TFT Guide - Part 1 - Flat Panel Displays - THG.RU

Introduction

The current development of the market for flat panel displays will almost certainly remind some vendors of bygone days when profit margins and demand were still attractive. A rapid rise in demand, lacking investments in production capacities and persistently high production reject rates lead to an ideal situation for vendors. A potential customer is obliged to pay a lot for a flat panel display in order to save desktop space and energy. However, this is a situation that will only prevail for a limited time as the market changes its direction and prices are subject to the usual dynamic market forces.

Part 1 of the TFT Guide provides you with an overview of the market situation, prices and trends concerning the TFT market. Newcomers and professionals alike will find something useful in this article. We will cover topics such as functionality, the most important characteristics of flat panel displays and technologies in detail here. The article is rounded off with useful tips for buyers.

Parts 2 and 3 are dedicated to the technically-minded readers. We’ll report on current technologies used to increase the viewing angle, the latest digital interfaces (DFP and DVI) and on the relationship of pixel spacing and the maximum possible diagonal dimensions of a display.

At a later point in time we will report on the most important companies in the field and will showcase various models. We’ll keep an eye on prices and will report on changes as they occur.

The Market Situation

The immense success of notebooks gave the development of flat panel displays an unsurpassed boost. Obviously, this also had an effect on normal flat panel displays. Flat panel displays (often referred to as TFTs or LC displays) have been a hot topic of discussion above all in Europe and Japan. This presence in the media is rather surprising, as the sales volume of devices for 1998 was very far from that of CRTs. But so what? – when you take into consideration that the market leader for CRTs sold more than that in just one week. On the other hand, there is an enormous demand for TFTs and one that can currently not be met. That means that the situation concerning TFTs is tense, such as has become rare in the PC market and something that would normally be resolved quickly. Well, things are different for the TFT market and several explanations for the sparse supply include the low availability of glass, the restricted production volumes of the vendors and the unwillingness of the manufacturers to lose money by investing in what they see as a high risk business. The fact remains that the majority of TFTs are used in a business environment, especially where desktop space really is critical, or where noise levels, heat dissipation and health factors play an important role.

A number of requirements have to be met before TFTs can become successful in the home users segment:

Prices must be at the same level as CRTs
Minimum size 15″ with a resolution of 1024 x 768 pixels
Availability
Standardized interfaces for digital TFTs
Quality and functional compatibility for ALL applications

Manufacturing and Yields

The construction and manufacturing of active matrix TFTs is just as complicated as the way in which they work. Many different materials and very thin layers of glass are used, and that on a diagonal surface of up to 30 inch! The rejection rates for production are correspondingly high. Extremely thin transistors are applied to the devices which control its color display elements during later operation. Above all, the spacing between the layers must be absolutely correct. As in the wafer and chip manufacturing business, the production of TFTs is related to a specific yield. Although manufacturers have very different opinions on this topic, it remains a fact that smaller models are easier to manufacture and their rejection rate is lower. The manufacturers are therefore all confronted with the common problem of either producing larger TFTs for flat panel displays or smaller ones for notebooks and other application fields. Larger models mean higher profits but at the price of lower yields whereas the quantities for notebooks are much higher, with a higher yield. Currently – in the second quarter of 1999 – both fields are in short supply – notebooks are apparently being produced and then stocked while the manufacturers are constantly waiting for TFTs in order to be able to finish and supply the machines and the demand for 15″ flat panel displays (and larger devices) can not be completely met.

Current Price Situation and Price Trends

Prices range between twice to three times those of CRTs. This means that a 15.1 inch LC display (corresponding to a 17″ tube display) cost around $850 and $1,300. An 18.1 inch TFT (corresponding to a 21″ tube display) will cost between $2,800 and $3,500.

A short-term negative trend became apparent in the first part of 1999 where prices rose slightly. Many flat panel manufacturers increased their prices for 15″ panels from $500 to $600. This development stands in contrast to the general trend of the IT market but the high demand makes this pricing policy currently possible.

It’s somewhat curious that without the burden of the poor financial situation of the manufacturers, 15″ panels could literally cost about $80. In theory, flat panel displays could then cost less than CRTs. However, drastic changes aren’t likely to occur until capacities are reduced or the primary demand for displays shifts away from notebook production.

