Editor's Note 2005: The AMLCD is
the most widely used display technology in currrent new production
military aircraft and upgrade packages for legacy aircraft. Click here for a discussion of CRT
The emergence of high
performance Thin Film Transistor Liquid Crystal Display (TFT LCD)
technology over the last decade heralds a new era in desktop displays
for computing applications. The long serving Cathode Ray Tube (CRT)
display is in the process of being knocked off its perch as the premier
display technology used in computing applications. Certainly the CRT
will be with us for many years to come, since it is mature and cheap,
but clearly the TFT LCD is the technology of the future.
The central issue at this time is price. The CRT
despite its many limitations is highly price competitive due to the huge
production volumes achieved, and the refinement in production processes
which comes with half a century of mass production. In the simplest of
terms, the CRT is to computer displays what the internal combustion
engine is to automobiles.
Despite this, the clear performance and capability
advantages of the TFT LCD have pretty much set the long term path for
displays. The enormous production and R&D investment made by the
Japanese and Koreans, in anticipation of an enormous consumer market in
computer displays and High Definition Televisions (HDTV), clearly
indicate that the CRT has only about a decade of life left in
CRT Displays -
The Technology Issues
The trusty CRT is the last piece of vacuum tube
technology to survive in the mass production marketplace (excluding the
specialised 2.45 GHz magnetron tube in your microwave oven). The
central idea behind the CRT is to produce an electron beam from a
heated cathode, accelerate that beam to tens of keV of energy, and
drive the beam into a phosphor material which emits light when so
excited. The beam, termed a cathode ray, is deflected magnetically (or
electrostatically in some applications) in a raster scan pattern, to
sequentially access individual pixels on the screen of the CRT. The
beam is swept rapidly across a horizontal line - a row of pixels,
typically at rates between 15 kHz (TV) and 100 kHz (hi-res monitors),
then swung back to the beginning of the line to trace the screen again,
while it is stepped down vertically, to address another row of pixels
on the screen. Vertical stepping rates are much slower, between 50-60
Hz (TV) up to 100 Hz (hi-res monitors). By controlling the brightness
of the beam as it addresses each pixel, we can paint a picture on the
Now this mechanism alone provides only for a monochrome
display, to achieve colour display three beams must be used, one for
red, green and blue each, with each beam current controlled individually
to achieve the desired hue, saturation and brightness desired for a
given pixel. The beams each hit red, green and blue emissive phosphor
dots or stripes on the face of the tube. A shadow mask, usually built
as a thin sheet of perforated metal behind the screen, separates the
three beams. Conventional tubes use triangular clusters of phosphor
dots, whereas the popular Sony patented Trinitron uses stripes of
Driving a CRT requires essentially five signals, three
video channels which carry brightness for the individual beams, as an
analogue voltage typically with a peak-peak swing of one Volt, and
horizontal and vertical synchronisation (sync) signals which control the
beam deflection circuits and hence, the addressing of pixels on the
screen. Sync signals are typically TTL level.
A graphics adaptor for a CRT will typically store 8-32
bits of brightness information per pixel in a dual ported video RAM
(VRAM) chip array. The CPU or accelerator hardware will write
calculated brightness values into each pixel location via one port,
while a RAMDAC (RAM controller and Digital to Analogue converter on one
die) will extract the stored pixel value, decode it, and convert it to
analogue voltages to directly drive the cable into the CRT display.
Many RAMDACs also generate the sync signals, which may be separate or
combined into a single TTL signal. Incompatibilities in sync signal
formats, polarities and timing have long been used by manufacturers to
lock competitors out of their markets, and the popular multisync
monitor will only perform where the sync signal format is known and
While purists may grumble at the superficiality of this
treatment of CRT displays, it serves to illustrate both the simplicity
of the CRT interface, and the basic interface model which is common to
most LCD displays.
Other refinements to the interface have evolved in
recent years, mainly to accommodate power saving and boot time probing,
and a VESA standard exists to define additional serial TTL status
monitoring and control signals between adaptors and CRT displays.
