6.2. Video systems − monitors, video cards, devices of video data processing

Basic units of the video system of PC there are a monitor and video card. Monitor, or display, − device for information representationon on the screen of computer. Exactly through a monitor all visual information is perceived from a computer. A health and first of all sight of operator depend on quality and safety of monitor.

Computer monitors in the process of operation use three base colors: red (R), green (G), blue (B) (fig. 6.2). For their denotation apply abbreviation RGB. A monitor with the complete range of colors will have 256 gradations of every primary color, id est it can recreate 256 х 256 х 256 = 16 700 000 colors (such monitor is called "fullcoloured").

In modern PC use three basic types of monitors: СRТ (Сathode Ray Тиbе) − monitors with cathode or electron-ray tubes; LCD (Lіquid Сrystal Dispiay); РDР (Рlаsmа Dispiay Рапеl).

A major element of СRТ - monitor is kinescope (fig. 6.3) which is yet called an electron-ray tube (ERT). A kinescope consists of vacuum Fig. 6.2. Distribution of primary hermetic glass tube. One of colours on computer monitor ends of tube narrow and long − it is a mouth, and other − wide and sufficiently flat − it is a screen. From a frontal side the inside part of glass tube is covered by luminophor – substance, which radiates the light under its borbadment by charged particles.

Fig. 6.3. Kinescope: (1 − glass; 2 − luminophor coverage; 3 – fastener; 4 – clamp)

For creation of image in coloured СRТ-monitor they use three electron guns, from where three electron flows go out accordingly. Every gun answers one of primary colors and produces the beam of electrons under the action of the strong electrostatic field. On a path to the forehand of tube the flows of electrons pass through the intensity modulator and system of acceleration, where they get considerable enough energy, a part of which is spent on luminescence of luminophor.

Distinguish ERT with the delta-shaped and planar placing of electron guns. ERT with the planar placing name also tubes with erection of rays, as action of magnetic-field of Earth on the planar placed ray is almost identical, that is why in case of tube position change relative to the field of Earth it is not needed to do its additional adjustings.

The brightness of certain color is determined by intensity which falls on that or other point of ray. For the correction of electron beams (that they got on the necessary point of luminofor) use a special mask. Through a metallic mask or grates the electron beams get on the internal surface of glass screen of monitor, which is covered by varicoloured luminophor points. Thus they get on the different particles of luminophor, which shine primary colors with different intensity and as a result an image is formed with a neccesary color.

It is needed for the receipt of the high-quality coloured image, that ray, intended for red luminophor elements, not influences on the luminophor of green or blue color and vice versa. As electron guns are on certain distance one from other, then the angles of arrival of electron beams differ insignificantly, that enables create mask so that a necessary ray got on the corresponding point of luminophor, and other rays were closed by a mask. In the kinescopes certain types of masks are used (fig. 6.4).

Shadow mask. In this technology the coloured element consists Fig. 6.4. Types of masks in monitors: a − ERT with a shadow mask; b − ERT with aperture grates; 1 − mask; 2 − luminophors; 3÷5 − accordingly green, red and blue rays; 6 − strings

of three grains, placed in the tops of correct triangle. The sharpness of image is determined by distance between the geometric centers of neighbours of elements. A shadow mask consists of metallic net (usually from the alloy of iron and nickel) before part of glass tube with luminophor layer. This net provides hit of electrons on certain luminophor element. Minimum distance between the luminophor elements of identical color is called dot ріtch, which is the important parameter of image quality. The dot pitch is measured in millimetres. The less value of dot pitch, the higher quality of the recreated image. So, in the model of Ніtасhі СМ77І the step of shadow mask presents a 0,22 mm for horizontals and 0,14 mm − for vertical lines.

Slot Mask. Luminophor elements are placed in vertical elliptic cells, and a mask is made out of verticals. Minimum distance between two cells is called a slot pitch. A slot mask is applied in monitors company NЕС (under the name "Сrоmа Сlеаr") and monitors of firm Раnаsоnіс with the tube Рurе Flat. A firm LG in the monitors uses the flat slot tube Flаtrоn with a step 0,24 mm.

