PC and peripherical IT course YBET

Ecran cathodique et LCD

PC and periphericals Hardware training  

16. Cathodic and flat monitor (TFT, LCD - LED and Plasma) technologies.

1. Technologies of the flat panel displays - 2. DSTN - 3. TFT - 4. LCD - 5. LED - 6. Plasma - 7. transflectifs 's monitors - 8. Cathode monitor - 9. Comparison of technologies.

Two types of screens (monitors) are currently used: cathode tube (CRT) and liquid crystals (derived TFT, LCD and LED) but also plasma and OLED (based on a different technology). Let us begin with the total characteristics.

First is the size (dimension). We can forget old the 11 ", 12 ", 14 " and 15 " to arrive at the current standard: 17 ", in spite of the weak price difference with screens 19 ". These dimensions correspond to the distance between 2 corners opposed in diagonal. Out of screen LCD, it is the real size, in the old cathodic ones, the tube is partly hidden, reducing the effective size of posting. Old monitor use a report/ratio 4:3, now with the Wide format (also used for notebooks) take again a 16:10 report/ratio, as in television.

Normalizes Horizontal resolution X vertical resolution
VGA 640 X 480
SVGA 800 X 600
XVGA 1024 X 768
SXGA 1280 X 1024
HDTV 1920 X 1080
HDTV more 1920 X 1200
QXGA 2048 X 1536

First caracteristic is the maximum resolution, the number of points displayed. All the current monitors are minimum SVGA. The maximum resolution represents the maximum number of points which it can post. It indicates the number of pixels (points) displayable to the screen following the width and the height. Out of screen TFT and LCD, the resolution to be used is that maximum. As an inferior, the quality of the image is reduced. Into cathodic, the maximum resolution is seldom used, the image becoming often fuzzy or even scintillates.

On the other hand, the number of posted colors depends mainly on the graphics card (in particular of memory RAM and its configuration. The old graphics cards did not accept all the palettes according to the resolution, the current models yes but with losses of performances for the high-resolutions. The human eye decrees only 16 million colors (14 bits).

The vertical frequency (of line) in Hz gives the rafaishing frequency of each line of the screen. Located between 30 and 100 Hz with higher frequencies for the big sizes. The graphics board has also a maximum sampling rate related to the RAMDAC, if the vertical frequency of the screen is lower than that of the graphics card, the screen takes down in frequency, requiring a manual parameter setting on the level of the configuration of the graphics card.

A another characteristic is the frequency image which varies with the resolution. Expressed in Hz, it represents the number of times that the image is posted a second. The characteristics minima are of 72 hertz to avoid flutters of the image, tiring with long. More this frequency is high, better is posting. This explains also the problems with the very old interlaced screens, disappeared end of the year 90.

Display consists of a multitude of points, the number depends on the resolution. The smoothness of the contour of these points is called dot pitch or not mask. More it is weak, better is the quality of the image. If a standard screen is characterized by a dot pitch of 0,28 mm, the best goes down to 0,22 Misters the dot pitch depends on the type of cathode tube (and the type of grid), on technology used for the LCD and the size of the screen. The dot pitch represents the distance separating two points from different color in the same pixel.

Monitors use standards of energy saving (extinction of the screen by the computer after a certain time of inactivity) like the DPMS and others. Other standards manage the radiation (emission) electromagnetic armature by the cathode tube, they are initially the standards LR (Low Radiation) like the DPR 2, followed, mainly under the impulse of NOKIA and the Swedish trade unions, by the TCO92 and TCO95 which integrates in more the DPMS and of the standards of recycling. The TCO99 integrates new standards of energy saving. The last in date, Energy Star takes again different output but energy management, also. It is adapted to the electricals appliance but also to the buildings.  The majority of the standards of radiation do not have any more a raison d'être with the flat panel displays.

Controls (luminosity, contrast, shift of the image,…) are currently done in a digital way, using a menu on the screen.

1. Flat panel displays

This monitors do not use any more one tube, but matrices of liquid crystals, a little as if one inserted a LED in each point to be posted. This reduces the thickness of the screen (design) but also consumption related to the high voltage used in old technology.

The principal distinctions between technologies are the response time (in ms), the rate of contrast between the black and the white between two contiguous points (the best allows until 1000:1), the angles of vision horizontal and vertical (until 170°), the luminosity (expressed in candela per square meter (300 cd/m ² for example), the maximum frequency of transfer (typically 75 Hz) and the dot pitch.

The second difference is the type of signal used. Into cathodic, the signal analogical, therefore is subjected to electromagnetic external interferences. In a flat panel display, the signal used is numerical (even if the current screens still often use connector VGA requiring of retransformer the analogical one numerically). Approximately, the signal is converted by the graphics card into analogical signal via the ramdac to be retransformé numerically in the screen. Cable DVI is numerical, from where interest to connect your LCD via this cable.

