A monitor, the most important output device, is a computer display screen. Early monitors displayed white or a single color on a black background, but current monitors can display as many colors as the human eye can distinguish. There are two kinds of monitors: CRT and LCD.
A CRT is a cathode ray tube monitor, which means it works like a television. At one end of the tube is a device that produces a stream of electrons. At the other end is a screen that has been coated with substances that briefly glow different colors when struck by electrons.
A device that aims the electron beam scans the screen left to right, top to bottom, and the beam is turned on or off to make some of the screen glow or not. CRT screens are inexpensive, but because the electron beam has to be shot from well behind the screen, monitors of this type tend to be very deep and thus take a lot of space on the desktop. Also, CRT screens are gently curved instead of flat, which means that the images they display appear to bend at the edges.
An LCD is a liquid crystal display. Liquid crystal, as the name implies, has properties of both a liquid and a solid. The important feature for its use in monitors is that it turns solid when exposed to an electrical current, and it is opaque when in the solid state but clear when in the liquid state. A two-color LCD display uses a grid of liquid crystal cells in front of a bright light source, and current is sent to the cells that should be darker. More advanced displays can vary the level of transparency to make a gray-scale image. For a color display, filters are placed over the display to divide it into red, green, and blue columns.
CD monitors were originally designed for laptop computers, where a CRT was too big to be practical. Now, they are gaining popularity with desktop computers because they take up less space and the screens are perfectly flat. A printer outputs text or images to paper. Different types of printers exist for different needs. A printer used for a term paper should be fast, while a printer for photographs must print in high-quality color.
A dot-matrix printer works by pressing an inked ribbon against the paper with a set of pins. The ribbon used is like that in a typewriter. As the print head moves across the paper, pins on the head fire out, pressing the ribbon against the paper to make a dot.
These dots make up the image on the paper. As the print head reaches one side of the paper, the paper is advanced the height of the print head, and the print head continues in the opposite direction. Dot-matrix printers, which are inexpensive to purchase and operate, are very fast, but the output quality is poor because it's easy to see the dots in the image. They are often used by companies for items like billing statements, which are printed in bulk.
An ink-jet printer works by spraying ink at the paper. It is much slower than a dot-matrix printer because it makes more passes to produce the same size image, but it has several advantages. It produces a better-looking image than a dot-matrix printer because the image is not made of distinct dots. Also, some ink-jet printers can print in color, with red, yellow, and blue inks in separate reservoirs and a separate black ink reservoir because producing black by mixing all three colors wastes a lot of ink.
Ink-jet printers are inexpensive to purchase, but their cost per page of use is higher than a dot-matrix. Also, the images they produce can smear if the paper is handled soon after printing because the ink is still wet. These are the default printers for home users and for business users who only print occasionally.
A laser printer works using electrostatic principles to transfer ink to the paper. It's not as complicated as it sounds. A drum inside the printer rotates past a laser beam that is scanning back and forth. The drum is made of a material that becomes electrostatically charged where it is struck by the scanning laser. Anyone who has taken clothes from a hot dryer knows that static electricity can make things stick together. That's how it is used here. The drum is sprayed with toner, which is a powder form of ink that has been given the opposite charge of the drum. The toner sticks where the laser hits on the drum.
A piece of paper is then rolled against the drum, transferring the toner to it. If the process stopped at this point, the image wouldn't stay on the paper long because there's only static electricity holding the toner in place. The last stage in the cycle is the fuser, which is a set of very hot rollers that melt the toner into the paper. If you've ever wondered what a laser printer is doing when you turn it on and a blinking light indicates it isn't ready yet, it is waiting for the fuser to reach the right temperature.
The print quality of laser printers is excellent. Although they are more expensive than ink-jet printers for an initial purchase, the cost of ink can be less because expensive toner cartridges last much longer than ink-jet cartridges. Also, because the ink is fused into the paper, rather than sprayed on wet like an ink-jet, the image doesn't smear. In addition, laser printers are fast. They are often used for business communications, where print quality counts, although prices have gone down enough that home users who want higher quality purchase them also.
While most laser printers produce black-and-white output, some can produce color. These printers work by running the paper through the whole process four times, one pass each for red, yellow, blue, and black toner. Color laser printers are very expensive. For high-quality color printing, though, few printers can match a dye sublimation printer.
Sublimation means going from a solid directly to a gas, such as a block of dry ice turning into a fog without melting into a puddle first. A dye sublimation printer works by heating a ribbon so that the solid ink inside turns into a gas and then seeps into the paper. The ribbon has strips of different color inks in it to produce the different colors and has a back-and-forth print head like a dot-matrix or ink-jet printer.
Print quality is excellent with dye sublimation printers. In many cases the result is indistinguishable from a traditional photograph. Because the ink seeps into the paper as a gas and then dries, it tends to spread out a little so that the adjacent colors mix together. In contrast, a color ink-jet printer makes little dots of different colors. If you stand away from the image, the colors mix together, but move too close and the painting becomes just a bunch of dots. Dye sublimation makes a smoothly colored image. However, dye sublimation printers are very expensive. They are only needed for professional quality photographic printing.
Not all computer output is visual. Some of it is aural. Most computer systems are capable of producing sound through speakers. Computer speakers are no different in construction than those in a normal stereo system. Computers use sound to enhance or reinforce what's happening on the monitor. For example, most e-mail programs make a little "ding" sound when a new piece of mail arrives. This sound gives the user a cue without having to interrupt the user's work. Computer games also use sound to more fully envelop the user in the game's reality.
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02122011
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