Any hardware device that the computer uses to present information to the user can be categorised as an output device. Kinds of information might be sound data memory images
Obvious examples are the monitor and the printer.
3 Monitors / Visual Display Units
The monitor that shows a user what is going on with a computer. Older ones look a bit like a television screen modern monitors look flat these are Liquid Crystal Display (LCD) or Thin Film Transistor (TFT) displays.
4 CRT Monitors
The screens that look like small televisions are CRT monitors. Cathode Ray Tubes (CRT) consist of a vacuum tube enclosed in glass. One end of the tube contains an electron gun the other end contains a screen with a phosphorous coating.
When heated the electron gun emits a stream of high-speed electrons that are attracted to the other end of the tube.
5 CRT Monitors (2)
The light-streams will hit a point on the phosphor coating momentarily and cause the phosphorous molecules to vibrate giving off light.
The phosphor chemical has a quality called persistence and the rate of persistence indicates how long this glow will remain on-screen.
6 CRT Monitors (3)
In colour monitors the mix of red green and blue light will determine the colour emitted in the light of the phosphor glow.
The electron beam sweeps the screen from left to right in lines from top to bottom in a pattern called a raster.
7 CRT Monitors (4)
The horizontal scan rate refers to the speed at which the electron beam moves across the screen.
The electron beam must continue to sweep the screen to maintain an image - a practice called redrawing or refreshing the screen.
You should have a good match between persistence and scanning frequency so that the image has less flicker (if the persistence is too low) and no ghosts (if the persistence is too high).
8 CRT Monitors (5)
Most CRT displays have a refresh rate of about 70 hertz (Hz) refreshed 70 times a second.
A basic depiction of a CRT raster 9 CRT Monitors (6)
The scan rates expected by your monitor should match those produced by your video card.
Most CRT monitors are multiple-frequency monitors with a variety of popular video signal standards.
10 CRT Monitors (7) Pixels and raster lines for a Cathode Ray Tube 11 LCD Screens
LCD (liquid-crystal display) displays have low-glare flat screens and low power requirements than CRTs. (5 watts versus nearly 100 watts for an ordinary CRT monitor.)
There are three basic LCD choices passive-matrix monochrome passive-matrix colour and active-matrix colour.
12 LCD Screens (2)
In an LCD a polarising filter creates two separate light waves. In a colour LCD there is an additional filter that has three cells per each pixel - one each for displaying red green and blue.
The light wave passes through a liquid-crystal cell with each colour segment having its own cell.
13 LCD Screens (3)
The liquid crystals are rod-shaped molecules that flow like a liquid. They enable light to pass straight through but an electrical charge alters their orientation changing the orientation of the light passing through them.
14 LCD Screens (4)
The diagrams below are simplified 3-D cross-sections of a LCD display cell
Light coming out See them animated at http//plc.cwru.edu/tutorial/ enhanced/files/lcd/tn/tn.HTM 15 LCD Screens (5)
In a passive-matrix LCD each cell is controlled by electrical charges transmitted by transistors according to row and column positions on the screens edge.
In an active-matrix LCD each cell has its own transistor to charge it and twist the light wave.
This provides a brighter image than passive-matrix displays because the cell can maintain a constant rather than momentary charge.
16 LCD Screens (6)
The best colour displays are active-matrix or thin-film transistor (TFT) panels in which each pixel is controlled by three transistors (for red green and blue).
Active-matrix-screen refreshes and redraws are immediate and accurate with much less ghosting and blurring than in passive-matrix LCDs (which control pixels via rows and columns of transistors along the edges of the screen).
17 LCD Screens (7)
Active-matrix displays are also much brighter and can easily be read at an angle.
An alternative to LCD screens is gas-plasma technology typically known for its black and orange screens in some of the older Toshiba notebook computers but have developed as clear colour monitors since then.
18 The Light of an LCD Monitor
Most computer Liquid Crystal Displays are lit with built-in fluorescent tubes above beside and sometimes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. This is known as a backlight.
19 The Light of an LCD Monitor (2)
A typical laptop display uses a tiny Cold Cathode Fluorescent Lamp (CCFL) for the backlight. One of these small tubes is able to provide a bright white light source that can be diffused by the panel behind the LCD.
20 The Light of an LCD Monitor (3)
The fluorescent light lights up the cells of an LCD display. The moving molecules of the cells are aligned by electrical current applied to the cells transistors to filter or bend light so that colours and brightness are detected by the human eye.
21 Video Cards
A video card is a circuit board that fits to the motherboard to support the monitor by providing signals that operate your monitor. Most video cards follow one of several industry standards
MDA (Monochrome Display Adapter)
VGA (Video Graphics Array)
CGA (Colour Graphics Adapter)
SVGA (Super VGA)
EGA (Enhanced Graphics Adapter)
XGA (eXtended Graphics Array)
IBMs PS/2 systems appeared first around 1987.
