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When the computer processes the data (or after the computer finds the solution to the problem), it displays the results (or communicates the solution to whoever posed the question) by use of Output devices.

The term Output is used to describe all that comes out, from the computer memory, or from the processing stage of a data processing system to the external environment.

Output therefore, involves receiving information (processed data) from the computer through a suitable device for external use.

Output devices provides the user with the results from the computer.


  1. Transmit the intermediate results & final results to the users.
  2. Convey messages, e.g. error messages, to the operators.
  3. Provide immediate response to queries/ questions.
  4. They are used when writing onto the secondary storage media.
  5. Accept the results produced by the computer (which are in coded form & hence cannot be easily understood) & convert these coded results to human readable form.

The output produced by computers can be put into 2 broad categories: –

  • Human-readable output, which serves the informational needs of people.  This is a form of output that is readily understandable by human beings, e.g., printed or drawn output.
  • Machine-readable output, which is required/used for subsequent input to the computer.  In this case, the output may be temporarily stored on machine-readable media.

For example,

In the production of a payroll for a company, a payroll report is produced listing the amount each employee must be paid.  This report can then be used by the person responsible for issuing cheques.

At the same time, an updated employee file is produced, say, on a tape.  The tape will be used as input to the computer for the next payroll run.

The quality, validity and usefulness of the output is influenced by the output facility used.  The following factors/ considerations determine the choice of output equipment and media.

  • Suitability of the application.
  • The speed at which the output is required.
  • Whether a printed version is required.
  • The volume of the data.
  • Cost of the method chosen as compared with the benefits to be derived.


Output can further be classified as: –

  1. Hardcopy output.
  2. Softcopy output.


This is where the end results are displayed on a screen.  The user can see the results, but cannot touch them.

The output lasts for a short-time only, i.e., it is available only as long as it appears on the screen.

Examples of Softcopy Output devices.

  • Visual Display Units (VDU) / Monitor.
  • Audio Response Units.
  • Speakers.


Hardcopy implies that the output is permanent, i.e. it can be retained for an indefinite period.

The user can see & touch the results.

Hardcopy is desirable if the information or the results of the computer working is to be maintained for future reference/ use.

Examples of Hardcopy Output devices.

  • Printers.
  • Graph Plotters.
  • Microforms.

Exercise (a).

  1. (i). What is the difference between “Hardcopy” and “Softcopy”?

(ii). Name TWO output devices which produce temporary output.

(iii). Name THREE output devices which produce permanent output.

  • What are the factors, which determine the choice of output media and device?

Exercise (b).

  1. List FIVE different types of computer Output devices.


Visual display unit (or a Monitor) is a television-like screen, which displays the data that is being typed at a Keyboard.  It also displays the information that has been processed by the computer in a human-sensible form. 

In many cases, an ordinary television set can serve as the display unit.

The display is meant to provide a means of visually checking whether the information that has been entered is correct.

The output displayed on a monitor screen is called a Softcopy output.

Classes of Monitors.

  • Cathode Ray Tubes (CRT) – found in most desktop microcomputers.
  • Liquid Crystal Displays (LCD) – used by Laptops and Notebooks.
  • The resolution of a CRT is adjustable, while that of an LCD is often set. 
  • The CRT can display an almost unlimited no. of colours, but the LCD can display a limited no. of colours.
  • LCD’s are backlit by a series of light bulbs.  Notebooks use only 1 light bulb in order to conserve power, but desktop LCD’s can use up to 4 bulbs. 

These bulbs can be replaced by the user.  They also have a life span.  Therefore, after sometime, you will notice that the screen starts becoming dimmer.  Take the LCD to the vendor for a bulb.

Advantages of LCD over CRT.

  1. The screen of a LCD is much thinner & smaller than of CRT.

LCD screens come in 14 or 15-inch sizes.  A 15” LCD has a 15” viewable screen; a size that is only slightly smaller than a 17” CRT.

  • LCD’s have no flicker.
  • They consume low power than the CRT.

The performance of a computer Monitor depends on 3 factors: –

  • Size of the screen.
  • The number of colours it can display.
  • Monochrome monitors – display only 1 type of colour (Black for Background & White for Foreground).
  • Colour monitors – display a variety of colours.

Examples; Enhanced Graphics Adapter (EGA), Video Graphics Adapter (VGA), Super Video Graphics Adapter (SVGA).

The higher the no. of colours displayed, the more realistic the images.

  • The Screen resolution (or sharpness of the image), which is determined by the no. of pixels.

The Viewing angle – the picture appears clear when viewed straight on or a certain amount of degrees off the center; but when you are too far over to the side, the picture grows too dim to see.

A VDU can be used to display lines of text as well as graphics (images, pictures & drawings). 

When used for output of text, a typical display unit is made of 24 lines; each line consisting of between 40 – 80 characters.

When used for output of Graphics, the screen is considered to be composed of a no. of dots arranged in rows & columns.



Each dot is called a Picture element (or Pixel). 

