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Candidates should be able to:
- understand the need for input and output devices
- describe suitable input devices for a wide range of computer controlled situations
- describe suitable output devices for a wide range of computer controlled situations
- discuss input and output devices for users with specific needs.
Why does a computer system need input devices?
Input devices are needed so that data can be entered into a computer system, to be stored or processed.
Some input devices are manual, such as a keyboard or mouse, and some are automatic, such as a barcode reader or a webcam. Automatic input devices are generally more reliable, faster and more accurate than manual devices.
Why does a computer system need output devices?
Output devices are needed to present the information that is created as the result of processing by a computer system, or to control devices such as motors, relays, lights etc.
Output devices such as printers, monitors and speakers are designed to communicate information to the user. Motors, relays etc. are often used with control systems.
What situations are particular input devices suited to?
The mouse is used to control the movement of a pointer on the screen when it is moved horizontally over a flat surface. Buttons on the mouse let you select options from menus and drag objects around the screen etc.
Some types detect the movement of a ball under the mouse as it is moved. An optical mouse shines light down onto a surface and detects changes in the reflected light as the mouse is moved. Some models are now wireless.
- Suited to: A graphical user interface where pointing with a screen pointer is combined with selection using buttons to access items / menus / hyperlinks etc.
- Advantages: Ideal for use with desktop computers. Usually supplied with a computer so no additional cost. All computer users tend to be familiar with using them.
- Disadvantages: They need a flat space close to the computer. The mouse cannot easily be used with laptop, notebook or palmtop computers when not near a flat surface (these need a tracker ball or a touch sensitive pad called a touch pad).
A keyboard uses labelled buttons as input switches. The standard QWERTY keyboard is the commonest way to enter text and numerical data into a computer. Alternatives to fixed layouts include overlay keyboards and touch screen keyboards, both allowing easy customisation.
- Suited to: Entering text and number data into a computer system. Making selections on devices.
- Advantages: Reliable for data input of text and numbers. Usually supplied with a computer so no additional cost. Specialised keyboards are available for particular purposes.
- Disadvantages: Slow to enter data and prone to typographical errors with new users. Slow for accessing menus etc. and difficult to use if you want to move objects around the screen. Difficult for users unable to use a keyboard through paralysis or muscular disorder.
A tracker ball is like an upside-down mouse because the user rotates the ball and the main body part stays still. It has buttons for selecting items and menus etc. like a standard mouse.
- Suited to: situations were a mouse is needed but desk space is limited or a mouse is difficult to control.
- Advantages: Does not need a flat space close to the computer. Can be useful with laptops as they can be built into the computer keyboard or clipped on.
- Disadvantages: Not supplied as standard so an additional cost and users have to learn how to use them.
Barcode scanner: A barcode uses a pattern of dark and light parallel lines of different thickness to encode a number. They also include a checksum digit to ensure the barcode has been read correctly. The scanner scans a beam of laser light over the barcode and detects the distinctive pattern of reflected light from the barcode.
- Suited to: quickly reading numerical codes from labels and packaging.
- Advantages: Fast and accurate capture of numerical data.
- Disadvantages: Limited to numerical data. Can increase packaging costs as multi-pack products require a different barcode. If the barcode is misread, the data may have to be entered manually.
MICR (Magnetic Ink Character Recognition):
- Suited to: reading numerical data printed on cheques with magnetic ink using a special MICR font.
- Advantages: Fast and accurate. Not affected if the printed numbers are obscured or overwritten with normal inks.
- Disadvantages: Expensive to purchase the printers and readers, although this may be seen as an advantage as it is a deterrent against fraud.
OMR (Optical Mark Reader):
- Suited to: quickly reading numerical data entered as marks on a pre-printed sheet such as a lottery ticket form or a class register.
- Advantages: Fast and accurate.
- Disadvantages: Limited to numerical data.
OCR (Optical Character Recognition):
- Suited to: converting the text on a scanned image or photograph into text that can either be edited (such as in a word processor) or that can be looked up in a database (such as an address/postcode).
- Advantages: Fast and reasonably accurate.