What is a TFT? – Getting to know the Technology

Modern display technologies are currently classified as either cathode ray tube monitors (CRTs) or flat panel displays. Tube devices are large and take up a lot of space, flat panel displays – i.e. devices without a tube – as the name states, are flat and space-saving. The flat panel display category itself encompasses a number of very different technologies such as LCDs (Liquid Crystal Displays), plasma displays, LEDs (Light Emitting Diode) and various other devices. Within these technologies, one can distinguish between flat panel displays that emit light and those that use back light that passes through them.

We will discuss those flat panel displays that – from the current point of view – seem to be the most purposeful; so-called TFT-LCDs. These devices belong to the group of displays that use back light passing through them. STN and DSTN (passive matrix LCDs) are also used, but nowadays only in very low-priced notebooks.

Figure 1: Overview of the different flat panel display technologies. Active matrix LCD’s have prevailed on the market.

How TFTs Work

TFT stands for ‘Thin Film Transistor’ and describes the control elements that actively control the individual pixels. For this reason, one speaks of so-called ‘active matrix TFTs’. How are images produced? The basic principle is quite simple: a panel with many pixels is used whereby each pixel can emit any color. To this purpose, a back light is used which is normally comprised of a number of flourescent tubes. In order to light a single pixel, all that needs to be done is for a small ‘door’ or ‘shutter’ to open to let the light pass through. The technology that makes this possible is of course more complicated and involved than the simple explanation above. LCD (Liquid Crystal Display) stands for monitors that are based on liquid crystals. Liquid crystals can change their molecular structure and therefore allow varying levels of light to pass through them (or they can block the light). Two polarizer filters, color filters and two alignment layers determine exactly how much light is allowed to pass and which colors are created. The layers are positioned between the two glass panels. A specific voltage is applied to the alignment layer, creating an electric field – which then aligns the liquid crystals. Each dot on the screen (pixel) therefore requires three components, one for red, green and blue – just as for the tubes within cathode ray tube devices.

The most common devices are Twisted Nematic TFTs. The following sections explain the way in which such TFTs work. A number of different technologies obviously exist. These are explained in part 2 – Viewing Angle Technologies.

Figure 2a: How a Standard TFT (Twisted Nematic) Display works

When no voltage is applied, the molecule structures are in their natural state and twisted by 90 degrees. The light emitted by the back light can then pass through the structure.

Figure 2b: How a Standard TFT (twisted nematic) works

If a voltage is applied, i.e. an electric field is created, the liquid crystals are twisted so that they are vertically aligned. The polarized light is then absorbed by the second polarizer. Light can therefore not leave the TFT display at this location.

Architecture of a TFT Pixel

The color filters for red, green and blue are integrated on to the glass substrate next to each other. Each pixel (dot) is comprised of three of these color cells or sub-pixel elements. This means that with a resolution of 1280 x 1024 pixels, exactly 3840 x 1024 transistors and pixel elements exist. The dot or pixel pitch for a 15.1 inch TFT (1024 x 768 pixels) is about 0.0188 inch (or 0.30 mm) and for an 18.1 inch TFT (1280 x 1024 pixels) it’s about 0.011 inch (or 0.28 mm).

Figure 4: Pixels of a TFT. The left upper corner of a cell incorporates a Thin Film Transistor. Color filters allow the cells to change their RGB basic colors.

The pixels are decisive and the smaller their spacing, the higher the maximum possible resolution. However, TFTs are also subject to physical limitations due to the maximum display area. With a diagonal of 15 inch (or about 38 cm) and a dot pitch of 0.0117 inch (0.297 mm), it makes little sense to have a resolution of 1280 x 1024. Part 4 of this report covers the relationship between dot pitch and diagonal dimensions in more detail.

What causes the unpleasant Scaling Errors?

Pixels are in a fixed location and therefore define the resolution of a TFT without any geometrical problems. In other words: the maximum number of pixels corresponds to the maximum resolution. But what about lower resolutions? What happens if you have to switch to a lower resolution as is often necessary for games, video playback and other applications? In this case it is important that the electronics scale the ‘smaller’ image up to the size of the maximum size of the display panel. If the circuitry can’t handle this task efficiently, the result will be distorted and not exactly ergonomic. From a technical point of view, this is not as easy to handle as for CRTs.