Ареrture Grill contain the series of strings, that consist of luminophor elements, built as vertical stripes of three primary colors. Such system provides the high contrasty of image and proper saturation of colors. Mask which is set in tubes of firm Sony (Міtsubishi, View Sonic), is thin foil which vertical thin lines are plotted on. Basic advantage of this technology it is the saturated colors gamma. In addition, screen in monitors is less convex, than in monitors with shadow slot masks.

The electron flow can deviate in vertical and horizontal planes, that provides a successive its hit on all field of screen. The beam deflection takes place due to deflector system.

Every pixel which is used for the generation of image in a point has certain co-ordinates on the plane of monitor screen. It is however impossible to compare the size of step in the tubes of different types. So, the step of points in tubes with a shadow mask is measured bias, while step of the aperture grate − on horizontals. Therefore at the identical step of points a tube with a shadow mask has a less density of points, than tube with an aperture grate. For example, step a 0,25 mm in an aperture grate almost equivalent to 0,27 mm − in a shadow.

Operational principle of LCD-monitors (Liquid Сrystal Dіsрlау) is based on the polarization phenomenon of light flow. Screens of monitors are made of a substance which is in the liquid state and has properties, typical for crystalline bodies. The molecules of liquid crystals (Liquid Сrystal) can change the orientation under the action of electrons, owing to properties of light ray change also, that passes through them.

LCD - monitor has a few layers, where a basic role is carried out by two panels (they are made of very clean glass material which sodium is withdrawn from), that contain a thin layer of liquid crystals between themselves. On panels there are grooves which send crystals, giving a special orientation to them. Grooves are placed so, that they are parallel on every panel, but perpendicular between two panels. Longitudinal gooves appear as a result of placing on the glass surface of thin tapes of transparent plastic which is then processed. Impacting with gooves, molecules in liquid crystals are oriented identically in all cells.

Turn of polarization plane of light ray is unnoticeable for the eye, that is why there was a necessity to add to the glass panels two other layers which are polarization filters. So, filters transmit a that light beam, which polarization axis answers set only. Therefore at passing of polarizator a light beam will be weak depending on a angle between its polarization plane and axis of polarizator.

In default of voltage a cell is transparent, as the first polarizator skips only light with the corresponding vector of polarization. Due to liquid crystals the vector of light polarization returns, and to the moment of beam passing to second polarizator it is already returned so, that passes through second polarizator without obstacles (fig. 6.5, а).

At presence of electric field the turn of polarization vector occures on a less angle, that is why second polarizator becomes only partly transparent for a radiation. If potentials difference will be such, that the turn of polarization plane does not take place in liquid crystals, then the light ray is consumed by the second polarizator, and screen at illumination behind or at the front it will Fig. 6.5. Operation principle of LCD-monitor seem black (fig. 6.5, b)

without voltage (a) and under voltage

Thus, a screen of LCD-monitor is an array of little cells − segments (pixels), by which it is possible to manipulate for displaying of information. For the leadingout of color display it is necessary lighting from behind of monitor so that light go out of the back-end of LCD-display. It is needed in order to look an image with the proper quality, even if environment pale.

As well as in traditional ERT tubes the pixel LCD-monitor is formed from three primary colors − RGB. A color appears as a result of use of three filters, which distinguish from a radiation of white light source a three basic components and value changes of certain electrical charge which causes the turn of crystal and change of brightness of corresponding light flow. Combining three primary colors for every point or pixel of screen, possibility of recreation of any color appears.

There are a few types of LCD-monitors:

STN (Super Twisted Nematic) − technology of STN enables to increase the torsion angle (twisting angle) of crystals orientation inside LCD-display from 90 to 270°, that provides the best contrasty of image in the case of monitor size increase;

DSTN (dual-scan twisted nematic) − crystalline screens with dual-scan. In this construction one double-layer DSTN-cell consists of two STN-cells, the molecules of which during operation turn into opposite sides. Light, passing through such construction in the "closed" state, loses greater part of the energy. A contrasty and discrimination are high enough, that is why possibility to make a colour display in which on every pixel are due three cells and three optical filters of primary colors appeared.

