Transcript Compression Techniques

Display Technology
Display Technologies
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The Technologies
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CRT
LCD
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PDP
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Dual Scan
Active Matrix
ALiS
PALCD
ThinCRT
LEP
CRT (Cathode Ray Tube)
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100 year old technology
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A glass bell envelope contains a vacuum and an
electron gun. By the application of a current, and
electron stream is created, which is fired through the
vacuum towards the inside face of the glass envelope.
Here it strikes a phosphor layer, which converts the
beam into visible light, colour being achieved through
mixing varying levels of light intensity from red, green
and blue phosphors.
CRT (Cathode Ray Tube) [cont.]
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As there’s only one electron gun, and one beam for
each colour, the screen needs to be refreshed
constantly. This is achieved by altering the angle of the
beam with a magnetic deflector coil, which deflects the
beam across each part of the screen from top left to
bottom right in a movement known as a raster. If
refresh rate is set at 75Hz and resolution is 1024x768
(XGA), this equates to painting 58,982,400 pixels per
second.
CRT (Cathode Ray Tube) [cont.]
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CRT Types
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FST (Flat square tube)
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much less curvature than standard CRT
uses shadow mask (smaller than screen size) to prevent distortion
and overspill of electron stream
Trinitron (also Diamondtron)
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to avoid light reduction caused by shadow mask, uses an aperture
grille, a series of narrow alloy strips.
grille improves light emission, but can move and is visible in
background
CRT (Cathode Ray Tube) [cont.]
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CRT Types (cont.)
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ChromaClear
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Developed by NEC to combine shadow mask and aperture grille
technologies. Uses vertically aligned slots in shadow mask
focusing onto rectilinear phosphors.
EDP (Enhanced Dot Pitch)
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Developed by Hitachi, focuses on the phosphor implementation.
Rather than the standard equilateral triangle model, reduces
distance between dots on the horizontal to create isosceles
triangles of oval rather than round dots, giving a greater density of
dots and hence better resolution.
CRT (Cathode Ray Tube) [cont.]
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Advantages of CRT
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robust, well-known technology
high-quality resolution and image control
Disadvantages of CRT
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size (footprint) on monitors
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short or mini-neck tubes possible, but exacerbates distortion
problems
analogue technology
LCD (Liquid Crystal Display)
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Work by polarisation of light
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Liquid crystals don’t emit their own light, but depend on a
cold cathode backlight being passed through a sandwich of
glass, liquid crystal and polarising filters, at right angles to
each other.
The liquid crystal molecules need to be aligned to allow the
light to refract along the chain and out the other side. By
anchoring the long crystal molecules to each side of the
screen by grooves in the glass, their natural state creates the
necessary alignment.
LCD (Liquid Crystal Display) [cont.]
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When a current is applied to any screen element, the
molecules lose the necessary alignment, so any light is
blocked by the opposing polariser.
Colour is produced in similar way to CRTs, with
individual liquid crystal cells for red, green and Blue.
Unlike phosphors, which emit light, the liquid crystals
filter the light, allowing only their corresponding
colours through.
LCD (Liquid Crystal Display) [cont.]
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LCD Types
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Basically, there a re two types of LCD technology that
have become successful over earlier, less efficient
techniques, passive and active matrix.
Dual Scan (passive matrix)
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the matrix refers to the underlying layer of conductors, used
to activate the screen elements.
In passive matrix, this is usually made up of a lattice of
conductive strips running from edge to edge of the display
LCD (Liquid Crystal Display) [cont.]
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As these strips are relatively long, the time taken to activate each
element is longer than in active matrix models. This means that it
takes longer to refresh the screen, an effect that increases with the
size of the screen and leads to submarining and the need to use
mouse trails
To improve the performance of passive matrix, dual scan LCD
splits the conductor matrix into two sections, each of which are
addressed separately by drivers down both sides of the screen.
Dual scan maintains the low power requirement of passive matrix
but increases refresh rate, with the result that many contemporary
notebook displays use this technology
LCD (Liquid Crystal Display) [cont.]
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LCD Types [cont.]
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Active Matrix - TFT (Thin Film Transistor)
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uses a much more complex conductor array, replacing the
lattice with a grid of independent transistors that lie on a
layer beneath the screen elements.
Far more complex to manufacture, but much faster because it
independently addresses the liquid crystal cells.
Viewing angle is wider, as transistor position obstructs
backlight less than conductor strips.
Much more expensive and has a higher power drain.
LCD (Liquid Crystal Display) [cont.]
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Advantages
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Much smaller footprint than CRT
Lighter, so appropriate for portables
Digital technology (analogue connectors - new DVI)
Disadvantages
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More expensive than CRT (but less so now!)
Poorer resolution
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both types based on 100 dpi, but IBM Monet now gives 150 dpi
and IBM Roentgen will give 200dpi
Plasma Display Panels
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PDP
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Plasma Display Panels
Based on the principle that certain gases emit light when
subject to an electric current
John Logie Baird first considered this technology as a
possibility for producing an image on a screen
Original plasma screens were monochrome, low resolution
and very power hungry. They worked by capturing neon gas
between two plates that were etched with conductive lines.
