An output device is any peripheral device that converts machine-readable information into people-readable form such as a monitor, printer, plotter and voice output device. Output devices are used to display input data that has been processed by the computer into useful output information. Through its output system, the computer presents the data it contains in a form that the user can understand. The presentation is typically in one of four forms: graphics (e.g ., charts and photographs), text (e.g ., words and numbers), video (e.g ., photographic images in motion), or audio (e.g ., music and speech). Probably the most common output devices are monitors for producing temporary displays (soft copies) and printers for producing paper copies of documents (hard copies).
COMPUTER DISPLAY (MONITORS)
A computer display is also called a display screen or video display terminal (VDT). A monitor, or screen, can display graphics, text, and video output. To display video in the monitor, the computer needs a video card (or graphics card) for converting the digital signal (i.e ., data in the form of Os and 1s) from the processor to an analog signal (i.e ., data in the form of a continuous electrical wave) that the monitor can interpret.
A monitor is a screen used to display the output. Images are represented on monitors by individual dots called pixels. A pixel is the smallest unit on the screen that can be turned on and off or made different shades. The density of the dots determines the clarity of the images, the resolution.
Screen resolution: This is the degree of sharpness of a displayed character or image. The screen resolution is usually expressed as the number of columns by the number rows. A 1024 X 768 resolution means that it has 1024 dots in a line and 768 lines. A smaller screen looks sharper on the same resolution. Another measure of display resolution is a dot pitch.
Types of Monitor
There are two types of computer monitor based on the colour display. They are
- Monochrome monitor: A monochrome monitor has two colours, one for foreground and the other for background. The colours can be white, amber or green on a dark (black) background. The monochrome monitors display both text and graphics modes.
- Colour Monitor: A colour monitor is a display peripheral that displays more than two colors. Color monitors have been developed through the following paths.
- Colour Graphic Adapter (CGA): With a CGA monitor, it is harder to read than with a monochrome monitor, because the CGA (320 X 200) has much fewer pixels than the monochrome monitor (640 X 350). It supports four colours.
- Enhanced Graphic Adapter (EGA): It is a video display standard that has a resolution of 640 by 350 pixels and supports 16 colors. EGA supports previous display modes and requires a new monitor.
- Video Graphic Adapter: This is a video display standard that provides medium to high resolution. In a text mode, the resolution of this board is 720 by 400 pixels. It supports sixteen colours with a higher resolution of 640 by 480 pixels and two hundred and fifty-six colours with 320 x 200 pixels.
- Super VGA: This is a very high resolution standard that displays up to 65,536 colours. Super VGA can support a 16.8 million colours at 800 by 600 pixels and 256 colors at 1024 by 768 pixels. A high-priced super VGA allows 1280 by 1024 pixels.
Types of monitors based on technology
Monitors come in two main varieties based on the technology, they are
A) Cathode-ray tubes (CRT): CRT (Cathode Ray Tube) computer monitors were the most common computer monitors until flat panel screens became affordable. Similar to older TVs, CRT monitors still typically have much better contrast ratios and viewing angles than other computer monitor alternatives.
B) Flat panel display: Portable computers such as a lap top use flat panel displays, because they are more compact and consume less power than CRTs. Portable computers use several kinds of flat panel screens:
- Liquid crystal displays (LCD): Flats-creen LCDs are generally used in notebooks and laptops. LCD (Liquid Crystal Display) computer monitors save a lot of space and can even be mounted on walls. LCDs are difficult to read in a strong light, because they do not emit their own light. Portable computers have brighter and easier to read displays. They provide a wide viewing angle and come in sizes typically ranging from 17-inches to 60-inches.
- Backlit LCDs: This is a type of LCD display having its own light source provided from the back of the screen. The backlit makes the background brighter and clear, as a result the texts and images appear sharper. However, this is still less clearer than CRTs.
- Active Matrix LCDs: This is an LCD display technique in which every dot on the screen has a transistor to control it more accurately. This uses a transistor for each monochrome or each red, green and blue dot. It provides better contrast, speeds up screen refresh and reduces motion smearing.
- Gas-plasma displays: Gas-plasma displays are capable of larger displays with good resolution and can be in excess of 42 inches. Plasma computer monitors are less commonly used with computers due to their high operating temperatures, high power consumption, and fixed resolution. This is also called a gas panel or a plasma panel and is another flat screen technology. A plasma panel contains a grid of electrodes in a flat, gas filled panel. The image can persist for a long time without refreshing in this panel. The disadvantages of the gas plasma displays are that they must use AC power and cannot show sharp contrast.
