Output devices, applications and how they work
Introduction
We have seen a model of a computer system before. We know that for a computer to do useful things, we need to get data into it. To see the result of any processing, we need some output devices.
Introduction to output devices
There are lots of commonly used output devices available for a user to select from.
Dot matrix printers and how they work.
These are relatively slow and noisy and the quality of the hard copy is relatively poor compared to ink-jets and laser printers. They were very common a few years back in the early days of computing. Their uses are far more limited now but they do have one particular advantage. The hard copy is made by pins striking paper, which is why they are also known as 'impact printers'. That means that identical copies can be made of a printout by using carbon paper between sheets of paper (as opposed to printing out two copies of something - there is a subtle difference). This system is used by credit card companies to produce actual copies of receipts when a customer makes a purchase. After a customer's credit card is swiped and authorised, two identical copies of a receipt are printed using small dot matrix printers. Both copies are then passed to the customer, who signs the top copy. This puts a carbon signature on the bottom copy. The customer keeps one copy and the shop keeps the other. You cannot make actual carbon copies with ink-jets or laser printers although of course you can print out two copies of a document!
Ink-jet printers and how they work.
This type of printer 'sprays' ink onto paper. Different ink-jet printers work in different ways. Canon printers, for example, heat up the ink so that it explodes in bubbles onto the paper. The characters are still made up of dots, but the dots are so small (of the order of 50 microns diameter - a human hair is about the same diameter) that you can't see them and tell them apart. The dots are accurately positioned together to create characters and images but of course, you can use the printer software for an ink-jet printer to order your printer to use more dots per inch (or DPI). This gives you a higher resoultion e.g. 1200 by 800 DPI, very important for high-quality photos, for example but this uses more ink than lower resolution settings. If you are only printing out text, then 100 DPI is usually fine.
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- An ink-jet printer can't produce carbon copies.
- An ink-jet printer can produce very high quality black and white as well as colour copies for a very low cost.
- An ink-jet printer is a good choice for low volume applications such as small businesses or homes.
Laser printers and how they work.
These types of printers are in widespread use. A laser beam scans across a drum inside the printer to 'paint a pattern of static electricity corresponding to whatever it is you are printing out. The static electricity attracts powdered ink called toner onto the page and then a special unit bonds the toner onto the paper so it is permantently fixed. (This is also how a photocopier works.)
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- A laser printer produces very high quality black and white hardcopy.
- A laser printer costs more to buy and run than ink-jets although costs have been steadily falling in recent years.
- The price of colour laser printers has been falling to make them within reach of individuals and small businesses.
- Refills are expensive compared to ink-jets.
Plotters.
Plotters are widely used in some industries.
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- Plotters are used to plot very large drawings such as those needed by engineers and designers whereas standard printers commonly print up to A4 (and sometimes A3).
- Plotters produce very high quality, very accurate, colour drawings.
- Plotters are relatively expensive compared to printers.
Visual Display Units (VDUs).
Monitors are ideal for displaying data and information to users. They come in a range of sizes. Larger ones such as 21-inch screens, for example, would be ideal for engineers using Computer Aided Design software applications. 15-inch screens are perfectly acceptable for users using a range of generic applications. CRT (Cathode Ray Tube) monitors (similar to televisions) do take up a lot of space on a desk. Flat panel liquid crystal display screens, often referred to as TFT screens, save a lot of space by comparison. They are not quite as sharp as CRT screens, however, and are more expensive. TFT screens produce less radiation than CRT monitors. Excessive exposure to radiation is seen as a potential risk to computer users. They also use about half the power a CRT screen uses. If you multiply up the savings in power use in an organisation with thousands of computers, the cost-savings do become significant.
Speakers and how they work.
Some applications such as burglar alarms, factory warning systems and monitoring equipment make use of audio output. Some applications also require sound, such as video-conferencing, using your computer to make phone calls, listening to DVDs or CDs and playing games. There are different ways that audio output from a computer can be achieved.
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- The cheapest option is simply a pair of speakers powered by the computer. They will plug into the back of the computer, in the speaker output. The quality and level of sound will be perfectly adequate for many applications but they cannot produce a very loud output and cannot produce a very high quality sound.
- You could also buy a pair of speakers that come with their own power supply. Although more expensive, they produce a higher quality sound and greater volume.
- It is perfectly possible to connect an amplifier to the back of a computer and then pass the amplified signal to some speakers. This is a much more expensive proposition but does produce hi-fi quality sound.
- In some noisy environments such as factories, klaxons (sometimes known as 'sirens') are common. These can be computer controlled and can produce a very loud sound that can be heard over noisy machinery.
Speakers work by sending an electrical current to a coil (3). The current varies according to the sound you want to play. A coil that has a current flowing through it becomes an electromagnet, which is attracted and repelled by a permanent magnet (1). The parts above are all contained within a speaker cone (3) that has a thin membrane called a diaphragm (4). The constant act of the coil being magnetised and demagnetised causes the cone to 'pump' sound out of the speaker by causing the diaphragm to vibrate.
Headphones.
There are situations where a user wants to listen to sound in a public place but doesn't want to disturb others. For example, a user in a library might want to listen to CDs on a computer, or a telesales operator might need to concentrate on what a customer is saying. They work in the same way as speakers.
3D printers
There are lots of short but very interesting videos you can watch to see the possibilities of this technology, which involves designing an object using some software and then sending it to a 3D printer to print out the actual object. It does this by printing out very thin layers of the object one at a time, and binding each layer with the previous one. Some examples of the use of technology include;
Shooting the world's first 3D-printed gun.
Driving a 3D-printed car.
Printing out replacement legs.
Printing out replacement ears and replacement skin.
3D printing in the high street