- Disadvantages: Can have difficulties with poor quality images, complex fonts or with hand-written text.
- Suited to: inputting data or accessing icons and menus on a graphical interface. Text and numbers can be entered using an on-screen keyboard.
- Advantages: Allows the screen to be used as both an input and output device. Many touch screens now allow multi-touch for even greater interaction.
- Disadvantages: Limited on small touch screens such as those on Smartphones by the size of the finger, unless a stylus is used.
- Suited to: situations where a microphone can be used to input text or make selections from menus.
- Advantages: Voice recognition telephone menu systems save on staffing costs. Can be quicker than entering text via a keyboard. Idea for disabled users and hands-free applications.
- Disadvantages: Many systems require the software to be trained to recognise the user’s voice. There can be errors when the software does not recognise words, either due to regional accents, background noise or the limitations of the software.
- Suited to:
- precise movement (in 2 axis) input for software such as a flight-simulator.
- controlling physical devices such the tilt/pan on CCTV cameras, an electric wheel-chair or robot vehicles direction.
- Advantages: Simple and cheap technology.
- Disadvantages: Not capable of the same fine control offered by a computer mouse, touchpad or trackerball.
- Suited to: cursor control, object dragging and menu selection tasks as an alternative to a mouse or trackerball where space is limited.
Advantages: Usually built into the computer so makes efficient use of space at no extra cost.
Disadvantages: Can be harder to use than a mouse, particularly when trying to combine button selection and movement.
MIDI (Musical Instrument Digital Interface):
- Suited to:
- Situations where musical data such as the note, duration, volume etc. needs to be recorded digitally, rather than just digitising the audio signal.
- This data can then be easily manipulated, combined and played back on a computer or another MIDI equipped musical instrument
- Advantages: Makes it easy to store and manipulate musical data from any MIDI equipped musical instrument.
- Disadvantages: Still requires the user to play a musical instrument if they want to record data. MIDI equipped musical instruments will be more expensive then standard ones.
What situations are particular output devices suited to?
These produce a hard-copy of text or images. There are a wide range of printers available but they tend to fall into four main groups:
- Laser printers: powdered plastic toner is charged and attracted to paper that has a pattern of the opposite charge matching the required image/text. The toner is then melted to bond with the paper.
- Suited to: situations were high quality, fast printing is required, but not of photographic quality.
- Advantages: Printing costs per page are low and printing is fast. Double sided and colour printing are available.
- Disadvantages: A higher initial purchase cost and printing is not possible on glossy photographic paper.
Inkjet printers: fine nozzles in a printer head spray microscopic drops of coloured ink onto the paper surface, line by line. Basic models use 3 colours plus black while premium models have up to 6 colours.
- Suited to: situations were photographic quality printing is required and speed is not essential.
- Advantages: Purchase costs are very low and these printers offer very high quality borderless colour printing of photographs on both plain and glossy papers.
- Disadvantages: Slow printing speeds and the printing costs per page are high as the ink cartridges are relatively expensive to replace.
Dot-matrix printers: characters are formed from a pattern of pins. These pins are forced against a coloured ribbon to print the character. As the pins physically make contact with the paper they can produce multiple copies if multi-layer carbon paper is used.
- Suited to: situations such as printing receipts were fast, low quality printing is required and/or multiple copies of the printout are required such as in warehouses.
- Advantages: Purchase costs and running costs are very low and multiple copies of the printout can be generated.
- Disadvantages: Low quality text and image printing. Noisy due to the impact of the pins on the paper.
- Plotters: one or more pens physically draw directly onto the paper. In some designs the paper remains stationary and the pen moves via two arms, one controlling the X movement and the other the Y movement. In other designs the pen just moves from side-to-side as the paper is moved backwards and forwards on a roller.
- Suited to: large drawings.
- Advantages: fast output and the image size is only limited by the design of the plotter.
- Disadvantages: cannot produce photographic images and filled areas are either very slow to draw line by line or limited to hatching. Expensive to purchase.
- Suited to: displaying text, images or video in a wide range of sizes, colours and resolutions. In a laptop or Smartphone the screen is built into the device and therefore portable. Touch screens allow the monitor to become an input device.
