Output devices

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Output devices 

An output device is any piece of computer hardware equipment used to communicate the results of data processing carried out by an information processing system (such as a computer) which converts the electronically generated information into human-readable form.

Classification of output devices 

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classification of output devices

Soft copy devices

Soft copy refers to intangible output that can be seen or heard i.e. screen display or sound. Examples include: monitors, LCD projectors, and speakers.


A monitor or display (also called screen or visual display unit (VDU) is an electronic peripheral device used to display information in the form of text, pictures and video, enabling the user to monitor what is going on in the computer. Examples of monitors include: CRT-cathode ray tube, LCD-liquid crystal display, GPD- gas plasma display.

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CRT monitor

A cathode ray tube or CRT

CRT is traditionally used in most computer monitors and the advent of plasma screens, LCD, DLP, OLED displays, and other technologies. As a result of CRT technology, computer monitors continue to be referred to as "The Tube". A CRT works by moving an electron beam back and forth across the back of the screen. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. By drawing many such lines from the top to the bottom of the screen, it creates an entire screenful of images.

 A Liquid crystal display (LCD)

LCD is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It uses very small amounts of electric power, and is therefore suitable for use in battery-powered electronic devices. Example: TFT-thin film transistor  
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LCD Screen
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A plasma display 

A plasma display is an emissive flat panel display where light is created by phosphors, excited by a plasma discharge between two flat panels of glass. The gas discharge contains no mercury a mixture of noble gases (neon and xenon) is used instead. This gas mixture is inert and entirely harmless. The glass panels seem to be vacuum sealed, because when they are broken the plasma breaks up, seemingly from the addition of air to the space.
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Plasma Display

Surface-conduction electron-emitter display (SED)  

SED is a flat-panel, high-resolution display. Some SEDs have a diagonal measurement exceeding one meter (approximately 40 inches). 
The SED consists of an array of electron emitters and a layer of phosphor, separated by a small space from which all the air has been evacuated. Each electron emitter represents one pixel. The SED requires no electron-beam focusing, and operates at a much lower voltage than a CRT. The brightness and contrast compare favorably with high-end CRTs. Prototype electron emitters have been developed with diameters of a few nanometers. SED technology can offer unprecedented image resolution.
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Surface-conduction electron-emitter display (SED) 

Digital Light Processing (DLP) 

is a technology used in projectors and video projectors. In DLP projectors, the image is created by microscopically small mirrors laid out in a matrix on a semiconductor chip, known as a Digital Micro mirror Device (DMD). Each mirror represents one pixel in the projected image. The number of mirrors corresponds to the resolution of the projected image: 800x600, 1024x768, 1280x720, and 1920x1080 (HDTV) matrices are some common DMD sizes. These mirrors can be repositioned rapidly to reflect light either through the lens or on to a heat sink.
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Digital Light Processor

An organic light-emitting diode (OLED) 

 is a thin-film light-emitting diode(LED) in which the emissive layer is an organic compound OLED technology is intended primarily as picture elements in practical display devices. These devices promise to be much less costly to fabricate than traditional LCD displays. When the emissive electroluminescent layer is polymeric, varying amounts of OLEDs can be deposited in rows and columns on a screen using simple "printing" methods to create a graphical color display, for use as computer displays, portable system screens, and in advertising and information board applications. OLED may also be used in lighting devices. OLEDs are available as distributed sources while the inorganic LEDs are point sources of light.
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 Common names used with screen display

  • Pixel- stands for picture elements. These are tiny dots which form images displayed on the screen.
  • Color depth-in computer graphics, color depth or bit depth is the number of bits used to indicate the color of a single pixel in a bit mapped image or video frame buffer.
  • Resolution-this is the number of pixels per inch on the screen usually measured in dots per inch (dpi) or bits. The higher the resolution, the more the number of pixels per square inch, hence clearer the images.
  • Refresh rate-The refresh rate (most commonly the "vertical refresh rate", "vertical scan rate" for CRTs) is the number of times in a second that a display hardware draws the data. If a screen has a low refresh rate, images tend to flicker hence causing eye strain.
  • Display size- is the measure in inches as the diagonal length of the screen measured from top right to bottom left.
  • Direct X- This is a software that enhances the multimedia capabilities of your computer. Direct X provides access to the capabilities of your display and audio cards, which enables programs to provide realistic three-dimensional (3-D) graphics and immerse music and audio effects.

 Video Graphic Adapters (VGA) or Video Card

It is the standard monitor or display interface used in most PCs. Therefore, if a monitor is VGA-compatible, it should work with most new computers. The VGA standard was originally developed by IBM in 1987 and allowed for a display resolution of 640x480 pixels. Since then, many revisions of the standard have been introduced. The most common is Super VGA (SVGA), which allows for resolutions greater than 640x480, such as 800x600 or 1024x768. A standard VGA connection has 15 pins and is shaped like a trapezoid.
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Video Graphic Adapter

 Examples of video adapters include:

  • Color Graphic Adapter (CGA)-display text and images up to 16 colors
  • Enhanced Graphic Adapter (EGA)- an improvement of CGA but also displays in 16 colors
  • Video Graphic Array(VGA)-display text, graphics and video using 256 colors
  • Super Video Graphic Array(SVGA)- displays text and graphics using more than 16 million colors, has a minimum resolution of 800x 600 pixels
  • Extended Graphic Array (XGA)- has a resolution of up to 1024 x 1280 pixels and is popular with 17 and 19 inch monitors.
  • Super Extended Graphic Array(SXGA)-has a resolution of 1280 x 1024 pixels and is popular with 19 and 21 inch monitors.
  • Ultra Extended Graphic Array(UXGA)- is the latest and highest standard

Sound output devices

Produce sounds such as peeps, audio or digital. Examples include: speakers. Further, the sounds from the computer can be heard from the built-in case speaker, or the speakers which are plugged into the sound card. 
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Sound Output

Light emitting diodes (LED)

These are indicators that display light when electric current is passed through them. They are used to give warnings the same way a motorist would use signals to indicate when he/she is over taken or taking a turn.
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Light Emitting Diode

 Hard copy output devices

 Hardcopy refers to tangible output that can be felt such as paper. Examples include: printers, plotters and facsimile (fax).