It introduced the Video Graphics Array (VGA) display.
The technology for the very popular VGA had/has particular hardware to ensure reliability and versatility.
23 VGA (2)
On a motherboard VGA is implemented by a single custom VLSI chip. (Very Large Scale Integration.)
The VGA BIOS (Basic Input/Output System) is the control software residing in the system ROM for controlling VGA circuits. (NOT related to the BIOS chip associated with the startup procedure of a personal computer.)
24 VGA (3)
This software can initiate commands and functions able to display up to 256 colours on screen from a palette of 262144 (256K) colours.
Because the VGA outputs an analog signal you must have a monitor that accepts an analog input.
SVGA offers different resolutions but 1024 x 768 resolution is described as a resolution capable of detailed work. (If working with graphics.)
Most microcomputer monitors support at least one video standard.
The IBM EGA (Enhanced Graphics Array) appeared first around 1984.
It was a colour monitor displaying 16 colours in a range of either 320 x 200 or 640 x 200 pixels.
27 Back to VGA Briefly
Unlike earlier video standards which are digital the VGA is an analog system.
Why are displays going from digital to analog when most other electronic systems are going digital
Why then did IBM decide to change the video to analog The answer is colour.
Digital display generates different colours by firing the red green blue (RGB) electron beams in on-or-off mode.
28 Back to VGA Briefly(2)
You can display up to eight colours (2 to the third power).
Analog displays as digital use RGB (Red Green Blue) electron beams to construct various colours but each colour in the analog display system can be displayed at varying levels of intensity - 64 levels in the case of the VGA.
29 Back to VGA Briefly(3)
This versatility provides 262144 possible colours.
For realistic computer graphics colour is often more important than high resolution - because the human eye perceives a picture that has more colours as being more realistic.
A printer is a device that accepts text and graphic output from a computer and transfers the information to paper card plastic or photo-sensitive paper.
Printers vary in size speed sophistication and cost. In general more expensive printers are used for higher-resolution colour printing.
31 Printers (2)
Computer printers are usually of two types
impact printers or
32 Impact Printers
Impact printers typically have a hammer or a dot-matrix that strikes paper through a ribbon containing ink.
These two examples are old technology though dot-matrix is still often used by many organisations for such things as sample data output or computer program prints.
33 Impact Printers (2)
Dot-matrix printers were cheap to buy and operate for banks and hospitals so that is where you often still see them.
An OKI Dot Matrix Printer (_at_1980s()) 34 Non-Impact Printers
Non-impact printers examples are the laser printer and the inkjet printer.
The laser allows toner (an ink powder) to stick to paper as it rolls past a drum.
The inkjet sprays ink from an ink cartridge at very close range to the paper as it rolls by.
35 Printing on a Laser Printer
When a computer sends the data of a print job to a laser printer it is routed through a central controller a small computer inside the printer which manages the printer.
The controller may place several jobs at once into a queue then printing them.
When the controller has determined what is going to be printed the process of preparing the printing drum begins.
36 Printing on a Laser Printer (2)
The drum inside a laser printer can hold an electric charge. Close to the drum is a transfer corona roller or wire which can negatively or positively charge the drum. In most laser printers the drum starts out positively charged although this process can also work in reverse. The controller manipulates a small laser reflected from a mirror to write on the drum with a negative charge creating an electrostatic image. This charge causes powdery ink (toner) to be attacted to selected paper areas as a sheet rolls over the drum.
37 Printing on a Laser Printer (3)
The paper that is fed through the printer is given an even stronger negative charge by the transfer corona wire before being rolled past the drum. The electrostatic image on the drum will transfer to the paper.
Then it is fed through a fuser which heats the toner and causes it to bind with the fibres in the paper.
38 Printing on a Laser Printer (4)
The laser receives the page data - the tiny dots that make up the text and images - one horizontal line at a time. As the beam moves across the drum the laser emits a pulse of light for every dot to be printed and no pulse for every dot of empty space.
The laser does not actually move the beam itself. It bounces the beam off a movable mirror instead.
39 Printing on a Laser Printer (5)
Meanwhile the drum passes a discharge lamp which will expose the entire surface of the drum and erase the electrostatic image.
The transfer corona wire applies another positive charge and the printer is ready for the next page or job.
40 Inkjet Printers (Bubblejet)
With the thermal bubble or bubblejet resistors create heat which then creates a bubble in the ink. The bubble expands and forces ink out from the nozzle. Eventually it will collapse drawing more ink into the cartridge. On average a bubblejet printer will have a range of three hundred to six hundred nozzles.
41 Inkjet Printers (Piezoelectric)
The piezoelectric utilizes small crystals in the nozzles which will vibrate under the influence of an electric current this in turn pushes ink out and draws more ink into the cartridge. The drops of ink that come from the piezoelectric type printer are significantly smaller than those of the bubblejet printers allowing for greater control over the image quality.
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