- OLED (Organic Light Emitting Display): Computer monitors are much thinner and brighter than LCD or Plasma screens. OLED monitors can also be placed on transparent surfaces, such as glass, allowing the user to see through them when not active.
A printer is an output device that produces text and graphics on paper. All printers have three main components: the printing mechanism itself, paper feed mechanism, and the control and interface electronics.
Printers can be divided into two main groups.
1) Impact printer: Impact printer produces text and images when tiny wire pins on print head strike the ink ribbon by physically contacting the paper. Examples of impact printers are dot matrix printer, line printer and character printer or daisy wheel printer.
(a) Dot-matrix printer: Dot-matrix printer is an impact printer that produces text and graphics when tiny wire pins on the print head strike the ink ribbon. The print head runs back and forth on the paper like a typewriter. When the ink ribbon presses on the paper, it creates dots that form text and images. Higher number of pins means that the printer prints more dots per character, thus resulting in higher print quality.
Advantages of dot matrix printer
- Can print on multi-part forms or carbon copies.
- Low printing cost per page.
- Can be used on continuous form paper, useful for data logging.
- Reliable and durable.
Disadvantages of dot matrix printer
- Limited print quality.
- Low printing speed.
- Limited colour printing.
(b) Line Printer: A line printer uses a long drum imprinted with raised characters. As the drum rotates, words are formed on the page. The drum moves in a horizontal direction, printing one line at a time. Line printers have limited font availability and no graphic capability. Due to their design, they require special paper with prepunched holes on the sides that attach to the printer by raised grooves. The line printer is still found in some businesses due to its durability and ease of use.
Advantages of line printer
- It is a High Speed impact printer.
- Less expensive.
- Print entire line at a time.
Disadvantage of line printer
(c) Daisy-Wheel Printers: It uses a wheel as a print head. When the wheel rotates, hammers strikes the backside of the spoke and press it against the paper to print a character.
Advantage of daisy-wheel printer
- It can print letter quality characters.
Disadvantages of daisy-wheel printer
- Printing speed is slow.
- It cannot print graphics.
2) Non-impact printer: Non-impact printer produces text and graphics on paper without actually striking the paper. Examples of non -impact printers are inkjet printer, laserjet printer and thermal printer.
(a) Inkjet printer: Inkjet Printers are non-impact printers which print text and images by spraying tiny droplets of liquid ink onto paper. They are the most popular printers for home use. Currently, most inkjet printers use either thermal inkjet or piezoelectric inkjet technology. Thermal inkjet printers use aqueous ink which is a mixture of water, glycol and dyes. These inks are inexpensive but they can only be used on paper or specially coated materials. Piezoelectric inkjet printers allow the use of a wider range of inks, such as solvent inks, UV-curable inks, dye sublimation inks, and can print text and graphics on different uncoated materials.
Advantages of inkjet printers
- Low cost.
- High quality of output, capable of printing fine and smooth details.
- Capable of printing in vivid color, good for printing pictures.
- Easy to use.
- Reasonably fast.
- Quieter than dot matrix printer.
- No warm up time.
Disadvantages of inkjet printers
- Print head is less durable, prone to clogging and damage.
- Expensive replacement ink cartridges.
- Not good for high volume printing.
- Printing speed is not as fast as laser printers.
- Ink bleeding, ink carried sideways causing blurred effects on some papers.
- Aqueous ink is sensitive to water, even a small drop of water can cause blurring
- Cannot use highlighter marker on inkjet printouts.
(b) Laser printer: Laser printers are non-impact printers which can print text and images in high speed and high quality resolution, ranging from 600 to 1200 dpi. Unlike inkjet printers, laser printer use toner (black or colored powder) instead of liquid inks. A laser printer consists of these major components: drum cartridge, rotating mirror, toner cartridge and roller. The drum cartridge rotates as the paper is fed through. The mirror deflects laser beam across the surface of the drum. Laser beam creates charge that causes the toner to stick to the drum. As the drum rotates and presses on paper, toner is transferred from the drum to paper, creating images. Rollers then use heat and pressure to fuse toner to paper. Colored laser printers add colored toner in three additional passes.
Advantages of laser printers
- High resolution.