Why? In the case of CRTs, the electron beam can be adapted to the new resolution by simply changing the deflecting voltages. Besides, it basically doesn’t matter if the beam happens to hit a point between two pixels occasionally. This is quite a different matter in the case of TFTs: due to the active control of every individual pixel, complex scaling electronics are required to recalculate the data for smaller resolutions. With whole number scaling factors (e.g. a factor of 2 when scaling 800 x 600 up to 1600 x 1200) it’s fairly simple: the height and width of each pixels are doubled. The displayed image is correctly shown. Things become harder when scaling from 800 x 600 to 1024 x 768. The scaling factor is then 1.28, i.e. not a whole number (integer). It’s no longer possible to uniquely assign data to a single pixel in every case. The electronics therefore have to decide whether to activate one pixel or two. Mathematical rounding-off errors then lead to unpleasant effects when displaying text (see figure below). State-of-the-art electronic components can reduce this effect using a trick (see Advanced Scaling) in order to reduce the optical impression: if data can’t be uniquely assigned to a pixel, then the pixel’s display intensity is reduced.

Figure 5: Scaling using the character “m”. Scaling factors with fractional numbers often cause visual distortion.

What’s important when evaluating systems? First, an explanation of the most important concepts

Interpretation of the Screen Diagonal Size

The visible diagonal size of a tube monitor is always smaller than the tube’s actual diagonal size. TFT panels however, do not have an edge area. The specified diagonal size is therefore the same as the visible diagonal size. This means for example that a 15.1 inch flat panel display has a visible diagonal that is the same size as a 17″ CRT monitor.

Viewing Angle

This is still a critical point as not every flat panel display has a viewing angle that you will be used to from using a standard CRT monitor. As the light from the back light has to pass through polarizer filters, liquid crystals and the so-called alignment layers, it has a certain directional property, i.e. most of it leaves the monitor with a vertical orientation. If a viewer looks at the display from a steep side angle, it may seem dark or may have color distortion. This effect may be useful for bank cashpoints, but it is otherwise generally unwanted. It took the best part of a year before manufacturers started to implement improved technologies for better viewing angles. IPS (in-plane switching), MVA (multi-domain vertical alignment) and TN+film (twisted nematic and retardation film) are the leading techniques used. They increase the viewing angle to 160 degrees and more, which corresponds to the same value as that which CRTs have. The maximum viewing angle is regarded as the point at which the contrast ratio has dropped to 10:1 of the original best value (i.e. from a viewpoint which is directly vertical to the display surface).

Contrast Ratio

The contrast ratio is derived from the maximum and the minimum values for brightness. The further apart these values are, the better. This doesn’t represent a problem to tube monitors which have contrast ratios in excess of 500:1 and therefore have photo-realistic quality. Conjuring up a black picture on a tube monitor is therefore not too difficult. Again, this is a different matter for TFTs. The brightness of the back light flourescent tubes can hardly be changed and they are always on when the unit is in operation. In order to display a black image, the liquid crystals have to block the light from the back light completely. However, it is physically not possible to do this perfectly – some light will always seep through. The manufacturers are therefore still working on this. Acceptable values for the human eye are values over about 250:1.

Brightness

This is something where TFTs lead. The maximum brightness is principally determined by the flourescent tubes that are used (back light). Brightness values between 200 and 250 Candela per square meter are not a problem. Although technically possible, brighter values are pointless as the display would otherwise practically blind the viewer.

The maximum brightness for CRTs is about 100 to 120 cd/m². Higher values are hardly possible due to the enormous acceleration voltages required by the cathode ray guns, which in turn have negative side effects with regard to higher emission values and reduced phosphor lifetime.

Pixel Errors

These occur due to defective transistors and are seen on the screen as disturbing color points. Due to the inoperable transistor, light is either never passed through a pixel, or it is constantly lit. The disturbing effect is enhanced if the errors occur in groups on areas of the display. Unfortunately, a standard does not exist that defines the maximum number of permissible pixel errors or groups of pixel errors on the display. Each manufacturer has his own definition. Between three and five pixel errors are normal. Buyers to whom this is important should check this when purchasing a machine as the errors occur during manufacturing and can not be repaired. One consolation: the number of such errors will not increase later, provided you don’t press the display surface with your finger or other objects.

Response Time

Many TFTs still have problems with regard to moving (animated) images. The cause is the response time of the liquid crystals. Values between 20 and 30 milliseconds are typical for modern systems. By comparison: a standard movie produces 25 images per second, i.e. a single frame has to be shown for 40 milliseconds. As the liquid crystals are very slow to respond, fast sequences, e.g. the view from a jet fighter flying through a valley or the credit banner of a film appear to be ‘smudged’. However, it would be unfair to say that TFTS are unusable for video playback, the response times are generally sufficient.

