Introduction Most basic component of a Personal Computer for interactivity & displays. Monochrome VDU’s Greyscale VDU’s Colour VDU’s Adapter Card and Cable
Creation of Pictures Whole screen is composed of tiny dots called Pixels made of Phosphor.
Creation of Pictures Pictures are drawn on the screen by making some pixels bright and the remaining dark.
Creation of Pictures The attributes are stored in the display RAM of the adapter card. A picture on a VDU screen is refreshed about times per sec.
Cathode Ray Tube
Cathode Ray Tube
Cathode Ray Tube A fine beam of electrons aimed at a phosphor coated screen creates a glowing pixel. The beam is accelerated by high potential differences and the Accelerating Electrode. The beam is focussed by the Focussing Electrode. Brightness of the dot is controlled by varying beam current. With the help of deflection coils the beam can be positioned anywhere on the screen.
Colour CRT Colour monitors display combination of red, green and blue each in an independent intensity Each pixel consists of three dots: red, blue and green (triad). Each of the three beams should hit the corresponding dot but not in between the triads.
Colour CRT To ensure that beams strike only the triads, a shadow mask is used. The shadow mask is made of Invar (64% iron & 36% nickel) which does not change its dimensions when heated.
Colour CRT In a colour monitor if one has a two-level (on-off) intensity control for each colour, there can be a total of 8 possible colour shades. R R R R G B X B G X X X white magenta yellow red X X X X G B X B G X X X cyan blue green black
Scanning With the help of deflection coils the beam is swept from left to right and back, and then top to bottom and back to create a pattern called Raster During the hori- zontal and verti- cal retraces the beam current is switched off.
Scanning The horizontal and vertical scanning is strictly maintained at constant frequencies by the adapter with the help of synchronisation pulses. In mono monitors, the adapter sends two sync pulses and an intensity signal. In colour monitors three intensity signals are sent along with two sync pulses.
Multisync Monitors Monitors capable of syncing to video signals within a range of frequencies are called Multisync Monitors. Outside this range the images may be unclear or the monitor itself may get damaged.
Interlacing Each frame is split into two parts consisting of odd and even lines. The first frame is displayed and then the second frame is displayed shifted so that its lines fit between the lines of the first frame. This succeeds in lowering the frame rate without increasing the flicker.
Monitor Specs Horizontal Scan Rate: No. of horizontal lines displayed by monitor in one sec. and controlled by the horizontal sync signal generated by adapt. Vertical Scan Rate: No. of frames displayed by monitor in one sec. at a given pixel addr. and controlled by the vertical sync signal by adapter. If we consider a vertical refresh rate of 60 Hz and a screen of 800 lines per screen then horizontal refresh rate is calculated : 800 lines X 60 times in 1 sec. i.e lines in 1 sec. i.e. 48kHz
Monitor Specs Dot Pitch: Shortest distance between the centers of two neighbouring dot triads. Measures the resolution of the monitor. Usually 0.25 to 0.4 mm. Pixel Addressability: Total no. of pixels that can be addressed in the video frame buffer, ie. (Horizontal-pixels)X(Vertical-pixels) Aspect Ratio: Ratio of length and breadth of the viewable area. Normally 4:3 for monitors and 16:9 for movie theatres. Size: The longest diagonal length of the monitor in inches.
Monitor Specs Colour Depth: It is the no. of discrete intensities that the video card is capable of generating for each colour, which determines the maximum no. of colours that can be displayed. This depends on the total no. of RGB bits used to generate a pixel. With a colour depth of 8 bits we can have a total of 2^8=256 colours. An improvement of 8 bit colour is called High Colour whereby 15 or 16 bits are used to attain or colours simultaneously. 24-bit colour addressing is called True Colour whereby 16.7 million colours can be displayed.
Monitor Specs Relation between pixel addr., aspect ratio and monitor screen size: Pixel Aspect ratio Monitor Size Addressability 14” 15” 17” 20” 640X480 4:3 O O G G 800X600 4:3 A O O G 1024X768 4:3 NR A O O O=Optimal, G=Grainy, A=Acceptable, NR=Not recommended
The VGA Adapter The Video Graphics Array is a standard established by IBM. Needs a VGA Adapter and a compatible monitor.
The VGA Adapter The VGA adapter contains several components which includes: Display Memory: Used to store screen display data where the CPU writes the attributes of each pixel and is later used by the adapter to generate RGB signals. Graphics controller: Can perform logical functions on data being written to display memory. RAMDAC: For converting digital signals of the CPU to analog signal for the monitor.
VGA - Display RAM Video cards always have a certain amount of RAM. This RAM is also called the frame buffer. Two RAM features are significant: How much RAM? That is significant for colour depth at the highest resolutions. Which type of RAM? This is significant for card speed. Video card RAM is necessary to keep the entire screen image in memory. The CPU sends its data to the video card. The video processor forms a picture of the screen image and stores it in the frame buffer. This picture is a large bit map.
