How Computers Work: Input and Output

The central processing unit is the unseen part of a computer system, and users are only dimly aware of it. But users are very much aware of the input and output associated with the computer. They submit input data to the computer to get processed information, the output.

Sometimes the output is an instant reaction to the input. Consider these examples:

Input and output may sometimes be separated by time or distance or both. Here are some examples:

The examples in this section show the diversity of computer applications, but in all cases the process is the same: input-processing-output. We have already had an introduction to processing. Now, in this chapter we will examine input and output methods in detail.

Input: Getting Data from the User to the Computer

Some input data can go directly to the computer for processing. Input in this category includes bar codes, speech that enters the computer through a microphone, and data entered by means of a device that converts motions to on-screen action. Some input data, however, goes through a good deal of intermediate handling, such as when it is copied from a source document and translated to a medium that a machine can read, such as a magnetic disk. In either case the task is to gather data to be processed by the computer ‹sometimes called raw data and convert it into some form the computer can understand.

A keyboard is usually part of a personal computer or part of a terminal that is connected to a computer somewhere else. Not all keyboards are traditional, however. A fast-food franchise like McDonald's, for example, uses keyboards whose keys represent items such as large fries or a Big Mac. Even less traditional in the United States are keyboards that are used to enter Chinese characters.

A mouse is an input device with a ball on its underside that is rolled on a flat surface, usually the desk on which the computer sits. The rolling movement causes a corresponding movement on the screen. Moving the mouse allows you to reposition the pointer, or cursor, an indicator on the screen that shows where the next interaction with the computer can take place. The cursor can also be moved by pressing various keyboard keys. You can communicate commands to the computer by pressing a button on top of the mouse. In particular, a mouse button is often used to click on an icon, a pictorial symbol on a screen; the icon represents a computer activity-a command to the computer-so clicking the icon invokes the command.

A variation on the mouse is the trackball. You may have used a trackball to play a video game. The trackball is like an upside-down mouse-you roll the ball directly with your hand. The popularity of the trackball surged with the advent of laptop computers, when traveling users found them- selves without a flat surface on which to roll the traditional mouse.

Source Data Automation: Collecting Data Where It Starts
Efficient data input means reducing the number of intermediate steps required between the origination of data and its processing. This is best accomplished by source data automation ‹the use of special equipment to collect data at the source, as a by-product of the activity that generates the data, and send it directly to the computer. Recall, for example, the supermarket bar code, which can be used to send data about the product directly to the computer. Source data automation eliminates keying, thereby reducing costs and opportunities for human-introduced mistakes. Since data about a transaction is collected when and where the transaction takes place, source data automation also improves the speed of the input operation.

For convenience, we will divide this discussion into the primary areas related to source data automation: magnetic-ink character recognition, optical recognition, data collection devices, and even directly by your own voice, finger, or eye. Let us consider each of these in turn.

Magnetic-Ink Character Recognition
Abbreviated MICR, magnetic-ink character recognition is a method of machine-reading characters made of magnetized particles. The most common example of magnetic characters is the array of numbers across the bottom of your personal check.

Most magnetic-ink characters are preprinted on your check. If you compare a check you wrote that has been cashed and cleared by the bank with those that are still unused in your checkbook, you will note that the amount of the cashed check has been reproduced in magnetic characters in the lower-right corner. These characters were added by a person at the bank by using a MICR inscriber.

Figure 1: Flatbed Scanner
An inexpensive way to get entire documents, pictures, and anything on a flat Surface into a computer is by using a scanner. Scanners use optical recognition systems that have a light beam to scan input data to convert it into electrical signals, which are sent to the computer for processing. Optical recognition is by far the most common type of source input, appearing in a variety of ways: optical marks, optical characters, bar codes, handwritten characters, and images. Scanners use Optical Character Recognition software, described below, to translate text on scanned documents into text that is suitable for word processors and other computer applications.

Optical Mark Recognition
Abbreviated OMR, optical mark recognition is sometimes called mark sensing, because a machine senses marks on a piece of paper. As a student, you may immediately recognize this approach as the technique used to score certain tests. Using a pencil, you make a mark in a specified box or space that corresponds to what you think is the answer. The answer sheet is then graded by a device that uses a light beam to recognize the marks and convert them to computer-recognizable electrical signals.

Optical Character Recognition
Abbreviated OCR, optical character recognition devices also use a light source to read special characters and convert them into electrical signals to be sent to the central processing unit. The characters-letters, numbers, and special symbols-can be read by both humans and machines. They are often found on sales tags on store merchandise. A standard typeface for optical characters, called OCR-A, has been established by the American National Standards Institute.