In the flat panel displays, one finds 4 technologies (only the two last are used in TV):

2. DSTN, passive matrix.

The DSTN (Dual Scan Twisted Neumatic) lights only points located at the crossing of a line and a column (from where the mention of matrix) the ones after the others. Each point consists of liquid crystals by point which according to the swing angle makes pass the light of a luminous flagstone through three filters towards the screen. Liquid crystals are directly managed by an electrical signal coming from the inverter. This technology does not use transistors for control, reducing the response times and contrast of posting but with a weaker cost price. They do not allow either important angles of vision. Various firms have to try to improve it like Toshiba and Sharp with FastScan HPD (Hybrid Passive Display) which offers better response times (150 ms instead of 300 and one higher contrast of 40:1, for 30:1 for standard technology and the HPA (High Performance Adressing), similar to the HPD but developed by Hitachi.

These technologies are not used any more.

3. TFT, active matrix.

An active matrix screen TFT (Twin Film Transistor) also uses a dot matrix of liquid crystals with a retro-lighting (generally called the flagstone). Each points uses three crystals (one by color) and each crystal is managed by a transistor. The light is sent starting from a luminous flagstone through a polarizing filter before reaching the crystal matrix. While passing through a second polarizing filter, it is reversed with 90° and passes in a third filter before reaching the screen.

This technology allows an excellent contrast of the image (from 150 to 200: 1) and of the higher response times (25 to 50 ms).

4. LCD monitors and TV.

In the case of the TFT, liquid crystals are clearly dissociated. The LCD is similar but integrates the electrodes in two glass plates in which they are completely drowned. Between the two, one lays out a crystalline liquid similar to liquid crystals above but definitely more compact. It is also the difference in tension which will modify the alignment of the whole of the liquid but each electrode is managed by a transistor, which improves the response time. The large advantage compared to preceding technology is a contrast even higher (typically 1000:1), the increase the visual angle and… of the luminosity. It is the technology used currently for the standard screens and the television of small sizes.

For the remainder, operation is identical to the two preceding ones: 

Operation of a screen LCDThe case integrates a succession of superimposed layers. The first integrates a backlight which clarifies the surface of the screen uniformly. It is fed by a electronic boad called Inverter. This component converts the 12 volts DC into a tension of approximately 1000 Volts AC. In addition to the converter includes a piezo (quartz) ensures the frequency of the clock. It is often this bord which breaks down on screens LCD: image becomes dark. To check if it is well the inverter, use a flashlight to light the screen.

Then, one places on all the surface of posting a first polarizing filter followed by liquid crystals (TFT) or a crystalline liquid (LCD), composed of sticks. In the absence of electric charge, the crystals are folded up on them-even and prevent the light from passing. Between these 2 layers are a network of transistor (TFT) or roasts electrode (LCD) which control the position of the crystals electrically. A simple electric impulse and the stick is rectified, allowing the passage of the light. Each pixel is associated with 3 sticks (1 per color), each one controlled by its own transistor or electrode. The maximum resolution of the screen is function of pixels, that is to say number of associated transistors. For a LCD 15 " with a resolution of 1024*768, 2.539.296 transistors and crystals are used. These screens are black at rest.

5. LED technology

Three techniques are currently used:

  1. Almost identical to that of the LCD, only the backligh consists of LED

  2. Lighting is done while inserting LED on with dimensions one and either into the back: only advantage, reduction thickness of the screen. On the other hand, as the light is not direct, a loss of output.

  3. OLED, completely different, which uses neither of flagstone, nor from liquid crystals.  Each point receives a LED with triple entered (one by basic color). While exploiting the tension applied to each terminal, one lights each color more or less.

These three technologies have the advantage of consuming less than standard technologies LCD while allowing (for the third) posting of big size. The OLED probably in the short run will replace Plasma, too expensive to produce.

6. Plasma.

Last technology used for the screens, plasma, mainly for very great dimensions, in particular televisions. The principle uses a gas made up of xenon 10% and Argon 90% which emits a light when it is subjected to an electric tension. Each pixel gathers three cells (one by basic color), themselves containing a plasma of this ionized gas. Each point is illuminated in turn. Each cell allows 256 levels of lighting following the tension applied but produces only invisible light in the range of the ultra-violet. They are the luminophores associated with each cells which produce the three ranges of visible lights. The number of possible colors per point is of 2563, that is to say 16 million alternatives, the maximum distinction of the eye.

This technology is not mainly used for televisions and screens from 35 ". Compared to the LCD, it is more expensive, consumes more (even if consumption depends on the image: not in black and important in white), one weaker lifespan has (from 30 to 50.000 hours for 50 to 60.000 in LCD) and is sensitive to the burns of pixels when the image remains static. However, contrast is even higher and the speed response is also higher. In any event, the LCD is limited to sizes of screen lower than 50 ". The difference in choice between a LCD and a plasma depends finally on the size on posting.