VGA - Display RAM Amount of RAM The amount of RAM required depends on the no. of pixels and the colour depth: V. RAM=(H-addr)X(V-addr)X(C-dep)/8 For 640X480 pixels with 8 bit colour 640X480X8/8 = bytes ~ 1 MB For 1024X768 pixels with 24 bit color 1024X768X24/8= bytes~ 2 MB
VGA - Display RAM Type of RAM The high end cards use VRAM ( Video RAM). This is a RAM type, which is only used on video cards. A VRAM cell is made up of two ordinary RAM cells, which are "glued" together. VRAM also costs twice as much. The advantage is, that the double cell allows the video processor to simultaneously read old and write new data on the same RAM address. Thus, VRAM has two gates which can be active at the same time. Therefore, it works significantly faster.
VGA - Controller Chip The video card provides a support function for the CPU. It is a processor like the CPU. However it is especially designed to control screen images
VGA - Controller Chip Flat Video Cards The original VGA cards were said to be "flat." They were unintelligent. They received signals and data from the CPU and forwarded them to the screen, nothing else. The CPU had to make all necessary calculations to create the screen image. The solution to this problem was the accelerator video card, which appeared in the early nineties. All newer cards are accelerated and today they are connected to the CPU through high speed buses like PCI and AGP.
VGA - Controller Chip Accelerated Video Cards With accelerated video cards, Windows (and with that the CPU) need not calculate and design the entire bit map from image to image. The video card is programmed to draw lines, windows, and other image elements. This saves the CPU a lot of work in creating screen images. All video cards are connected to the PCI bus The AGP bus is an improved version of the PCI bus
VGA - RAMDAC All graphics cards have a RAMDAC chip converting the signals from digital to analog form. Traditionally monitors work on analog signals. The PC works with digitized data which are sent to the graphics adapter. Before these signals are sent to the monitor they have to be converted into analog output and this is processed in the RAMDAC:
VGA Modes Mode Type Resolution Chars Colours (Hex) 0,1 text 360x400 40x ,3 text 720x400 80x ,5 gfx 320x200 40x gfx 640x200 80x text 720x400 80x25 mono D gfx 320x200 40x25 16
Super VGA Anything better than 640X480 and 16 colours is an SVGA mode. Video Electronic Standard Association or VESA defined a std. interface for SVGA called VESA VGA BIOS Extensions determine what video modes are available and how video memory is accessed.
Super VGA A VESA video driver can integrate a VESA compliant software and a proprietary SVGA hardware. The video modes defined by VESA are: Mode # Pixel Addressability Colours 100h 640x h 640x h 800x h 800x h 1024x h 1024x h 1280x h 1280x
Video Display Standard Monochrome Display Adapter (MDA): First Video cards used in the earliest machines established by IBM. Mono text only standard at 80X25 chars. Each char. was 9 dots wide and 14 dots high giving resolution of 720X350 and refresh rate of 50 Hz. Did not support graphics. Obsolete nowadays.
Video Display Standard Hercules Graphics Adapter (HGA): A company called Hercules created a MDA- compatible card capable of supporting monochrome graphics and text. Support for the card was included in popular software packages like Lotus for displaying graphs and charts.
Video Display Standard Colour Graphics Adapter (CGA): IBM’s CGA standard supported different modes: text at 80X25 chars. in 16 colours and graphics ranging from monochrome at 640X200 to 16 colours at 160X200. Refreshes at 60 Hz. Obsolete nowadays having being replaced by EGA.
Video Display Standard Enhanced Graphics Adapter (EGA): IBM’s next standard was EGA which allowed graphical output of 16 colours at 640X350 and 80X25 text at 16 colours, all at a refresh rate of 60Hz. EGA are the minimum requirement for Windows 3.x. Obsolete nowadays being replaced by VGA.
Video Display Standard Video Graphics Adapter (VGA): IBM’s latest and most widely accepted standard. Supports 16 colours at 640X480 or 256 colours at 320X200. Older displays sent digital signals to monitor while VGA uses analog signals to allow for more colours. Now replaced by SVGA.
Video Display Standard Super VGA and Ultra VGA: Anything better than 640X480 at 16 colours is SVGA. Sometimes SVGA is referred as 800X600 and UVGA as 1024X768. Set of standards for SVGA laid down by VESA called VESA Bios Extensions. IBM followed VGA by XGA for its proprietary MCA bus.
Accelerated Graphics Port (AGP)
AGP - Overview A new bus was introduced in It is called AGP (Accelerated Graphics Port), and it is exclusively designed for video cards. AGP was designed with two purposes: To relieve the PCI bus of work with graphics data. To have better bandwidth. AGP was introduced with the Pentium II processor and Intel 82440LX chip set. However both Ali and VIA soon introduced chip sets for Socket 7 motherboards including AGP. So today AGP is found on most new motherboard.