ТFТ (thin film transistor) − monitors on thin-film transistors. The thin-film transistor (thick 0,1÷0,01 μm) − it the controlled elements by means of which every pixel is controlled on the screen. A pixel on the basis of ТFТ is arranged so: in a glass plate after each other three coloured filters RGB are placed. Every pixel is combination of three color cells or subpixel elements. ТFТ-screen consists of whole net of such pixels, where operation of certain color area of every pixel is controlled by a separate transistor, for example, in a display which has discrimination of 1280 х 1024, there are 3840 х 1024 transistors and subpixel elements. The size of point (pixel) for a 15,1" display ТFТ (1024 х 768) approximately presents 0,0188 inch (or 0,30 mm), and for 18,1" display ТFТ − about 0,011 inch (or 0,28 mm).

S - ТFТ − monitors Sиреr ТFТ. In technology S-ТFТ use simple metallic electrodes, set on a bottom glass plate, which forces molecules to be revolved, which constantly are in plane, parallel to screen plane. Therefore an image on a display remains bright and clear even at the large viewing angle, arriving quality which is compared to the image on a ERT-screen.

Basic properties and differences of LCD-monitors. Real diagonal size of screen. The visible diagonal size of ERТ-monitor always less, than actual diagonal size of tube, as losses after the scope of monitor present close inch. In ТFТ-panel this area is not, so a diagonal size is such, that and visible diagonal. Thus, panel by a size 15,1" is equivalent to size 17" of ERТ-monitor. All the PC have standard monitors − 15 or 17". For professional work with graphic material use monitors of size 19, 21 or 22".

The contrasty ratio is determined of the values of maximal and minimum values of brightness. On СRТ-monitors this ratio presents 500:1, that enables to get photo-realistic quality of picture. For LCD this ratio has a far fewer value. It especially notedly during displaying of black color. On ERТ-monitor a black is formed simply enough − by the change of level of all colour constituents. For displaying of black a liquid crystals must fully block passing of light. However physically it not possibly and light partly passes through crystals. Presently acceptable values for LCD are 250:1 ÷ 300:1.

Brightness. The maximal brightness of TFT-displays is determined by possibilities of backlight lamp. Therefore to create brightness of 200 ÷ 250 cnd/m² is not problem, although in practice there is not a necessity. Maximal brightness of ERТ-monitor presents 100 ÷ 120 cnd/m². Creating the greater value of brightness is possible, but it negatively will influence on the operation term of luminophors. Presently brightness in ТFТ - and ERТ-monitors almost identical.

Viewing angle. This property is critical almost for all flat-panel displays. The maximal viewing angle equals to an extreme value at which the contrast ratio goes down to 10:1 from the defined value in case of perpendicular position to the screen plane. The serious lack of LCD-monitors is a viewing angle: even in the last models not always it is possible normally to see an image, if to look at a monitor under an angle. However this problem begins to solve: if before a viewing angle for LCD-monitor was about 45°, then presently there are models with the viewing angle to 170°.

Discrimination. Modern monitors provide minimal discrimination of 800 х 600 pixels. Maximal discrimination it is possible to define in СRТ-monitor, dividing the visible size of screen by the step of point, slot step or step of stripe. Yes, maximal discrimination of monitor by a size 17" with a slot mask, by the step of points 0,25 mm and by the screen size 320 х 240 mm presents 1478 х 1109 pixels. It should be noted that СRТ-monitors during operation can use different values of discrimination in the full-screen mode, while LCD-monitors − only one. A less discrimination is possible for them in case of the use of part of screen. So, on a monitor with the discrimination 1024 х 768 at the discrimination 640 х 480 will be used only 66 % of screen. Application of the special functions of "overscan" on all screen results in the considerable diminishing of the clearness and distortion of image.