A Pixel is a screen dot & is a direct mapping of the information (e.g. character) in the Video RAM contained in the monitor’s Adapter card.

To display an image on the screen, the selected pixels are brightened or darkened.

The term Resolution is used to describe the no. of pixels per unit area of the screen.  E.g., the no. of pixels per cm2.

If there are few pixels per unit area, the display is said to be of a Low-resolution.  If there are many pixels unit area, we talk of High-resolution display.

The higher the screen resolution, the finer & the higher the no. of different images that can be displayed.

Note. Specific applications require certain resolutions to be able to run, e.g., Microsoft Windows.


Monitors do not have a direct impact on the performance, but have a significant impact on the use of a PC.  A bad quality monitor can hinder the use of an otherwise high-tech PC.

Display – Presentation of information such as by projection on a screen, an Audio message, a computer print-out, etc.

Display Adapter Card (also called Video Card or Graphics Card).

The real brain of a display operation is the Video Card.  It is inserted into the slot on your computer’s motherboard as an expansion card.  It then speaks to the monitor about what the computer is asking it to do. 

The Video card determines how fast graphics display, how many colours can be used, etc. 

For a Monitor to work to its best, it must be fitted with a Video card capable of bringing out the best in it.

The Video Card receives image data from the Processor in form of digital information & stores it in video RAM.  The digital information is then converted into analogue display signal, which is fed to the monitor.

How a Monitor Works.

The inner surface of the screen is coated with a Phosphorus material that emits/ produces light when struck by an electron beam.  Whenever the electrons hit the phosphor, it glows, producing images. 

When the Monitor is plugged into the Video card, it gets a scan frequency (or a signal) giving the timing of the screen redraws. 

The electron beam must cross the screen in synchronization with the scan signal of the Video card.  The beam starts at the top left of the screen, crossing to the right.  As it does this, it excites the phosphor dots.  On reaching the right side of the screen, it returns to the left side in order to refresh the line of pixels underneath the first one.  It continues this process down the screen, returning to the top to do it over again when it has finished the entire screen. 

During the process, the beam excites those phosphor dots, which the video card tells it to.  Therefore, the card gives instructions to the electron gun to excite some pixels; hence, forming pictures.

On a Colour monitor, each pixel contains 3 separate dots, one for each of the primary colours of light; Red, Yellow, & Blue.  Combining these colours together produces the range of colours that we all know.

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Screen burnout.

This is a term used to describe the damage caused to the inner surface of the screen display.

Inside the Monitor, an electron beam is aimed at a Phosphor-coated screen.  If a screen display is static, the electron beam continually strikes the same place on the screen surface, and eventually burns a hole in (or wears away) the Phosphorus coating.  This might take several days or weeks to occur.

Note. Damage to the screen cannot be repaired.  The damage is visible by turning & inspecting the screen surface closely.  If an image can be seen, then the screen has suffered damage (i.e. it has burned out).

Factors that determine how severe the screen burnout is.

  • Type of Phosphorus coating used on the screen’s inner surface.
  • Length of time the screen image remains constant.
  • The brightness & contrast of the screen.

How to avoid screen burnout.

Screen burnout can be avoided by applying the following common sense principles: –

  1. Switch the Monitor off, if you wish to be away over long periods.
  2. Turn down the screen brightness, if leaving the machine unattended.
  3. Using a “Screen Saver” program.

Types of Monitors (Displays or video Adapters).

  • Colour Graphics Adapter (CGA) display.

This was the original type of Monitor, but now it is obsolete. 

It supported colour, but could only display a limited no. of colours.  Usually, it supported 4 colours at a time.

It also ran at a low graphics resolution of 640 x 200 pixels.

  • Enhanced Graphics Adapter (EGA) display.

It is a low-grade adapter.  It was developed to improve the capabilities of CGA.

An EGA display works with a 9-pin connector and an EGA video card

They have a resolution of 640 x 350 pixels, which is better than that of CGA monitors.

They also offer two brightness levels for each primary colour dot, thus, displaying a wider range of colours.  EGA screens can show 16 different colours.

  • Video Graphics Adapter (VGA) display.

It was introduced by IBM in 1987 for use on earlier IBM PS/2 systems.  The VGA Video card contains all the circuitry needed to produce VGA graphics, and like all expansion cards, it plunges into a slot on the motherboard via an 8-bit interface.

VGA is able to display photographic quality images on a PC, i.e. it offers clean images at higher resolutions.  It is able to build an image that is 640 x 480 pixels in size.

With a VGA, a PC has to deal with 640 x 480 x 3 bits every time a picture changes.

The standard VGA can produce about 256 colours at a time from a palette of 262,144 colours.

The VGA can also be used in Monochromes.  It is able to translate colour graphics into graphics using 64 different shades of grey.  This, in effect, simulates colour on a monochrome monitor. 

VGA video card requires a VGA monitor, or a monitor capable of accepting the analog output of a VGA card.

  • Super Video Graphics Adapter (SVGA) display.