The strength of the CRT lies in maturity and cost, and
potentially excellent picture contrast/brightness/resolution and viewing
angle performance. In terms of picture quality per dollar, the CRT is
hard to beat.
But the CRT also has it warts. Weight, volume and power
consumption are all issues, and having a 21" monitor on your desk means
abandoning all hope of using it for anything else useful. More serious
failings also exist. One is the propensity of CRT guns to emit soft
X-rays, ostensibly absorbed by additives in the tube faceplate glass.
Nevertheless the potential for harmful X-ray exposure from prolonged use
of CRT displays has led to a significant tightening of emission
standards in recent years. The magnetic beam deflection system used also
produces problems, since strong magnetic fields will exist around the
neck of the tube, and picture quality is vulnerable to external
magnetic fields. The profusion of CRT monitors in Australia with
lopsided pictures testifies to a consumer base with no appreciation of
the fact that a picture geometry adjusted in the Northern hemisphere's
magnetic field won't do for the Southern hemisphere.
Other problems are also implicit in CRT designs. One is
their propensity to produce nasty RF interference in the neighbourhood,
by radiating amplified video and sync signals. The former effect,
termed Van Eck radiation, can be sufficiently intense to allow a third
party with suitable equipment to read the picture off your screen from
tens to hundreds of metres away. Beware, if a nondescript panel van
with antennas on its roof is parked in the neighbourhood !
Despite its failings the market for CRT displays has
seen steady growth this decade, with worldwide USD 11B sales in 1993
growing to a projected USD 18B this year.Clearly the incumbency of the
CRT will take some beating.
The LCD display is slowly penetrating the market, with
1993 sales of USD 2B growing to a projected USD 10-15B this year. We
can expect the market to swing around the turn of the century, if
current trends in LCD manufacture persist.
LCD Displays - A
The liquid crystal was discovered in 1888 by Renitzer,
an Austrian botanist, but it was not until 1968 that RCA Labs in the US
produced the first experimental display device. What makes liquid
crystals useful for displays is their optical behaviour, since they
posess a voltage dependent optical polarisation. Light rays passing
through such materials are twisted through 90 degrees in polarisation,
if no voltage is applied. If an electrical voltage is applied, the
molecules in the liquid crystal change their alignment and the light
rays are no longer twisted in polarisation.
If we sandwich a layer of liquid crystal between two
glass plates, put a transparent sheet electrode on either plate, and
then apply an electrical voltage of suitable magnitude to the
electrodes, this sandwich of sheets becomes an electrically controlled
optical polariser. If we then take two sheets of optical polariser,
rotating one ninety degrees to the other, and place them either side of
the liquid crystal/electrode sheet, we have a voltage controlled light
valve. With no voltage applied, polarised light passes through the first
polariser into the liquid crystal, is twisted by ninety degrees, and
exits the second polariser. By applying a voltage to the liquid
crystal, we force the molecules to untwist and the light rays retain
their initial polarisation and are stopped by the second polariser.
Voltage ON = Pixel OFF, Voltage OFF = pixel ON, and by varying the
voltage we can smoothly control the amount of light passed though this
sandwich panel. This is the basic physical model upon which all LCD
display technology is based.
Liquid crystals can be driven with voltage levels of
Volts to tens of Volts, and consume little power. However, they are
slow to twist when voltage is applied, and this is a major problem for
high performance displays intended to show rapidly changing images.
This is the reason why LCD based calculator and photocopier displays
were quick to emerge, but computer displays had to wait until the
To produce a matrix display suitable for bitmapped
computer graphics, we need to divide the LCD panel electrodes into a
matrix of individual rectangular cells, one per pixel (monochrome). By
controlling the electrical voltage across each cell, we can control how
much light is transmitted by that pixel. Typical displays use a flat
fluorescent backlight behind the LCD sandwich, to provide a diffuse
source of white light.