AGP - Overview The Accelerated Graphics Port (AGP) interface is a platform bus specification that enables high performance graphics capabilities, especially 3D. Fast floating-point performance smoothens the drawing of 3D meshes and animation effects and adds depth complexity to the scene. AGP technology accelerates graphics performance by providing a dedicated high-speed port for the movement of large blocks of 3D data between the PC's graphics controller and system memory.
AGP -Technology AGP includes several techniques: A kind of clock doubling mode, where the bandwidth is expanded to about 530 MB/sec. Possibility to utilize system board RAM for texture cache. A technology that expands the memory used by the graphics card, utilizing the ordinary RAM in the PC. This technology is called DIME by Intel (for Direct Memory Execute).
AGP - Reference
Digital Graphics Card Flat LCD monitors works on a purely digital input a digital graphics adapter, where the RAMDAC isn't used. There is a RAMDAC on the card since it is capable of controlling a traditional CRT monitor simultaneously with the LCD screen. The LCD monitor is digital in its nature, and it should receive the pure digital signal from the graphics processor. That way the image becomes as perfect as possible - extremely sharp!
Digital Graphics Card ATI has launched the Xpert LCD adapter. It gives three functions in one: A 15-pin analog SVGA connector for CRTs. A video output connector for televisions. A 20-pin connector for the digital monitor.
Digital Graphics Card
Liquid Crystal Display
LCD-General Principle Liquid crystals are transparent organic compounds with long rod-like molecules which in their natural state align parallel to each other. It is possible to control the alignment of the molecules of the LC by flowing them over finely grooved surface. Light in passing through LC’s follow the alignment of the molecules.
LCD-General Principle On passing electric current through the liquid crystal their molecular alignments can be altered and consequently the way light passes through them. Bright backlighting is required since liquid crystals do not generate light, they can only block it.
LCD-General Principle A layer of LC is sandwiched between two grooved surfaces with grooves perpendicular to each other. Light follows the molecular alignment and also twisted by 90 deg.
LCD-General Principle Natural light waves are oriented at random angles. A Polarising filter act as a set of parallel lines blocking all light waves apart from those oriented parallel to those lines. A second filter with lines at 90 deg. To the first would therefore totally block this already polarised light.
LCD-General Principle LCD’s consist of two polarising filters perpendicular to each other with a layer of twisted liquid crystal in between. Light passing through first filter is twisted by liquid crystal and passes out completely through second filter.
LCD-General Principle On applying an electric voltage across the liquid crystal molecules realign vertically allowing no light to pass through the second filter. The LCD then appears dark. Coloured LCD’s use additional red, green and blue filters over 3 separate LCD elements to create a multi-coloured pixel.
Types of LCD’s Liquid Crystal Displays are usually used in laptops and are expensive because of low yield (~65%). LCD’s can be of different types: Passive Matrix LCD’s Dual Scan LCD’s Active Matrix LCD’s Ferroelectric LCD’s
Passive Matrix LCD Rows of pixels are activated sequentially by activating the row transistors while the appropriate column transistors are activated. Thus a given row is activated for a short time during screen refresh resulting in poor contrast. PM’s are relatively inexpensive.
Passive Matrix LCD
Dual Scan LCD Contrast in a PM LCD is improved by spliting the screen in two parts, top and bottom and refreshing them indipendently. Dual scan displays are thus better than original PM LCD’s but they do not have the brightness and contrast of an active matrix LCD display.
Active Matrix LCD Instead of using one transistor for each row and column, AM LCD’s use one transistor for each pixel. Pixels can be activated more frequently giving better contrast and control over modulation. AM can produce higher resolution displays that can produce more colours but are more expensive.
Active Matrix LCD
Ferroelectric LCD FE LCD’s use a special type of liquid crystal which holds its polarization after being charged. This reduces the required refresh rate and flicker. FE LCD’s have faster response time of 100ns compared to ms for PM LCD’s.
Comparing CRT & LCD
Comparing CRT & LCD LCD monitors are based on a very thin screen as opposed to the bulky tube of a CRT monitor. Thus a 12” LCD takes a third of the desk space than a 14” CRT monitor.
Comparing CRT & LCD Viewing angle problems on LCD’s occur as it is a transmissive system which works by modulating the light which passes through the display while CRT’s are emissive
Plasma Display Panel PDP’s use a layer of gas sandwiched between two glass plates. Row and column electrodes run across the plates. By activating a given row and column the gas at the intersection is ionized giving off light. The type of gas determines the colour. PDP’s have excellent brightness.
Conclusion Future of VDU’s: Touchscreen panel HD monitors Reference Web sites: webopedia.internet.com plc.cwru.edu