- High print speed.
- No smearing.
- Low cost per page (compared to inkjet printers).
- Printout is not sensitive to water.
- Good for high volume printing.
Disadvantages of laser printers
- More expensive than inkjet printers.
- Except for high end machines, laser printers are less capable of printing vivid colors and high quality images such as photos.
- The cost of toner replacement and drum replacement is high.
- Bulkier than inkjet printers.
- Warm up time needed.
c) Thermal printer: Thermal inkjet printer uses heating element to heat liquid ink to form vapor bubble, which forces the ink droplets onto the paper through the nozzle.
Thermal printers use two types of printing technologies: direct thermal and thermal transfer printing. Traditional thermal printers use direct thermal method by pushing electrically heated pins against heat-sensitive paper (thermal paper). The coating on the thermal paper turns black in the areas where it is heated, producing characters or images. Direct thermal printers have no ink, toner or ribbon. These printers are durable, easy to use and cost less to print than other printers. However, the thermal paper is sensitive to heat, light, water, and abrasion and the text and images may fade over time. In thermal transfer printing, a thermal print head applies heat to a heat-sensitive ribbon, which melts ink onto paper and a wide range of materials to form text and images. The printouts can be extremely durable and can be stored over long period of time. Thermal printers are often used in cash registers, ATM and point-of-sales terminals. Thermal printer is not the same as thermal inkjet printer. The latter uses inkjet print technology by heating liquid ink to form vapor bubble, which forces the ink droplet onto the paper from the nozzle.
Plotters are large scale printers that are connected to a computer. They are typically used to print CAD (Computer Aided Drafting) drawings created by engineers and for banner advertisements and other large promotional materials. They are used mainly by industry and businesses who need a flexible, efficient and cost effective printing solution. There are several different models and manufacturers of plotters, including Gerber and Hewlett Packard, the computer company.
Types of Plotter
- Drum Plotter: A drum plotter is pen plotter that wraps the paper around a drum with a pin feed attachment. The drum then rotates the paper as pens move across it and draw the image. It was the first output device used to print graphics and large engineering drawings. There are two types of drum plotters, external and internal. With an external drum plotter, the paper is wrapped around its external surface, while the internal drum plotter uses a sheet of paper wrapped around its internal surface.
- Flat-Bed Plotter: A flat-bed plotter is a mechanical drafting device used with many CAD programs for designers. The paper remains stationary on a flat surface while a pen moves across it horizontally and vertically. This plotter may use several different colors of pens to create the graphics. The size of the graphic is limited to the size of the flat-bed plotter’s surface.
- Inkjet Plotter: The inkjet plotter creates an image by spraying small droplets of ink onto a piece of paper. A popular choice for advertising agencies and graphic designers, inkjet plotters are used generally for large outputs, such as banners and billboards and large signs often seen on roadsides. They are available in thermal or piezoelectric models. Thermal inkjet plotters use heat to apply droplets of ink, while piezoelectric plotters use charged crystals to apply the ink. Inkjet plotters tend to produce better quality graphics than other types of plotters.
Advantages of Plotters
- Speed: Plotters are faster than other types of printing machines, including the desktop printer. The technology allows precision printing at a higher rate of output, and there is typically less need for manual adjustments or maintenance during printing. This also increases the speed or rate of output of the plotter. For a business that is seeking a versatile, fast solution that can produce a large amount of deadline-oriented material, plotters are an optimal choice.
- Versatility: The versatility of plotters is another major advantage. A plotter can be hooked up to any computer. There are a number of plotter configuration options as well, depending on the model and series you buy. This allows you to print and manipulate the plotter in a number of ways, and it also allows it to be connected to any type of machine. Plotters also have their own interfaces in some cases, which allow the user to operate and control them without resetting the paper or having to maintain the plotter during operation.
- Precision: The precision of the plotter is the main advantage for engineering drawings. Plotters have advanced technology that allows them to print more precise lines. When printing a set of drawings for a bridge or skyscraper, it is imperative that each line be precise. Colour accuracy and picture quality are also improved with the overall precision of the plotter.
Disadvantages of Plotter
- They take up more space than regular printers.
- The price of a plotter is much higher than the price of printer.
Speakers are attached to computers for the output of sound. Sound cards are required in the computer for speakers to function. Speakers range from simple, two-speaker output devices to surround-sound multi-channel units.