The most trivial arrangement for an LCD display is the Passive
addressing scheme. Such arrangements use a row of vertical electrode
stripes on one side to provide horizontal addressing of pixels, and a
row of horizontal electrode stripes to provide vertical addressing of
pixels. The pixel at the intersection of the row and column stripe
electrodes with voltage applied to them is then addressed, i.e. turned
on or off. We can then in a very simple fashion use a pair of counters
to sequentially address each and every pixel, column by column in a row,
and row by row, repeating the scan ad infinitum.
While appealing in its simplicity, this scheme in
practice falls foul of the speed limitations of (Super) Twisted Nematic
((S)TN) liquid crystal materials, which require tens to hundreds of
microseconds to switch state once a voltage is applied. Therefore we
would cheat by addressing pixels by row, driving each column
concurrently with a control voltage and using the horizontal electrode
stripes to address row by row.
Let us assume that we are addressing a 640x380 pixel
display with a horizontal frequency of 27 kHz and vertical frequency of
70 Hz. We have 37 microseconds to address each row of 640 pixels,
assuming we drive all 640 columns with a specified analogue voltage,
and we revisit each pixel every 14 milliseconds - this means that we
have about 37 microseconds to drive each pixel, which has 14
milliseconds to wait before getting its next dose of electric field.
In practice passive schemes perform poorly, since the
short time in which the voltage is applied produces poor contrast,
narrow viewing angles and poor greyscale performance (i.e. a measure of
how finely we can control the transmission through the pixel), little
surprise since the pixel is being driven for less than 1% of the time.
This is exacerbated by a propensity to crosstalk between pixels, as the
electric field produced by applying voltage to a pixel can leak into its
neighbour, and in effect partially drive it, smearing the image. For
this reason passively addressed LCD panels never got beyond trivial
As is evident, a more sophisticated scheme was required,
to allow each pixel to be set to a voltage level and held there until
the next addressing cycle. Ideally this could be accomplished by
attaching a tiny capacitor to each pixel, and using a tiny switch to
connect the capacitor to the column drive voltage once per addressing
cycle. The TFT panel uses this method.
Active Matrix Thin
Film Transistor LCD Panels
In a TFT panel design, Thin Film semiconductor
technology is used to build a tiny transistor switch and capacitor for
each and every pixel in the LCD panel. Using plasma deposition,
sputtering, photolithographic and etching techniques similar in
principle to those used in monolithic chip fabrication, transistors,
capacitors and metal addressing lines are deposited on a glass
substrate to form the required structures. Much of the machinery used
for this task is derived from equipment used in conventional chip
The most commonly used addressing architecture for TFT
panels uses column "data" drivers to concurrently apply the required
voltages to every pixel in a row, which is selected by a row "scan"
driver. The scan driver turns the TFT switch on to charge the capacitor
of every pixel in that row. Once the TFT switch is turned off, the
capacitor holds the pixel at the set voltage (ie polarisation ->
transmissivity -> brightness ) level until the next refresh cycle.
Because the pixel matrix uses active TFT devices, it is termed an Active
Matrix TFT LCD panel.
Because each pixel is isolated once the TFT switch is
turned off, AM TFT panels have no pixel crosstalk problems, and
isolation of the column lines from the capacitors and LCD pixel means
that faster LC materials can be used, since there is not need for the
inertia used in the passive architecture. Higher speed allows for much
bigger panel sizes and resolutions, with superior contrast ratios,
greyscale resolution and better control of viewing angles. The AM TFT
panel is superior in every respect to the passive model.
While the use of TFTs solves many problems, it also
creates problems, since it is by any standard a finicky fabrication
process. The TFT switch is typically built as a MISFET (Metal Insulator
Semiconductor Field Effect Transistor), mostly using Silicon Nitride as
an insulator for the Gate, and doped Silicon for the transistor channel.
The FET Source drives the Thin Film capacitor, the column voltage drive
feeds into the FET Drain, while the Gate of the FET is connected to the
selector scan drive. When the vertical scan drive voltage is applied to
the gates of all TFT FETs in a row, they switch on and the voltage
applied to the column drive is switched through the FET channel via the
Source to the capacitor, charging it up. Readers familiar with DRAMs
will appreciate the conceptual similarity.