7. Transflectif monitor

The main issue of these bill-posters is posting in strongly lit mediums, used for example for postings out of window and outsides. For all these external applications, the solution is to increase the luminosity in full day and to decrease it when it makes sinks, but this technique has limits.

The solution used currently is the following one. In a standard flat panel display, the light outside (here in yellow) goes enlightened the liquid crystals part reducing the luminosity of the image. In a transflectif screen, one modifies the part “piloting LCD” while letting pass the light of outside (thus not of reverberation), then, a special filter is inserted between the luminous flagstone and the liquid crystals part which will return the light towards outside, increasing the luminosity.

This technique is used for PDA'scerre but also for installations of very big sizes. The luminosity passes from 200 to 300 cd/m ² to 1200 - 1500 cd/m ². This technology can be coupled with tactile technologies.

8. Cathodic

Diagram of operation of a cathode tube Out of cathode ray tube , two types are used: fixed frequencies (completely obsolete since 2000) and the analogical ones. The screens at fixed frequencies allow discrete values (generally three different fixed frequencies per resolution). The analog models detect all the frequencies lower than the maximum frequency to be parameterized on that of the graphics card. During a change of frequencies into analogical, the screen dies out a few seconds, time to find the adequate frequency.

The principle of operation of a cathode tube is identical to that of Televisions. The monitor contains an electron gun which produces an electron beam (1) projected on the flagstone (the posted part of the image) through an electromagnetic mechanism of positioning of the electron on the flagstone. It consists of two deflectors high voltage, vertical and horizontal (2 and 3 on the diagram). The end of the tube is covered according to the type of tube with a grid bored with holes (Shadow Mask) or tended wire (Trinitron) (4) which makes it possible to post only at one point given for a pixel of the screen. Once passed, the electron reaches the visible face of the tube covered with a luminophore layer following the three basic colors: red, green and blue. With the contact, the luminophore excites and produces a luminous flow. For a correct posting without losses, a luminophore must be regenerated every 13.33 ms, that is to say 75 times a second (75 Hertz), which corresponds to the frequency image of the screen seen higher

As the image is separate in three colors, the electron gun must send the electrons according to 3 filters. The trinitron use three guns directly. Each beam associated with a color is sent on a zone clean of the screen in the same pixel, sufficiently close relations for a juxtaposition and sufficiently distant not to interfere between them.

Three technologies (were) are used:

For the large screens, again standard of Invar Mask are used allowing qualities of posting identical, if not higher than Trinitron.

Flat panel displays CRT use a flat cathode tube. Visible surface is thus punt. In addition to the comfort of work, their precision of posting, coupled to an adapted cathode tube, make them perfect for many tasks: drawing, computer-aided design or final improvements of images, mainly for long work hours. The screens square corners post the image on the entirety of the tube, thus increasing the maximum size of posting.

Certain models are treated anti-reflecting and anti-static. This is done by recovering the cathode tube of a special film.

9. Comparison of technologies

9.1. Screens CRT with the TFT and LCD

The 2 types are characterized by the size from posting. The cathode screens are indicated by the dimension of the tube. The size of posting is thus lower than that announced. The table above shows approximately the correspondence of the differences in sizes.

Diagonal of flat panel display Diagonal of cathode screen
13,3 " 15 "
14,1 " 16 "
15,1 " 17 "
18,1 " 20 "
20 " 23 "

Even if technology TFT allows a better contrast than the DSTN, the cathode screens are generally higher (but that evolves/moves). They also make it possible to modify the resolution without loss of quality, the flat panel displays have an imposed resolution.

Except for LCD and plasma of high-end, returned colors is also higher for the standard CRT. This explains why the computer graphics experts still often use the latter.

On the other hand, a screen LCD has many advantages like the obstruction or the design. A last reason to use these screens is related to their lower consumption. In 2008, the majority of the manufacturers completely gave up the cathode tube screens.

9.2. Comparison of flat technologies.

One will forget the TFT and DSTN, obsolete, to concentrate on the LCD, various technologies LED and Plasma.

The most widespread technology is the LCD (the first two technologies LED are only adaptations). As favours, a cost price (thus of sale) weaker. The first technology LED which replaces finally the flagstone by LED with the advantage of consuming less but for a price of manufacture (for the moment) higher. This solution is mainly used for the laptops of high-end and some Netbook. The principal defect of these two technologies is related to the maximum size of posting (even if one can always put them one beside the other, technology used by Matrox with special graphics cards).

For large surfaces of posting (with share vidéoprojecteurs), the OLED and plasma are currently identical but the consumption of the second coupled with technological advances of components LED rendrent them almost obsolete in spite of currently higher contrasts. It is especially on the level of the very big sizes that the LED takes its take-off for the advertising displays. Several manufacturers have sleep and already stopped the manufacture of plasmas.

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