Output Devices Flash Cards

Speakers are used for sounds and music, including for listening to CDs from a computer and to prompt the user. But, perhaps more importantly, they are used for spoken output.
This may have a large impact as far as blind or disabled users are concerned, and this may also have an impact on the general computer user of the future.

When a sound is played on a computer, the CPU sends the digital data to the Digital Sound Processor on the computer's sound card. An on-board digital to analogue converter (DAC) then produces the analogue output that goes to the speakers.

The electric signal from the computer powers an electromagnet at the rear of the speaker. This electromagnet attracts and repels the cones in the speaker according to the electric signal from the computer. The cone generates different sounds by vibrating at different speeds, creating sound waves. This is the reverse process to a microphone where the sound waves vibrate the cone, which in turn generates an electric signal (in fact speakers can be used as microphones).

Sub-Woofer is a type of speaker that reproduces the lower end of the audio spectrum. • Surround Sound is also known as 3D sound; this multi-channel audio configuration delivers separate left, right, centre, rear left and rear right sounds, and sometimes a sub-woofer channel. • Dolby Digital is the standard for high-quality digital audio, used especially for DVD-ROM videos. Dolby Digital can deliver surround sound. • Frequency Response is the range of sound frequencies that the speaker can produce from the input signal. Values are given in Hertz (Hz). For example, most headphones have a frequency response of 20Hz to 18kHz. A larger frequency response results in improved sound reproduction accuracy. • Active Speakers are speaker units with a built in amplifier. They are often used with computers to increase the volume of the output without the need for a separate amplifier. They require their own power supply and often have their own volume, bass and treble controls. A typical unit is shown opposite.

These use fine pins striking a ribbon to make dots, which build up text and images. Letter quality printers use multiple strikes to build up text with overlapping dots. However they are very noisy and not very fast compared to modern printers. The paper is generally pulled through by means of holes either side of the page, which can be torn off afterwards. They used to be the most popular printers before inkjet and laser printers, but are now generally used when multiple carbon copies are needed.

These use tiny jets of ink, which form text or images from overlapping dots. It prints line by line. Print quality is determined by the dpi (dots per inch) and some inkjet printers are very nearly as good as laser printers. However they are generally slower than laser printers and cost more to run (for black and white), due to the amount of ink required. Their initial cost is often much less than laser printers and this is particularly evident in colour inkjets and colour lasers. A colour laser printer may cost £200 whereas a colour inkjet may cost £30 or less. Colour inkjet printers use four colours (CYMK) to make up colour images. Some can produce impressive results and the low cost makes colour printing more accessible to the home or small business user, particularly for printing photographs.

They give very high quality print. For an example of running costs, the HP1100 will print about 2000 sheets of paper from one new cartridge (which currently costs about £30) – a cost of about 2.5p per piece of paper. Colour laser printers are relatively quick and high quality but can be expensive to run

Like any device, printers need drivers to interface with the operating system. Every printer has its own driver, which must be installed before it can be used. This is because every printer operates slightly differently and uses different control commands. When a document is printed from an application like a word processor, the information is sent to the printer driver, which converts the information to the correct format for the specific printer being used.

1. The computer's printer driver sends information to the printer about where each dot goes. 2. The printer’s processor uses this information to shine a laser light beam across the surface of the printer drum, and the drum becomes positively charged in those places, but remains negatively charged elsewhere. 3. The drum rotates and the negatively charged toner jumps onto the places on the drum that are positively charged. 4, Meanwhile the paper is rolled through the paper train and given a stronger positive charge than the drum. When it comes into contact with the drum, the toner particles jump from the drum onto the paper.

There are many other less common types of printer. These include: • LCD & LED – These are similar to laser printers but light-emitting diodes or liquid crystals are used to produce the image on the drum. • Thermal printer – These work by melting ink onto sensitive paper. They are inexpensive and are commonly found in fax machines. • Braille printer – These print out a series of raised dots on paper for blind people to read from. Specialist Printers Specialist printers perform specific tasks – for example barcode label printers are available. Plotters allow production of large technical drawings using coloured pens (and are useful for very large drawings and drawing to a very high level of accuracy).

There are various things to weigh up when buying a printer: • Initial outlay. • Cost per average page (cost of cartridge/number of pages per cartridge). • Speed in PPM (pages per minute). • Quality (think about what it will be used for). • Paper sizes and thickness that can be printed. • Memory buffer size (needs to be larger if you’re doing large pictures).

Bitmapped Fonts are made up of a bitmap of dots (also see outline fonts). • CYMK (Cyan, Yellow, Magenta, and blacK) are the four colours most often used in colour printing. • Dithering is a process where small dots of colour are used to create shades of grey and hues of colour. • Font is a typeface of a particular size and a particular variation (e.g. italic or bold). Examples of some different fonts are Arial, Arial Black, Arial Narrow, Arial Rounded, MT Bold and Impact. • Outline Fonts are made up of characters described by lines and arcs, using a sequence of mathematical formulae. • A point is ¹/₇₂ of an inch, the traditional measure for measuring the size of text. This text, for example, is 12 points high. • Resolution is the number of dots of ink it takes to make a one-inch line. 600 dpi is generally high enough quality and a higher resolution would only usually be used for printing artwork and photographs. • A typeface is a design for the alphabet (and/or other characters). A design may have different styles distinguishing it such as its shape and boldness. Examples of typefaces are Arial, Courier, and Univers.

Plotters can produce very high-resolution drawings and are often used for very large diagrams, and may be used for other large graphics such as posters and maps. With the advent of colour laser printers, plotters are not being used so much, but they still have an advantage in that some plotters are accurate to hundredths of a millimetre. Also some plotters are used on huge pieces of paper.

The computer sends the information to be printed to the plotter. Plotters are essentially pens attached to motors. A plotter will usually have more than one (coloured) pen, and some have as many as eight pens. The plotter will then work in one of two ways: 1. The pens may only be able to move across the paper in which case the paper is fed through a fraction at a time and the pens either lifted up or held down. 2. The paper may be still and the pens move around the paper in all directions controlled by the motors. These are called flatbed plotters.

Because plotters are vector devices, drawing images by moving a pen in certain directions and not drawing each pixel individually, they must be commanded differently. Hewlett-Packard's HPGL is the most commonly used language to direct a plotter. Also, a file format called Computer Graphics Metafile (CGM) was developed for holding computer image information, such as those printed by plotters.
These standards have mostly been superseded now, with the development of large, high-quality inkjet printers.