TFT materials for AM TFT LCD panels are a science within
themselves. The most common material used is amorphous Silicon (a-Si),
common in Solar cell technology and decidedly inferior in electrical
performance to the monolithic Silicon used in IC fabrication. The
electrical conductivity and switching speed of a MISFET transistor both
limit the addressing speed of a TFT panel, and critically depend upon
the electron mobility (mu) of the transistor material. In this respect
a-Si is woeful by any standards. An improvement can be made by using
poly-Silicon (p-Si), which has two orders of magnitude higher mobility,
allowing for much faster charging of pixel capacitors. However,
conventional techniques for fabricating p-Si involve temperatures around
600 deg C, much higher than for a-Si, which tend to soften the cheaper
glass substrates used with a-Si, forcing the use of expensive Quartz
glass panel substrates. The latest technology for p-Si processing uses
Excimer UV laser annealing of the transistor structures, to achieve the
required p-Si properties without expensive glass substrates.
As is evident, the high cost of current AM TFT LCD
panels is a direct result of very complex fabrication processes, which
may produce often poor batch yields. The bigger the panel and its
number of pixels, the greater the odds that a processing defect will
occur rendering a pixel or row/column of pixels dead and thus resulting
in an expensive and useless reject. In practice automated production
machinery is used to process several panels together, with yields and
throughput set by the size of the machinery. A line which produces a
given defect density per square metre of panel, which processes a dozen
10" panels, will have a decidedly lower yield if it is used to make
three 20" panels at a time. The reader is invited to calculate this,
assuming a uniform density of defects per area !
At this point in the cycle we have made ourselves an AM
TFT LCD panel by combining the aforementioned production processes and
technologies. The panel alone however is not very useful without the
necessary electronic driver circuits which push signals on to the
panel's row and column addressing lines.
Driver Circuits for AM
TFT LCD Panels
The electronics used to drive a TFT LCD panel are no
less interesting than the innards of the panel. Two basic classes of
device are used - Column Drivers and Row or Gate Drivers.
The simpler of the two is the Gate Driver, which is
essentially a large register, with control and glue logic, which
controls an array of high voltage analogue drivers. Essentially, the
register propagates a "1" value along the rows in the TFT LCD panel,
selecting one of the total number of rows, and driving it with TFT Gate
control voltages with typical swings of 30-40 Volts. Since typical
designs only cover 120-128 rows per chip, these devices can be cascaded
to support vertical depths such as 768, 900 or 1024 lines by joining 6
or 8 Gate drivers respectively.
The Column Driver is by far the more complex circuit,
since it has to latch a digital number determining the brightness of a
pixel, and convert it to an analogue voltage to drive the TFT columns.
A typical design will use a 9-36 bit single ended or differential
TTL/CMOS bus for distributing the pixel values to the Column Drivers.
The number of bits available per pixel determines the gray scale
resolution of the display (and hence number of colours). Control signals
from a synchronisation circuit are used to consecutively latch binary
pixel values into the Column Driver. Once the pixel values are all
latched, they are fed into a discrete Digital to Analogue converter
(DAC) to produce drive voltages for each an every column. Due to LCD
non-linearity a Gamma-correction scheme is often used, with the voltage
levels for each Greyscale level produced by an external analogue
resistive divider circuit. In this manner, specific control can be
exercised over the display's Gamma correction. It is worth noting that
the discrete drive levels used simplify the DAC design down to an
analogue mux. Finally the output from the DAC is fed into drives which
feed the TFT columns. Like Gate Drivers, the Column Drivers are
typically cascaded, with 300-309 or 384 columns driven per driver. By
appropriately combining multiples of these values, common resolutions
such as 1024, 1152, 1280 or 1600 columns may be produced.
Some measure of the internal throughput on a column
driver pixel bus can be gained from the typical clock speeds of 65 MHz
and widths of 9 to 36 bits, producing bus throughputs of 73-293
Mbytes/sec. Not a trivial number, by any measure.
