Computer Output Devices : The information processed by the computer is translated into a form that we understand, and displayed by these machines. Normally, the output is classified as Softcopy Output or Sound Output and Hardcopy Output.
Softcopy refer to data that is shown on a display screen or is in audio or voice form. This kind of output is not tangible; it cannot be touched. Virtual reality and robots might also be considered softcopy devices. Hardcopy refer to printed output. The principal examples are printouts, whether text or graphics, from printers, plotters etc.
Display screens: also known as CRTs, monitors, or simply screens, differ in size, color, resolution, and video display adapter card. These are used for inputting the data or displaying the information after processing.
Size: monitor come in different sizes, from small screen built into palmtops and laptop to extra large monitors used for special purposes.
Color: Many monitors display color. These RGB displays can create 256 colors and several thousand variations on them by blending shades of Red, Green, and Blue (RGB). Monochrome displays show information using a single foreground color on a contrasting background color.
Resolution: All the characters and images on a monitor are made up of dot patterns; the number of dot, or pixels, per inch determines resolution, or the sharpness of the image. A higher number of pixels means a shaper image.
Video Display Adapters: to display graphics, a display screen must have a video display adapter attached with the computer. It is known as a video graphics card, and is a circuit board that determines the resolution, number of colors, and speed with images appear or the display screen. So far, there are three types of graphics cards introduced in the market.
VGA: video graphics array, support 16-256 colors, depending on screen resolution. At 320 x 200 pixels, it will support 16 colors and at 640 x 480 pixels, 256 colors. It is called 4-bit color.
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SVGA: super video graphics array, support 256 colors at higher resolution than VGA. It has two graphics modes: 800 x 600 pixels and 1024 x 768 pixels. It is called 8-bit color.
XGA: Extended graphics array, supports up to 16.7 million colors at a resolution of 1024 x 768 pixels. Depending on the video display adapter memory chip, XGA will support 256, 65536, ro 16, 777,216 colors. It is called 24-bit color or true color.
Types of screen: display screens are of two types i.e. Cathode-Ray Tubes and Flat-Panel Displays.
CRT (Cathode-Ray Tubes): the most common form of display screen is the computer or video display terminal. This same kind of technology is found not only in the screens of desktop computers but also in television set and in fight-information monitors in airport. A stream of bits defining the image is sent from the computer (from the CPU) to the CRT’s electron gun, where the bits are converted to electrons. The inside of the front of the CRT screen is coated with phosphor. When a beam of electrons from the electron gun (deflected through a yoke) hits the phosphor, it lights up selected pixels to generate an image on the screen.
How CRT screen work
Flat-Panel Displays: The flat-panel displays are much thinner, weightless, and consume less power to CRT. Thus, they are better for portable computers. Flat-panel displays are made up of two plates of glass with a substance in between them, which is activated in different ways. There are three types of technology used in flat-panel display screens: LCD (Liquid-Crystal Display) consists of a substance called liquid crystal, the molecules of which line up in a way that lighting behind the screen is blocked or allowed through to create an image. EL (Electro-Luminescent Display) contain a substance that glows when it is charged by an electric current. Gas-plasma display is like a neon bulb, in which the display uses a gas that emits light in the presence of an electric current.
Computer Output Devices: At present, EL and gas-plasma technology are more expensive thus are not used as often as LCD technology.
Printers: printers are used to print characters, symbols, and graphics on paper. They are divided into two categories:
- Impact printers
- Non-impact printers
Impact Printers: An impact printer forms characters or images by striking a mechanism such as a print hammer or wheel against an inked ribbon, leaving an image on the paper. Following are a few types of impact printers:
- Dot-matrix printer: it contains a print head of small pins, which strike an inked ribbon against paper, forming characters or images. Print heads are available with 9, 18, or 24 pins, with the 24-pin head offer the best quality prints.
- Daisy-wheel printer: this printer uses a mechanism is the shape of a series of petals arranged on a petal wheel., having a character at the end of each petal. A character comes into a print position by wheel rotation and an image is formed by the hammer strike on the desired character. It is slower than dot-matrix printer but better in quality.
- Line printer: this type of printer is normally used by mainframe and minicomputers. It prints a whole line of characters at once rather than a single character at a time. Some of these can print up to 3000 lines per minute. It is of two types: chain printer and band printer.
- Non-Impact Printers: Non-Impact printer forms characters or images without making direct physical contact between printing mechanism and paper. There are three types in this category.
- Laser Printer: it is similar to a photocopying machine and it use the principle of dot-matrix printers of creating images with dots. These images are created on a drum, treated with a magnetically charged ink-like toner (powder), and then transferred from drum to paper. The laser printer can produce high quality images of both text and graphics (ranging from 300dpi to 1200 dpi). Its speed varies from 4-32 text-only pages per minute for microcomputers and up to 200 pages per minute for mainframes.
- Ink-jet printer: ink-jet printer sprays small, electrically charged droplets of ink from four nozzles through holes in a matrix at high speed on to paper. It is cheaper compared to laser printer but lower in resolution (300-720 dpi) and is slower also (1-6) text-only pages per minute. It has another type of printer i.e. bubble-jet printer, which uses miniature heating elements to force specially formulated inks through print heads with 128 tiny nozzles.
- Thermal Printer: thermal printer uses colored waxes and heat to produce images by burning dots on to special paper. The colored wax sheets are not required for black-and white output. It produces a high quality printout but is quite expensive compared to other non-impact printers.
Plotters: A plotter is used to produce high-quality graphics in many colors and used for specialized applications i.e. architectural drawings, maps, graphs, and charts. Plotters are of two basic kinds:
- Flatbed Plotter
- Drum plotter
Flatbed Plotter: a flatbed plotter is the one, which has a paper lying flat on a table-like surface. The bed-size varies according to the need. One to four color pens move across the paper and the images are printed by the computer accordingly.
Drum Plotter: it works like a flatbed plotter with a difference that the paper is mounted over a drum, enabling a continuous output. A typical usage is to track an earthquake readings.
Sound output: Speakers are most commonly used to have this type of output.
Speaker: as we use microphone to input audio data to the computer, conversely we use speaker to get audio output from the computer. It works on the same principles to convert sound data into machine sable form. A variety of speakers are available in the market to satisfy the requirements of the users.