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9.
HOW
DOES PLASMA TV WORKS?
When voltage is applied between the electrodes, ionized gas collects at the
surface layer according to the polarity. The collected charge is called the wall
charge, and it produces an internal voltage difference (wall voltage). As a
result of the surface discharge, ultraviolet ray radiation occurs. This
activates phosphor dots and visible light is emitted. A colour display is
achieved by controlling the luminance from individual phosphors in the three
prime colours. To maintain the discharge, the polarity of the external voltage
is reversed. The applied voltage is lower than the initial voltage because the
wall voltage remains inside the panels, so once discharging occurs, it continues
unless it is stopped.
Courtesy: Philips
Gas Discharge Principle
The basic operating principle of the Plasma screen is that of light
generation by gas discharge and phosphor excitation, in the same way as in a
fluorescent lamp. The gas discharges take place in the picture elements or
pixels of the display . The total display, with the wide screen 16:9 aspect
ratio, consists of 852 x 480 pixels, each measuring 1.08 mm square, and has
dimensions of 920 x 518 mm (about 107 cm diagonal).
The front part of the screen consists of three layers. The first layer is the
anti-reflection and anti-glare front glass plate which is visible to the viewer.
Fixed to the back of this glass plate is a transparent dielectric
(capacitive) layer, behind which is a protective magnesium oxide (MgO) layer.
Behind this dielectric is a layer of barrier ribs which separate the front plate
from the rear glass plate.
On the rear glass plate is a matrix of embedded, addressable electrodes. The
pixels of the display are formed at the points where these embedded electrodes
cross. At these pixels, local gas discharges are ignited by high-voltage pulses.
This takes place in a low-pressure neon and xenon gas filling contained in a
separate chamber for each pixel. These gas discharges cause the emission of UV
light. The UV discharges are produced when the inert gas mixture is locally
converted into Plasma by the applied voltage pulses. The relatively low gas
pressure minimises the amount of energy that is needed to energise the pixels
and change the gas filling into a UV-emitting Plasma. However, voltages of
several hundred volts still have to be applied for this purpose.
Varying Intensity
This principle of tiny, individually addressable gas discharges is basically
how the Plasma display works. However, two `details' still need to be explained,
because the ability simply to turn a large number of individual pixels on and
off is in itself not enough to create the TV picture that we need. First of all,
the intensity of the light generated at each pixel needs to be modulated,
because even a black-and-white display is never simply black-and-white: it
consists of a wide range of grey levels covering the entire scale from black to
white. Secondly, and most obviously, the display needs to have the capability to
generate not just light but all the colours of the spectrum, to allow colour
pictures to be displayed.
As far as intensity is concerned, it is necessary for the intensity of each
pixel to be modulated, so that the required different grey scale levels can be
produced. However, the intensity of the discharge itself cannot be modulated, so
the solution has been found in pulse width modulation. The use of an 8-subframe
addressing principle allows each pixel to be re-addressed at up to 8 times per
cycle, thereby creating the effect of intensity variations. This technique
allows the apparent intensity of each pixel to be varied between 256 levels.
Creating Colours
To create
colours, each pixel consists of not one but three separate cells:
one each for red, green and blue. These activate the respective phosphors on the
rear glass plate, producing emissions of the desired red, green and blue light.
Each cell can reproduce 256 different intensity levels, which translate into 256
different colour contrast levels. Just as in a conventional CRT-based TV, all
desired colours can be created by mixing together different intensities of the
three primary colours red, green and blue.
The result is a colour palette totaling 256 x 256 x 256
colours, which adds
up to more than 1.6 million colours. The Plasma display has a resolution of 852
pixels horizontally and 480 pixels vertically, although since each pixel
consists of three cells the total number of light-producing cells is three times
as high.
Gamma Correction Circuit
Because the pictures transmitted by TV studios include a pre-correction to
compensate for the non-linear characteristic - or gamma characteristic - of
conventional CRT tubes, a colour correction has to be applied in the Flat TV
electronics. This done by a gamma circuit, which converts this standard
pre-correction to match the linear gamma characteristic of the Plasma display
technology.
Optimum Large-screen Picture Quality
While the Plasma technology itself gives brilliant colours and outstanding,
high-contrast reproduction, additional technologies are required to enable this
capability to be exploited to the full in a fully featured high-end TV set. In
fact the New Flat TV incorporates a range of advanced signal-processing
technologies to ensure optimal TV performance. Eliminating visible flicker is an
essential requirement.
In the New Flat TV, progressive scan refreshes the complete picture,
consisting of both even and odd lines, 50 times a second, effectively
eliminating any visible flicker and giving a perfectly stable picture. Philips'
expertise with the industry-leading 100 Hz Digital Scan technology is used in
the signal processing stages which are necessary to create optimal pictures from
a wide variety of signal inputs. For example a 625-line PAL signal, or a
teletext signal containing over 500 lines, has to be converted into the 480
lines (480 vertical pixels) of the Flat TV. In addition, advanced horizontal and
vertical filtering techniques are used in TV signal processing.
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