PDPs
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The conductive lines are at right angles to one another
(similar to LCD) and as current is passed along selected
lines, or channels, the gas in between glows and the image is
built up.
Modern PDPs contain a cocktail of gases that emit
ultraviolet, rather than visible, light, which is then used to
excite a phosphor layer, in a similar way to the electron
stream in CRTs. This gives far greater accuracy in the
resultant image while retaining the brightness and speed of
the original principle.
PDPs
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Using this current technology, PDPs can’t be made to
rival the resolution of LCD, so they haven’t been used
for monitors and notebook displays. However, they can
be made to a considerable size and their natural
brightness makes them highly suitable for large
advertising displays, such as noticeboards at airports
and train stations or product information screens at
trade-shows and exhibitions.
PDPs
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ALiS
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Fujitsu is developing a new plasma display type that
will overcome the low resolution restrictions of current
PDPs
Alternate Lighting of Surfaces uses interlaced rather
than progressive scans to increase the resolution of
screens. This is achieved by creating a sandwich of two
layers of the plasma display and removing the
conducting lines that separated the channels.
PDPs
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The result is that, as well as offering a higher level of
resolution, the displays are even brighter and it actually
only uses around half the drivers of the older
technology.
AliS has been developed to compete in the digital
television market, where a resolution of around 960
lines on screen is demanded.
PDPs
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PALCD
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A hybrid of PDP and LCD, the Plasma Addressed Liquid
Crystal Display.
Sony and Tektronix are currently working on making a
viable commercial PALCD.
Rather than use the ionisation effect of the contained gas for
the production of an image, PALCD replaces the active
matrix design of TFT LCDs with a grid of anodes and
cathodes that use the plasma discharge to activate LCD
screen elements
PDPs
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With the exception of the plasma activation, the rest of the
display uses standard LCD components.
PALCD displays can be built to provide 42” (105cm) and
greater presentation displays and televisions, the resolution
achievable being good enough to support digital television
though they won’t be used for monitors.
A significant advantage of PALCD is the lack of
semiconductor controls.This means that the displays can be
built in low-grade clean rooms, reducing costs.
Claimed to be brighter, and still thin.
New Technologies
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ThinCRT
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Developed by Candescent, who have replaced the electron
guns, deflection yoke and shadow mask of the standard CRT
with a perforated conductive sheet through which conical
cold cathode emitters, known as Spindt Cathodes, protrude.
Passing a current through the conductive sheet causes the
cathodes to emit a stream of electrons, which cause
phosphor elements to glow in exactly the same way as a
standard CRT.
ThinCRT
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Spindt Cathodes are very small, only 200 nanometres
each, so it takes several of them to activate individual
pixels onscreen. This allows a relatively high failure
rate, Candescent claims 20%, before any degradation
of the image is visible. As a result this technology is
more viable for mass production than LCD.
Advantages of ThinCRT
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Thin, a fraction of the depth of CRT, so more like LCD
and PDP
ThinCRT
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Offers the same, or better, quality of resolution as CRT, so
better than LCD and PDP.
Brighter, wider viewing angles, lower power consumption.
Potential Disadvantage of ThinCRT
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In order to function, the cathodes still need to reside in a
vacuum. In order to stop the glass envelope from collapsing,
Candescent has developed a non-conductive ceramic
compound which is made into 0.05mm spacers to keep the
sides apart. They shouldn’t interfere with the image but how
robust they are is currently an unknown factor.
LEP
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LEP
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Light Emitting Polymers
Conjugated polymers, such as polyprolle [which has
been known for over 100 years] and polyaniline, are
plastic materials with physical properties that confer
conductive properties.
In recent times these polymers have been used for
battery electrodes, transparent conductive coatings,
capacitor electrolytes and circuit-board plating.
LEP
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When Cambridge University discovered that certain
conjugated polymers, specifically
pphenlylenevinylene (PPV), could be made to emit light
in addition to carrying current, the idea of using LEP
for displays was born.
Cambridge Display Technologies have been
developing this technology, working on the display
technology itself, and the creation of stable red, green
and blue polymers based on PPV.
LEP
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Essentially the creation of the display itself is similar to, but
simpler than, LCD. On the surface of a layer of substrate a
transparent electrode layer is applied, on top of which a PPV
layer is applied, and then a second electrode layer. When
current is passed between the two electrode layers the
polymer will emit photons, which pass through the
transparent electrode layer and the substrate to the viewer.
The display is structured into cells, similar to the picture
elements of an LCD, and each cell can be addressed
individually using an active matrix of electrodes.
LEP
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LEPs are about to hit the market, probably initially as
backlight surfaces for handheld PCs and information
displays, in the market currently served by
monochrome LCDs, but they could have a major
impact in the future.
Adavantages of LEP
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Can be applied to very large surfaces, so far greater
than the current 42” PDP maximum.
Polymer can flex with the base surface
LEP
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Potential Disadvantage of LEP
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Far cheaper to make than competing technologies,
substrate doesn’t require to be glass, simple electrode
structure, easy to produce.
Lightweight
Longevity and efficiency of LEP still unknown factor
Philips has created a dedicated fabrication plant!