Although the information-processing application itself determines what the content of the output will be, the format of the output depends, in part, on the output device. For example, in the case of generating a simple report, the choices include

·                    Printing or video display.

·                    Text and/or graphics.

·                    Black and white or colour.

Which type of output to use is a deci­sion that depends on the desired use of the information, the cost of producing that information, and whether the re­quired technology is available. When se­lecting an output device, it is useful to understand the classifications of differ­ent output devices along with their capa­bilities and limitations. Output from a computer system comes in hard copy, soft copy, or ma­chine-readable form.

Hard copy refers to the recording of letters, graphics, or pictures on a hard medium, such as paper or film. Ex­amples of hard copy include printing, drawing lines with a plotter, or photo­graphically recording an impression on film or microfilm.

Soft copy refers to the use of elec­tronic signals to display a temporary im­age on a surface such as a video screen..

The most common hard-copy devices are printers. There are some common characteristics that should be examined when matching a printer to an application. They include:

·                    The ability to print images as well as text.

·                    The quality or the resolution of the printed image.

·                    The speed of the printer.

·                    Whether the printer requires special paper.

Printers are classified into two broad categories: impact and nonimpact.

The most common type of impact printer is the dot-matrix printer. The essential part of a dot-matrix printer is a print head, which contains a column of steel pins. Images are formed by driving various combinations of pins against a ribbon and onto the paper. The two main advantages of dot-matrix printers are their low cost and the flexibility with which they can produce images. A dot-matrix printer can print images as well as text. Because characters or images are pro­duced by a selected pattern of dots, these printers can also change the size and style of characters and add emphasis such as bold, italic, or underlining. With the addition of multicoloured ribbons, the printer can produce text and graph­ics in several colours.

The speed of dot-matrix printers var­ies considerably depending on what is being printed and the model of the printer. When printing draft-quality text, some dot-matrix printers can operate in the range of 200 to 400 characters per second. When printing near-letter-qual­ity text or graphics, low-

cost dot-matrix printers slow down to a speed of 40 to 80 characters per second.

Line printers are another class of impact printers, which are used prima­rily in large computer installations for high-speed printing of reports and forms. Unlike slower printers that print only one character at a time, these print­ers set up an entire line of print at once. Although they still print character by character or dot by dot, the process hap­pens so fast that it appears to be printing one line at a time.

A plotter is an output device that pro­duces an image on paper by controlling the motion of a pen carriage that draws lines. The plotter sees the output me­dium of paper as a series of X, Y coordi­nates. If you plotted similar coordinates on a piece of graph paper and drew lines connecting the points, you would have an approximate visual metaphor for how a plotter draws lines. The computer sys­tem sends the plotter a series of num­bers representing^, ^’coordinates in the desired pattern. The pen carriage then moves to the different positions while holding the pen on the surface of the paper. Curves are drawn by linking a series of very short straight lines to­gether.

Part 2.

Another category of printers is nonimpact printers. They operate on the same dot-forming principle as the dot-matrix printer; however, instead of using an impact mechanism, they use nozzles, heat, electricity, magnetism, or optical methods to transfer an image onto paper.

Inkjet printers form characters or graphics with a print head containing tiny nozzles or jets that spray drops of ink onto the paper. The effect is similar to dot-matrix printing.

An inkjet printer is capable of printing colour images. By using a separate nozzle and ink cartridge for each of the three subtractive primary colours—cyan (dark blue), magenta, and yellow—plus a fourth for black, up to six different hues can be printed. The colour inkjet printer is an improvement over the problems of ribbons with multiple colour bands that are required for colour dot-matrix print­ers. Very expensive inkjet printers pro­duce high-quality colour, which is well suited for low-volume publishing, proof copies and overhead transparencies.

Besides colour, other advantages of inkjet printers include quiet operation and high reliability, thanks mainly to fewer moving parts. A possible disadvan­tage is the problem of nozzle clogging.

A new variation of the inkjet printer is called a bubble-jet printer. With bubble-jet technology, the printing element is a computer chip with miniature openings, each with its own heating element. By heating the ink and forcing it through the opening, a small bubble is created. The bubble makes a more precise mark on the paper with less scattering of ink droplets than a conventional inkjet printer. The result is a sharper image.

Laser printers create images with a laser beam scanning across an electri­cally charged drum. As in a copier, toner or ink with an opposite charge sticks to the drum and is then transferred to pa­per by pressure and heat to create the finished image. In the past, laser printers were limited to minicomputer and main­frame applications that needed high-vol­ume, high-quality printing.

Laser printers offer near-typeset-quality text, medium-resolution graphics ca­pabilities, and higher speed than con­ventional impact or nonimpact printers. For example, most of the low-cost laser printers can print eight pages a minute at a resolution of 300 or 600 dots per inch. All can mix type styles and sizes as well as graphic images on the same page.

Laser printers are quite  different from conventional printers. They contain their own built-in microprocessor and RAM and ROM memories in order to produce high print speed and resolu­tion. For example, to print one page of graphic images at a resolution of 300 dots per inch, each square inch requires 90,000 dots. An 8-by-10-inch piece of pa­per would require 7.2 million dots. If each dot is equivalent to a bit, it would take slightly less than one megabyte of memory to store a single page. Because these printers process and print an en­tire page at a time, they are often called page printers.

A close cousin of the laser printer uses an array of light-emitting diodes (LEDs) instead of the beam of light emitted by a laser. LED printers feature a simpler de­sign than laser printers, which translates into higher reliability and lower costs.

A difference among laser printers is whether characters are produced by us­ing selected patterns of dots, called bitmaps, or by using mathematical de­scriptions, called outlines. Printers that feature the latter use a page-descrip­tion language—a language that is used to describe output to a printer or display device.

The language includes commands for describing individual character shapes, drawing lines and polygons, and scaling (i.e., making an object appear larger or smaller). When you print on a printer that is equipped with a page description-language, the language takes care of the behind-the-scenes technical details. It constructs a program describing the pages and sends it to the printer. When the printer receives the program, it ex­ecutes the commands and produces the output. The result is more precise con­trol over the way each page is printed and improved quality of the output.