Inertia. Duration of transfer of pixel from one state in other is called an inertia. СRТ-monitors have the least value of inertia − 10 ms. For LCD-monitors it is one of critical properties, that resulted in a visible delay in the process of displaying of animation plots. For modern ТFТ-displays the value of inertia presents 20 ÷ 30 ms. For example, for the normal revision of video it is needed to display 25 frames by 1 s, id est every frame can be reflected no more than 40 ms. Thus, ТFТ-displays assist the revision of videoplots on the whole.

Refresh rate. Frequency of reiteration of frames is called refresh rate. The eye of man see change of images as the movable image then, when frequency of image change not below, than 20 ÷ 25 Hz. However for operation with a monitor at a short distance to screen it is not enough of such frequencies, as the higher frequency of frames, the more steady image. For СRТ-monitors duration of glow of luminophor elements is small enough, that is why an electronic ray must pass through every element of luminophor layer often enough, that the noticeable twinkling of image was not. If frequency of screen bypass less, than 70 Hz, then to the inertia of visual perception it will be not enough in order to get unwinking image, that is why minimum safe frequency of frames is considered 75 Hz. Twinkling of image (flicker) resuins in the fatigue of eyes, headache and even to the visual impairment. The brightness of separate element of screen of LCD-monitor remains not-variable during all time between renewals of the pictures. The greater screen of monitor, the more noticeable twinkling, especially by peripheral (lateral) sight, as a visual angle of image increases. During work with LCD-monitor loading on eyes considerably less, id est the even plane of screen and absence of twinkling is designated. It is the lack of LCD-monitors, as a frequent change of emage, for example during viewing of films, influences on the inertia of pixels switching from one state into other, that results in distortion of image. Refresh rate of LCD-monitors not below, than 20÷50 Hz, that is why for implementation of the multimedia applied tasks it is better to use СRТ-monitors.

Beam convergence. In LCD-monitors every pixel is placed in the fixed matrix and switched separately, that is why there are no problems with beam convergence, unlike СRТ-monitors, where it is needed faultless operation of electron guns. Thus LCD-monitors provide the ideal clearness of image. So, the picture of 17-inch ТFТ-monitor at the discrimination of 1280 х 1024 excels in a clearness an image even of 19-inch СRТ-monitor, for which such mode is considered optimal.

Electromagnetic radiation. Variable electromagnetic fields which are created by the power module and all electric chart, in LCD-monitor radiates the same as СRТ-monitor. However LCD-monitor has a zero permanent potential of display, id est does not create round itself far more harmful permanent electrostatic potential.

Efficiency. LCD-monitor has power 15 ÷ 30 W, whereas 17-inch СRТ-monitor − 80÷130 W.

Overall dimension. LCD-monitors does not have a kinescope, instead of them compact LCD-matrix is used, that is why it is possible to make maximally compact monitors. If in ordinary СRТ –monitor a thickness equals to the screen diagonal approximately, then liquid-crystal monitors thin enough, their thickness reaches 5÷10 cm. For example, one 15-inch СRТ-monitor weighs 15 kg, and LCD-monitor in thick a 15 cm together with support − less than 6 kg

Absence of reflexes. Refleciton factor of light from the surface of LCD-monitor in three and more times less, than from the surface of kinescope with the most perfect presently coverage of antireflection − Sony FD Trinitron, Mitsubishi Diamondtron NF.

Front-end interface. By the method of transfer of signals between a video card and monitor distinguish digital and analog monitors. In digit monitors the color signal of RGB is passed in the discrete (digital) state by a separate conductor. Analog monitors operate with the video cards of standards VGA, SVGA and other They are capable to keep up discrimination of 640x480 pixels and anymore. In these monitors a signal is passed as a result of voltage change. An analog interface is used in all modern СRТ-monitors, there is a digital interface DVI (Digital Visual Interface) in LCD-monitors and plasma monitors. However majority of LCD-monitors have built-in analog interface for connecting to the most widespread analog outputss of video adapters. The interface of DVI foresees possibility of transfer, except digital data, analog signals in СRТ-monitors.