Printers are used to produce information on a piece of paper. Printers are classified according to the way their printing mechanism/technology and also according to their speed of printing.

Classification according to printing mechanism 

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Classification According to Printing  Mechanism

 Impact printer

They print using striking mechanism. They strike on a piece of paper in order to form an imprint on it.Examples  

Dot matrix printers 

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dot matrix

A dot matrix printer or impact matrix printer is a type of computer printer with a print head that runs back and forth, or in an up and down motion, on the page and prints by impact, striking an ink-soaked cloth ribbon against the paper, much like the print mechanism on a typewriter.

 Daisy wheel printers
This is an impact printing technology invented in 1969 by David S. Lee at Diablo Data Systems. It uses interchangeable pre-formed type elements, each with typically 96 glyphs (an element of writing: an individual mark on a written medium that contributes to the meaning of what is written.), to generate high-quality output comparable to premium typewriters such as the IBM Selectric, but two to three times faster. Daisy wheel printing was used in electronic typewriters, word processors and computers from 1972. According to Webster's, the daisy wheel is so named because of its resemblance to the daisy flower.
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daisy wheel printer

Non-impact printers

These printers are faster and quieter than the impact printers. They print using ink, thermal or laser mechanisms. Examples:

 Inkjet printers- Inkjet printer is a type of computer printer that creates a digital image by propelling droplets of ink onto paper.
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Thermal printer/direct thermal printer is a digital printing process which produces a printed image by selectively heating coated thermochromic paper, or thermal paper as it is commonly known, when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, producing an image. Two-color direct thermal printers can print both black and an additional color (often red) by applying heat at two different temperatures. They are mostly found in super markets to produce receipts.

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thermal printer

Laser printers - Laser printers have excellent print quality, low noise levels, high speed and the ability to print both graphics and text. Computer sends the data/image information to a processor within the laser printer which has photo-sensitive components which causes negatively charged ions to be produced which acts as hold-ups for ink drops. Light is then reflected, with the use of mirrors for accuracy and precision, onto where toner will be applied. This allows the toner to stick to the precise positions on the paper. 

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laser printer

Photo printer- Different from inkjet and laser printers in their printing mechanism. Whilst inkjet printers use a painting method for printing, thermal printers use heat to imprint the image on paper.
Thermal photo printers use stencils or color panels called dye panels. These dye panels only have one color each, so for every print, there are four dye panels used: black, cyan, yellow, and magenta. The printing process involves individual dye sublimation on paper. For instance, when a photo is currently being printed, the color panels will be individually imprinted on the paper.

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photo printer

Impact vs. non-impact printers

 Use inked ribbons
Use thermal, photo and electrostatic principles 
Multiple copy production possible
Multiple copy production impossible

Classification of printers according to the speed of printing 

kcse computer studies notes
classification of printers according to speed of printing

Character printers- provide one character at a time and are hence comparatively slow and less costly than the line or page printers.
Line printers- provide one whole line of print at a time. Hence they are more expensive than the character printers.

Page printers- page printers provide one whole page of print at a time, hence faster than both line and character printers, relatively more expensive and produce high quality printouts.

Factors to consider when purchasing a printer

  • Hardware cost-this is the initial cost of buying a printer. I.e. desk jets are cheaper than laser jets.
  • Running cost- this involves maintenance costs i.e. consider the cost of buying cartridges and toners and also the printing mechanisms. 
  • Software and networking features- is your printer compatible with the features of your computer. Most modern printers require higher memory and a higher computer speed.
  • Printing Speed – which kind of printer do you want? Character, Line or page printers? If you handle large volumes of data? Then a page printer will serve you better
  • Printing quality and reliability- presentable work and reliability are vital for the general output of a printer. Check whether it supports different paper sizes
  • User needs- user’s expectations and needs are vital in any printer purchase i.e. card printers, photo printers, receipt printers, publishing etc.
  • Availability of running materials- do some feasibility study on the availability of running materials in the market for they should be readily available.


The plotter is a computer printer for printing vector graphics. In the past, plotters were used in applications such as computer-aided design, th7ough they have generally been replaced with wide-format conventional printers. Pen plotters print by moving a pen or other instrument across the surface of a piece of paper. This means that plotters are restricted to line art, rather than raster graphics as with other printers.
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 Exercise Questions

  1. What are the main differences between a daisy wheel printer and a laser printer
  2. Differentiate between impact and non-impact printers
  3. What are video adapters? And what are they used for?
  4. Draw and label the surface of a disk platter of the hard disk
  5. Differentiate between a flash disk and a memory card
  6. List the three types of soft copy output
  7. Discuss the four types of registers
  8. What are platters? What are they used for?
  9. List six factors to consider when purchasing a printer
  10. List any four types of flat display monitors
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