Monitors are also known as visual display unit (VDUs). They come in different styles and quality levels. Screen colour quality, resolution and clarity are just some of the features that affect how desirable a particular monitor is to you.
Picture elements: Although the picture on a computer monitor may look sharp and clear, it is made up of many illuminated dots known as picture elements or pixels. The dots are usually so tiny that you would not normally notice them individually – you just see the whole picture. If you could actually see the picture elements, the picture would look jagged.
Advantages of monitor screens:
Disadvantages of monitor screens:
- Enables the use of a graphical user interface.
- Interactive, on-screen use of a computer is more natural and intuitive than programming.
- Screen size can limit the extent and detail of what can be seen. For example, it is possible to refer to several printed pages at the same time. It could be much more difficult to arrange this on a monitor.
- Screen displays can be difficult to read for people who have impaired sight.
Types of printer:
- An impact printer usually strikes through an inked ribbon, making marks on the paper. A non-impact printer uses a non-striking method to form the image on the paper.
- Inkjet printers use tiny dots of ink sprayed onto the paper, forming the shapes of characters and pictures. Inkjet printers are quiet and produce good quality output. They usually produce colour prints, and are very popular for home and small-office use.
- Laser printers use laser light to make patterns of ink on a drum. This drum then transfers the ink to the paper, and finally a heating process fuses the ink to the paper. Laser technology gives excellent quality and prints quickly. Laser printers are widely used but are more expensive than inkjet printers, and often only print in black and white.
- Thermal printers use heated wires to mark dots on the surface of a heat-sensitive paper.
- Thermal dye transfer printers use special coloured dyes heated into a gas. This process gives the best quality of colour printing, but is very expensive and requires special paper.
- Thermal wax transfer printers use molten wax, forming tiny dots of different colours on the paper. These coloured wax dots blend to produce tones.
Buffers and spooling: Printers are much slower than the computers and networks that send information to them to be printed. So that the computer does not have to wait for the printer to finish printing, most printers have a buffer built into them. A document sent to the printer will be quickly saved in the buffer so that the computer can do other tasks while the printer is printing the document.
A buffer will only hold a few short documents, and on large networks many users may be sharing the same printer. To avoid users having to wait, documents are first put into a queue on a server and then sent for printing in turn. This queuing process is known as spooling.
Print Quality: All these types of printers use very small dots to produce pictures. The smaller the dot, the better the quality of the picture. An important measure of print quality is the number of dots per inch (dpi). The higher the dpi, the better the picture. In addition, for the best-quality output the dots must be accurately placed.
Printers and paper:
Printers can use different types of paper:-
- Many printers use sheets of A4 paper.
- Some printers have a long stream of continuous paper flowing through. Usually the paper is perforated, so that it can be torn easily to produce separate pages of output. There may be several sheets together, either impregnated with carbon or with carbon sheets between them, so that multiple copies produced at ones.
- In pre-printed stationery, certain information – for instance, the name of a company – has already been printed on the paper. This is usually the case with utility bills, which are printed with a high - volume single-coloured printed on forms that already have coloured logos and other symbols printed on them.
Photo-printers: Photo-printers are specially designed to print digital photographs. A memory card storing pictures taken by digital camera can sometimes be plugged directly into the printer, and the pictures printed. In other cases, the camera can be directly connected to the printer.
Advantages of printers:
Disadvantages of printers:
- Printed output can be viewed without the need for a computer.
- Many people and organizations still use paper-based communications and similar legacy methods.
- Printing is slow and expensive compared with electronic communications and storage.
- Paper is bulky and deteriorates in storage.
- Printed materials are more difficult to access and distribute. For example, printed photographs can be viewed by a limited number of people compared with digital photographs displayed on a TV screen, and it is more difficult and expensive to send copies to others.
Multi-function printers: Printers are now being built in combination with scanners, and sometimes with fax machines. Combining a printer with a scanner produces the effective functionality of a photocopying machine, as well as providing the separate functions of scanning and printing.
Advantages of multi-function devices:
Disadvantages of multi-function devices:
- A multi-function device takes up much less space on the desktop than the individual devices it replaces.
- A multi-function device is usually less expensive to buy than all the individual devices it replaces.
- A multi-function device will be used more before it becomes obsolete.
- A multi-function device can be replaced more frequently, having provided good value, so that more modern technology is available for use.
- If part of the device develops a fault, all the functions may be unavailable.
- Only one person at a time can use a multi function device.
A plotter draws lines on paper using differently coloured pens. In a flatbed plotter, the paper is held still while the pen moves. Other plotters have the pen moving from left to right, while the paper goes forwards and backwards. Yet others use wires to draw charged patterns on special paper, then fuse toner onto the electrically charged patterns. An upright plotter can handle long sheets of paper, because of the way the paper flows, and can produce long continuous lines. Plotters are frequently used in computer-aided design.
Advantage of plotters:
Disadvantage of plotters:
- Much larger and longer sheets are printed.
- Printing is very slow
- Plotters are usually more expensive than printers.
4. Data projector
A multimedia projector or data projector projects an image that would normally be displayed on a computer screen onto a larger, separate screen. This allows the image on the screen to be shared with an audience in a large room. Multimedia projectors are almost always used when giving computer-based presentation. Multimedia projectors are also used with home entertainment systems, enabling people to watch TV and DVDs and to play computer games.
Interactive whiteboards are large touch sensitive panels used as screens for multimedia projectors. They are operated as touch screens. Some use capacitive or resistive technology but others use an optical curtain. Breaks in this curtain are detected and so the computer knows which part of the interactive whiteboard has been touched.
Speakers are output unit of sound. This is sued to play music, to make telephone calls using VoIP, to listen to voicemail, to play video and DVDs for entertainment or education and to listen TV and radio.
Speakers are also needed for speech synthesis, where a computer reproduces human speech. The voice need not be recording and could be computer – generated. For instance, you can have text-reading program that takes a word-processed document in electronic form and reads it aloud.
Advantages of speakers:
Disadvantages of speakers:
- They allow computer systems to be used for a wide range of multimedia applications.
- Interaction with a computer using voice recognition and speech synthesis is more natural and intuitive than programming.
- Applications that use speakers can be difficult to operate for people who have impaired hearing.
- Voice recognition systems can be difficult to set up and use.
6. Control devices
In control applications, computers make events happen using control devices or actuators. Examples of actuators are valves, heaters, coolers and motors. These can be powered by a range of different sources of energy, including electricity and compressed air. Actuators also include devices such as buzzers and alarms, which can warn us about various events: a burglar has entered through the window etc.
Advantages of control devices:
- Actuators enable computer to perform physical tasks in the real world. For example, computers can control a central heating system or a mechanical digger.
Disadvantages of control devices:
- Actuators perform physical operations when instructed by a computer. This can be dangerous for people if their presence is not known to computer.