Pixel errors. There are "dead points" on some LCD-monitors. It takes place as a result of transistor defects, id est a certain transistor can not control a light flow. It always blocks light, or always skips it. According to a standard, a presence to five "dead pixels" on new LCD-monitor is assumed.

Turn of screen. It should be noted and such feature of some LCD-monitors, as possibility of screen turn on 90° with the simultaneous automatic turn of image. So, presently the sheet of format А4 can be fully placed on the screen, without a necessity to use the vertical scrolling to see all text on a page.

Operation principle of РDР (Рlasma Display Panel) of plasma monitors (fig. 6.6) consists in the guided cold discharging of rarefied gas which is in the ionized state (cold plasma). Plasma screens make, filling space between two surfaces, by rare gas, for example by an argon or neon. By a operating element (pixel) which forms a certain point of image, there is a group of three subpixels. Every subpixel – it is separate micro-camera on the walls of which there is a fluorescence substance of one of basic colors. Pixels are in cross points of transparent electrodes which create a rectangular net.

In order that luminescence of pixel took place a controlled variable voltage of rectangular form is given at two ortogonal to each other electrodes in the point of which crossing there is a necessary pixel. Under its action in a gaseous medium an electric discharge arises which radiates light in an ultraviolet range, which predetermines luminescence of luminophor particles in a range, visible a man. The brightness of luminescence of luminophor is determined by the size of the controlleded voltage.

Fig. 6.6. Operation principle of РDР – plasma monitors :

1 − camear with coverage; 2 − external glass; 3 − electrode of feed; 4, 8 − layer of dielectric; 5 − protective layer; 6 − scanning electrode; 7 − dividing partitions; 9 − internal glass; 10 − electrodes of control

High brightness (to 500 knd/m²) and contrasty (to 400:1) in default of shaking − basic advantages of plasma monitors. In addition, the high clearness of image is kept on all working surface of screen.

So, angle relative to a normal, under which an image is visible on plasma monitors considerably more, than in LCD-monitors. Besides a plasma panels dont create the magnetic fields which are the guarantee of harmless influence on the state of health, and small duration of regeneration enables to use them for displaying video- and telesignal.

It should be noted high firmness of РDР-monitors to the electromagnetic fields, that enables to use them in industrial conditions, for example a powerful magnet, placed next to a display, does not influence on quality of image.

Therefore plasma technology of new generation ideally does for creation of large screens. It enables to produce flat and easy monitors in thick an about 90 mm. Therefore, in spite of large screen, they can be set in any place for information transfer.

The basic lacks of monitors of such type is high enough consumed power, which grows at the increase of monitor diagonal and subzero discrimination, predefined by the largeness of picture element. In addition, properties of luminophor elements quickly become worse, as a result a screen becomes less bright (the term of exploitation of plasma monitors presents 100 000 hours, id est about 5 years).

Video card, or video adapter, controls directly a monitor. It carries out the role of device which converts a digital signal, id est picture, created by a processor, on analog videosignals (mainly RGB) which displaies on the monitor of computer. Structurally the video card it is a pay which is set in a slot for expansion of РСІ or АGР.

To properties of video adapters such parameters belong:

• capacity of videomemory;

• bit of video adapter;

• supported discrimination in pixels for horizontal and vertically;

• type of bus;

• operation modes (text, graphic);

• maximal amount of colors;

• maximal frequencies of string and frame scanning;

• possibility of the programmatic loading of character generator.

A modern video card has such basic constituents: videomemory; set of microcircuits (videochipset); Video BIOS; RАМ - RАМDАС (RАМ Digital Analoque Converter); sockets for connecting to the bus and peripheral devices.

