors.
- To
erase selected part of an image, entire screen needs to be erased and
modified pictures needs to be redrawn.
A
DVST monitor has both disadvantages and advantages compared to the refresh CRT.
Because no refreshing is needed, very complex pictures can be displayed at very
high resolutions without flicker. Disadvantages of DVST systems are that they
ordinarily do not display colour and that selected parts of a picture cannot be
erased. To eliminate a picture section, the entire screen must be erased and
the modified picture redrawn. The erasing and redrawing process can take
several, seconds for a complex picture. For these reasons, storage displays
have been largely replaced by raster systems.
Difference
between DVST and CRT:
|
Sr. No. |
Basis |
CRT |
DVST |
|
1. |
Use of Refresh Buffer |
To store the picture definition, it
uses refresh or frame buffer. |
To store the picture definition, it
uses grid which is called as storage mesh. |
|
2. |
Form
of storage of picture definition |
It
stores the picture definition in the form of intensity values for different
points on the screen. |
It
stores the picture definition in the form of positive charge distribution on
storage mesh. |
|
3. |
Refreshing |
Refreshing is required. |
No refreshing is required. |
|
4. |
Colors
Produced |
It
displays wide range of colors. |
It
does not display colors. |
|
5. |
Use of Electron guns |
It uses three electron guns i.e., Red,
Green and Blue. |
It uses two electron guns i.e., primary
and flood gun. |
|
6. |
Flickering |
In
this the problem of flickering is observed; if the persistence of phosphor
and refresh rate does not match. |
In
this there is no problem of flickering is observed. |
|
7. |
Modification of picture |
In this the user can modify the
particular portion of picture at any time. |
In this the user cannot modify the
particular portion of picture at any time. For modification, entire picture
must be erased and redrawn. |
|
8. |
Use
of Collector mesh |
To
control the flow of electrons, it does not use collector mesh. |
To
control the flow of electrons, it uses collector mesh. |
|
9. |
Contrast |
It has higher contrast than DVST. |
It has poor contrast. |
|
10. |
Application |
CRT
is widely used to display animations, realistic scenes and graphics etc. |
DVST
is only used for the line drawing applications. |
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