SVGA is much more advanced than VGA.

In most cases, one SVGA card can produce millions of colours at a choice of resolutions.  It is able to show 256 colours at a resolution of 1024 x 768 pixels.

SVGA is able to support 1024 x 768 x 3 bits potentially changing 50 times per second.

Note.  EGA, & VGA monitors are not interchangeable.  A VGA monitor won’t even plug into a CGA or EGA card.  A SVGA monitor is not supposed to work with a standard VGA card.

Advantages of VDU/ Monitors.

  • The speed of output is fast – a Monitor displays the output almost instantly/ immediately.
  • It displays the information enabling the operator to visually verify/confirm if the data is correct.
  • Enables the operator to monitor his/her performance & improve productivity.
  • Used when saving the information to a secondary storage media.
  • Minimizes paper work, hence reducing the cost incurred on stationery.
  • Hardware costs are minimal since no extra equipments are needed as long as the VDU is available.  The screen is cheap, if bought as individual device.

Disadvantages of VDU.

  • Produces softcopy output, i.e., the output is temporary & can get lost when the power is switched off.
  • It is impossible to produce multiple copies.
  • It causes fatigue to the user’s eyes, especially when stared at for a long duration.
  • Can lure computer operators into not keeping hardcopy records.
  • Screen might not allow the viewing of the full area of data.

Factors to consider when selecting/ buying a Monitor: –

  1. Compatibility with the adapter card: The monitor must be capable of displaying the pictures that the display adapter card can generate.  Otherwise, the display will be unstable.

The Video card must be compatible with your computer’s bus. 

  • Memory:  You should have at least 2 MB of Video memory, but if you have a larger monitor, start with 4 MB of memory. Look for a video card that has room for expansion in the future in case you have to buy a larger monitor.
  • Monitor size:  This is the distance diagonally across the face of the monitor.  The larger the monitor size, the more the viewable picture area.
  • Resolution and Refresh Rate:  Both features depend on each other.  They work hand-in-hand to produce a clean image.

Refresh Rate (sometimes referred to as Scan Rate) is the no. of times per minute that a computer screen image is renewed (or the rate at which each pixel on a screen is re-drawn).  The Refresh Rate is measured in Hertz (Hz).

Make sure you know the refresh rate of your monitor.  The recommended refresh rate is 60 Hz & above.  The higher the refresh rates, the better. 

A low refresh rate results in an image that flickers (shines unsteadily), resulting in eye-strain. 

The standard for flicker-free images is 85 Hz.  To detect flicker, look slightly above or to the side of the monitor.  Sometimes, it helps to be in a darker room.

Your Video card plays an important role in all this.  If your card cannot provide support for the resolutions and refresh rates of the monitor, the picture will look degraded.  When pairing a video card with a monitor, at least make sure that it is capable of delivering a 72 Hz refresh rate at any resolution supported by that monitor.

Note. In order to stop images fading on the screen, they must be refreshed (sent to the screen) at least 50 times per second.  Each pixel on the screen has 3 bits of information (corresponding to Red, Green, & Blue) attached to it, and all of them have to be redrawn at high speed.

The higher the resolution, therefore, the more strain is placed on the performance of the monitor’s Adapter card.


VOICE OUTPUT DEVICES (Audio Response Units -ARU).

An ARU converts data/ information from the computer memory (which is in electronic form) through various specialized additional circuitry into waveforms/ sound for the receiver to hear. 

The sound can be spoken language, musical notes or beeps.  This output is obviously Softcopy.

Voice output is useful where reading is not necessary or is impossible and where fast output is required.

For example, Voice output is used:

  • As a learning aid.
  • In emergency situations for messages.
    • In answering services, e.g. Post office talking clock.

When an ARU is used to produce speech, it is called a Speech Synthesizer.

A Speech synthesizer is a useful form of output especially when communication with a computer is made using telephone lines.  A user dials the computer & makes an inquiry.  The computer output is passed through the Speech synthesizer, which is located near the computer.  The output is converted to a spoken reply, which is sent to the user over the telephone line.

Speech synthesizers are being included in many consumer products.  For example,

  • A Bathroom scale with a synthesizer can tell a person his weight, and whether he has gained or lost weight.
  • In Cameras, they can tell you if your film or exposure is set wrongly.
  • A washing machine with a synthesizer can tell you if and when to add more detergent, or the fabric conditioner.

Advantages of Voice Output Devices.

  • It is very fast, making it useful in emergency situations to relay messages.
  • Can be used for distant communication, especially if done over Telephone lines.
  • Useful where reading is impossible – can be used by visually disabled people.
  • Errors are easily corrected.  For example, when used in a washing machine, it can tell you if and when to add more detergent, or the fabric conditioner.

Disadvantages/ Limitations of Voice Output Devices.

  • The output is not permanent.
  • It may be boring, especially for prolonged output.
  • Cannot be used by people with hearing problems.
  • If the message is conveyed through beeps, it may be hard to understand.

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