Cathode Ray Tube (CRT) models are increasingly being replaced by flat-screen Liquid Crystal Display (LCD) models.
Simple LED and LCD screens are used as displays in numerous devices such as embedded computers and calculators.
- Advantages: allows a computer to output a wide range of static and moving information in up to 16 million colours and at a high resolution. A wide range of monitor sizes are available, from a few cm up to several metres.
- Disadvantages: CRT models are heavy and therefore non-portable, as well as being less power efficient and far more bulky. LCD screens, unless glass-fronted, are easily damaged.
- Suited to: output of voice or music. Headphones allow listening to be carried out in private.
- Advantages: simple cheap technology.
- Disadvantages: speakers built into computers tend to be poor quality. Trailing wires can be a problem for external speakers and headphones although wireless systems exist.
- Suited to: simple warnings and indicators although LED’s can be arranged to form segments of a text or numeric display.
- Advantages: LED’s use little power so can usually be powered directly from a computer system.
- Disadvantages: Lights generally need to be controlled through a relay due to their higher power consumption.
- Suited to: outputs that require linear or rotary motion. Rotary motion is usually through an electric motor while linear motion can be electrical, hydraulic (using hydraulic fluid) or pneumatic (using pressurised air).
- Advantages: Computer controlled stepper motors and servos offer highly accurate movement.
- Disadvantages: A relay is usually need for a control system to operate a motor as they require a higher current than a computer can deliver directly.
How can input and output devices be used by users with specific needs?
Blindness and visual impairment
- For partial blindness, screen colours can be set to high contrast options or customised for the colour-blind. Areas of the screen can be magnified and fonts can be set to larger than standard sizes.
- For complete blindness, a voice synthesiser can be combined with text recognition software to read out text from menus, documents, web pages and even images of scanned text.
- For colour impairment, special screen colours can be used.
- Braille printers can print the tiny bumps used in Braille directly onto paper..
- A Braille reader raises and lowers tiny bumps under the fingertips allowing text to be read without having to print it out on a Braille printer.
Braille keyboards or Braille keyboard overlays can input text and numbers.
- Voice recognition software and a microphone can be used to input text and control some computer operations.
- Research on using video cameras to replace the eye completely, or as a retinal implant, is starting to be developed. These would potentially be linked directly to the optic nerve allowing the brain to interpret the electrical inputs from the light sensors as an image.
- GPS equipped Smartphones with voice recognition, turn-by-turn navigation software and speech synthesisers allow access to unfamiliar areas.
Deafness and hearing impairment:
- Videophones and video conferencing allow sign language and lip reading to be used in distance conversations using video.
- Digital hearing aids can amplify and clarify sounds, as well as linking to induction loop hearing systems in telephones or public building.
- Flashing lights can be used to replace auditory signals and alarms as outputs.
Subtitles can be included in video content and some software can output subtitles in real time from spoken audio.
- A Voice Output Communication Aid (VOCA) is a device that is programmed to output synthesised speech from a special keyboard that either uses symbols or allows normal text to be entered and then speaks a completed word or sentence.
Muscular disorders or coordination problems can make using input devices such as a mouse and keyboard difficult or impossible to control. The alternatives below can be used separately or in combination with each other to meet the particular needs of the user.
- A joystick can also be used to control devices such as an electric wheelchair. It can can also be easier to control than a mouse when used to navigate software menus or input text/numbers using an on-screen keyboard.
- A puff-suck switch is operated by sucking or blowing through a tube. It can used as a replacement for clicking buttons on a mouse.
- A foot mouse replaces a conventional mouse and is operated by the feet. Usually one foot controls the movement and the other one the clicking of the buttons.
- Touch screens can be easier to use than a mouse and keyboard as input devices.
- Motors in prosthetic limbs can be controlled by linking sensors to severed nerves.
- An eye tracking camera can be located on the face or on the computer screen. It can detect where on the screen the user is looking and therefore control a cursor. Blinking can then used to operate mouse clicks.
- Research on using sensors on the skull to directly detect brain patterns and then control hardware and software is currently showing signs of progress.