Contemporary TFT LCD drivers are typically made using
conventional monolithic IC fabrication methods, and packaged using Tape
Automated Bonding (TAB) techniques for low cost. The TAB packaged
drivers are glued on to the glass substrate around the edges of the
panel and the leads are connected via one of many possible techniques to
the metal column and row leads fabricated on the glass. It is hoped
that the next generation of p-Si based technology may allow the driver
circuits to be made of TFTs and directly fabricated on the glass
substrate with the rest of the panel. Significant cost savings will then
The internal interface to the driver circuits for a TFT
LCD panel, as used for instance in a laptop, can be wholly digital,
bypassing the need for a RAMDAC in the graphics adaptor, as is used in
99% of contemporary desktop computers. Basically a bus is required to
carry data for each pixel, and a clock and sync signals are needed to
tell the panel drivers where to put each pixel value.
Should the TFT LCD panel be used in a monitor, then an
Analogue Digital Converter (ADC) will be needed to digitise the incoming
analogue RGB video so it can be placed upon the pixel data bus. Unless
an expensive ADC is used, some loss of colour resolution may occur. To
complete the monitor design, typically a microprocessor will be
embedded to control the setup of the monitor, using Non Volatile RAM or
EEPROM to save Gamma correction and brightness, contrast and colour
The observant reader will note that up to this point we
have not explained how colour is produced. This was intentional.
The basic technique for making a colour AM TFT LCD is to
simply triple the number of TFT/capacitor/electrode cells (until now
referred to as pixels), and produce a three colour RGB optical filter
layer, so that each pixel is really made up of three individual
subpixels, one for each colour, and additional Column Drivers to select
these. A rectangular pixel is split into three subpixels, each with a
coloured transparent rectangle per subpixel. Monstrously complex ?
Indeed, since we have just had to either triple the number of Column
Drivers or build in a clever multiplexing scheme so that we can select
which colour cell we are to drive.
AM TFT LCD Monitor
The market for high resolution AM TFT LCDs has steadily
grown in recent years. The first volume market for these displays was
in military avionics, since "glass" cockpits are the current trend and
price was not an issue where weight and volume are a high priority.
These early displays had sizes of about 6" and resolutions between 512
and 800 pixels square. The technology quickly proliferated into laptops,
as Japanese and Korean manufacturers mastered 8-10" 800x600 and
1024x768 resolution displays. We are now seeing rapid growth in the
monitor market, with the wide availability of 10", 12", 13.8", 14",
14.5" and 15" panels offering resolutions of typically 800x600 or
1024x768 pixels with 16-bit colour resolution.
The latest generation of production monitors have sizes
of 16.1" and resolutions of 1280x1024 pixels at 16-bits. We can expect
at least one 21.3" 1600x1280 16-bit colour monitor to enter volume
production this year, with several manufacturers claiming 20-21" class
products in the pipeline.
Because LCD panels utilise the whole screen size for the
picture, the diagonal size can be deceptive in comparison with CRT
sizes. A 14" LCD provides similar viewing area to a 16-17" CRT monitor.
Therefore a 21.3" inch LCD is closer in size to a 24" CRT.
I had the pleasure recently of using a 14" LCD monitor
and can happily state that the assertions of the vendor community are
mostly correct, since the device provided picture quality comparable to
a top end CRT, and much better picture sharpness and geometry. At
roughly three times the cost of a CRT, it is expensive viewing but for
many applications where space, weight, power consumption and eye
fatigue are an issue, arguably a justifiable expense.
The OEM AM TFT LCD monitor typically uses standard
analogue VGA/SVGA/XGA/SXGA 15 pin I/O, dissipates 30-50 Watts and is a
direct drop in replacement for a CRT monitor, with the caveat that
speeds and resolutions will be restricted in comparison with a
multisync CRT. That aside, the only impediment to wider market
penetration is and will continue to be cost.
The TFT LCD Monitor is here to stay, and without any
doubt the best is yet to come.
Click here for more ...