Videomemory is the first constituent of video card. A basic purpose of videomemory is temporal storage of picture which hatches on screen of monitor. Every picture has a certain volume which is measured in bytes (it also typical and for an image on the screen). Thus, the greater capacity of memory, the best divisions and depth of color on a monitor.

For video cards without the graphic accelerating use the same models of dynamic memory, that and for main memory, or memory model, specially worked out for storage of videodata. Presently apply SDRAM and DDR SDRAM, frequency of which not less, than frequency of chip and presents 125 ÷ 300 Mhz. Most popular video cards with the capacity of videomemory 32, 64 and 128 Mb. The feature of the memory use in video cards is 128 - or 256-bit busses of data between memory and graphic processor which enables quickly to transfer the large volumes of data.

Videochipset − it is a set of microcircuits of the video system which determines facilities of video cards. A standard is not on composition and parameters of components of this set of microcircuits, that is why the sets of different producers of video cards differ substantially, that affects functional facilities and quality of image processing. Presently these microcircuits it is possible to join in one − picture processor.

In some video adapters the special accelerators were entered − fasters and graphic processors. As well as any microprocessor, graphic chip has certain parameters: internal frequency presents 120 ÷ 300 Mhz. As well as central processor unit, it is possible to overclock videochip.

Majority of videochips is made by technology 0,25 or 0,18 μm, but already began to make chips by technology of 0,13 μm.

Chip is connected with the local videomemory by an internal bus. Its width is an important speed parameter and usually coincides with the width of interface of chip and memory. In majority of chips this parameter presents 128 ÷ 256 bits, and of the low cheaps − 64 bits.

Distinguish fasters of double- (2D) and three-dimensional (3D) graphic. Faster of two-dimensional graphic − this videochip, that can hardwarily accelerate a processing of "flat primitives", intended for imaging of flat images, for example graphic interface Windows and others like that. Faster of three-dimensional graphic can create a construction in the two-dimensional plane of screen projections of some three-dimensional objects, creating the illusion of three-dimensional image thus.

One of important parts of video processor − RАМDАС (digital-analogue converter), which desplays content of frame buffer on a screen. Frequency of RАМDАС specifies on that, what maximal discrimination and what frequency of frame scanning a video adapter can operate at. Therefore quality of image depends on possibilities of RАМDАС (frequency, bit and others like that).

On a video card there is video BIOS: the main memory, in which the screen fonts, service tables are writtenin and others like that. It should be noted that BIOS is not used by videocontroller directly − a central processor appeals only to it. BIOS is needed only for the primary start of adapter and operation in the mode of МS DOS. Operating systems with graphic interface (Windows or OS/2) does not use BIOS for the adapter control − they control it by means of drivers.

In addition, on a video card usually one or a few disconnectors are placed for intraconnections, one of which is called Feature Connector and it is used for a grant to the peripheral devices of access to the videomemory and image. For example, it is possible to connect a television set, hardwear decoder МРЕG, device of fade-in and others like that.

One of important parameters there is a attached interface, id est what bus a video adapter is connected to. A firm Іntel developed the bus АGР (Ассеlerated Graphics Port), which, essentially, is a separate channel between a video-adapter and by computer memory. This firm embeds the standard АGР Рrо, which assumes possibility of process control of maps feed with a large energy consumption.

Modern video adapters have about 16,7 million colors and even anymore. The discrimination of Suреr VGA presents 800 х 600 pixels and anymore, id est supports the text and graphic modes regardless of colors amount. Usually for a video card they specify maximal discrimination and frequency of regeneration, for example, 2048 х 1536 pixels and 75Hz. These two characteristics must answer to properties of monitor.

Operation principle of video adapter is based on that central processor of computer forms an image (frame) as an array of data and writes down it into videomemory, in particular frame buffer. After it part of videochip, which is called a graphic controller, consistently, bit by bit, line by line, reads out content of frame buffer and transfers it into RAMDAC. It, in turn, forms analog RGВ-signal, that together with the clock signals is passed on a monitor. Scanning of videomemory takes place synchronously with moving of ray on the screen of monitor.