Color Quality – Preparing the analog Input Signals

In comparison to digital flat panel displays, those models that are equipped with a standard VGA connector have to translate the analog image signals back into digital form first, and this can lead to a loss in color quality. Some manufacturers insist on using low-performance A/D converters, which can only handle 18 bit values (3 x 6 bits each for red, green and blue). Consequently, only 262,144 colors (pseudo RGB) can be displayed. True Color modes however require at least 16.7 million colors.

Advantages and Disadvantages of TFT Displays

As you’ll almost certainly be familiar with the characteristics of a classical tube monitor, we’d like to emphasize the most important differences between TFTs and CRTs at this point:

TFTs offer very good focus characteristics due to the active control of pixels by transistors. Another advantage compared with CRTS is the absence of geometry and convergence errors due to the technical nature of TFTs. Why don’t TFTs flicker? It’s simple. They don’t use an electron beam that has to scan left-to-right on each line of the screen. The lights are effectively turned off for a short time on CRTs when the electron beam flies back from the bottom right to the top left corner of the display (blanking). In contrast, the pixels of a TFT are never switched off, they simply change their intensity continuously.

The following table summarizes all the most important checkpoints.

Flat Panel Displays (TFTs)

Tube Monitors (CRTs)

Brightness

(+)

170 to 250 cd/m²

(~)

80 to 120 cd/m²

Contrast ratio

(~)

200:1 to 400:1

(+)

350:1 to 700:1

Viewing angle (contrast)

(~)

110 to 170 degrees

(+)

over 150 degrees

Viewing angle (color)

(-)

50 to 125 degrees

(~)

over 120 degrees

Convergence errors

(+)

none

(~)

0.0079 to 0.0118 inch (0,20 to 0,30 mm)

Focus

(+)

very good

(~)

satisfactory to very good

Geometry/linearity errors

(+)

none

(~)

possible

Pixel errors

(-)

up to 8

(+)

none

Input signal

(+)

analog or digital

(~)

only analog

Scaling for different resolutions

(-)

none or by low-performance interpolation methods

(+)

very good

Gamma (color tuning for the human eye)

(~)

satisfactory

(+)

photo realistic

Uniformity

(~)

often brighter at the edges

(~)

often brighter in the center

Color purity/color quality

(~)

good

(+)

high

Flickering

(+)

none

(~)

not visible over 85 Hz

Response time

(-)

20 to 30 msec

(+)

not noticeable

Power consumption

(+)

25 to 40 watts

(-)

60 to 150 watts

Space requirements/weight

(+)

flat design, light weight

(-)

require a lot of space, heavy

Table 1: Comparison between CRTs and TFTs

Legend: (+) positive (~) average, acceptable (-) negative

The ideal TFT – What to consider when buying

Want to buy a flat panel display? The first thing you should do is to consult both the vendor and the manual in order to check that the following requirements are met. You should only consider a purchase if the following requirements are fulfilled:

Brightness	better than 200 cd/m²
Contrast ratio	better than 300 : 1
Pixel errors	under 5
Viewing angle	over 140 degrees

Table 2: The most important purchase considerations

Where’s the future taking us? New Technologies

Two important developments are currently in progress. The first is that the panel manufacturers are working to improve the viewing angle. Parallel to improving standard TFTs (twisted nematic) by implementing a film, some manufacturers are investigating different terrain. What advantages new technologies such as IPS (in-plane switching) and MVA (multi-domain vertical alignment) will really bring us, is discussed in the article on viewing angle technology. The second trend is definitely towards digital control. The second technical article covers digital interfaces in detail.

Summary

Flat panel displays offer excellent focus and sufficient color quality for standard office applications such as word processing and spreadsheet calculations. TFTs also have a lot to offer in terms of ergonomics: less desktop space required, a third of the power consumption of standard tube monitors and of course, lower emission values. TFTs are not suitable for graphics designers who require photorealistic displays. The response time for current models is certainly not ideal for users who mainly play on their PC, whereby video playback, DVD’s and presentations are handled well enough by today’s TFT devices.

Flat panel displays will only find their way into the home when prices fall and of course, when their availability is improved.