The handheld wand reader is a popular input device for reading OCR-A. There is an increasing use of wands in libraries, hospitals, and factories, as well as in retail stores. In retail stores the wand reader is connected to a point-of-sale (POS) terminal. This terminal is somewhat like a cash register, but it performs many more functions. When a clerk passes the wand reader over the price tag, the computer uses the input merchandise number to retrieve a description (and possibly the price, if not on the tag) of the item. A small printer produces a customer receipt that shows the item description and price. The computer calculates the subtotal, the sales tax (if any), and the total. This information is displayed on the screen and printed on the receipt; notice that both screen and printer are output, so the POS terminal is a complex machine that performs both input and output functions. Finally, some POS terminals include a device that will accept a credit card, inputting account data from the magnetic strip on a customer's charge card.

The raw purchase data becomes valuable information when it is summarized by the computer system. This information can be used by the accounting department to keep track of how much money is taken in each day, by buyers to determine what merchandise should be reordered, and by the marketing department to analyze the effectiveness of their ad campaigns.

Bar Codes
Each product on the store shelf has its own unique number, which is part of the Universal Product Code (UPC). This code number is represented on the product label by a pattern of vertical marks, or bars, called bar codes. (UPC, by the way, is an agreed-upon standard within the supermarket industry; other kinds of bar codes exist. You need only look as far as the back cover of this book to see an example of another kind of bar code.) These zebra stripes can be sensed and read by a bar code reader, a photo- electric device that reads the code by means of reflected light. As with the wand reader in a retail store, the bar code reader in a bookstore or grocery store is part of a point-of-sale terminal. When you buy, say, a can of corn at the supermarket, the checker moves it past the bar code reader. The bar code merely identifies the product to the store's computer; the code does not contain the price, which may vary. The price is stored in a file that can be accessed by the computer. (Obviously, it is easier to change the price once in the computer than to have to repeatedly restamp the price on each can of corn. ) The computer automatically tells the point- of-sale terminal what the price is; a printer prints the item description and price on a paper tape for the customer. Some supermarkets are moving to self-scanning, putting the bar code reader-as well as the bagging-in the customer's hands.

Although bar codes were once found primarily in supermarkets, there are a variety of other interesting applications. Bar coding has been described as an inexpensive and remarkably reliable way to get data into a computer. It is no wonder that virtually every industry has found a niche for bar codes. In Brisbane, Australia, bar codes help the Red Cross manage their blood bank inventory. Also consider the case of Federal Express. The management attributes a large part of the corporation's success to the bar-coding system it uses to track packages. Each package is uniquely identified by a ten-digit bar code, which is input to the computer at each point as the package travels through the system. An employee can use a computer terminal to query the location of a given shipment at any time; the sender can request a status report on a package and receive a response within 30 minutes. The figures are impressive: In regard to controlling packages, the company has an accuracy rate of better than 99 percent.

Handwritten Characters
Machines that can read handwritten characters are yet another means of reducing the number of intermediate steps between capturing data and processing it. In many instances it is preferable to write the data and immediately have it usable for processing rather than having data entry operators key it in later. However, not just any kind of handwritten scrawl will do; the rules as to the size, completeness, and legibility of the handwriting are fairly rigid.

In a process called imaging, a scanner converts a drawing, a picture, or any document into computer-recognizable form by shining a light on the image and sensing the intensity of the reflection at each point of the image. Scanners come in both handheld and desktop models. The electronic version of the image can then be stored, probably on disk, and reproduced on screen when needed. Businesses find imaging particularly useful for documents, since they can view an exact replica of the original document at any time. If a text image is run through an optical character recognition (OCR) program, then all words and numbers can be manipulated by word processing and other software. The Internal Revenue Service, using imaging and also OCR software that can recognize characters from the image, is now scanning 17,000 tax returns per hour, a significant improvement over hand processing.

Another way to keep photos computer accessible is to have film that was shot with a conventional camera processed onto optical disk instead of prints or slides. Professional photo agencies keep thousands of images on file, ready to be leased for a fee. Typically, a couple of dozen thumbnail-size images can be displayed on the screen at one time; a particular image can be enlarged to full-screen size with a click of a mouse button.

Data Collection Devices
Another source of direct data entry is a data collection device, which may be located in a warehouse or factory or wherever the activity that is generating the data is located. As we noted earlier in the chapter, for example, factory employees can use a plastic card to punch job data directly into a computerized time clock. This process eliminates intermediate steps and ensures that the data will be more accurate.

Data collection devices must be sturdy, trouble-free, and easy to use because they are often located in dusty, humid, or hot or cold locations. They are used by people such as warehouse workers, packers, forklift operators, and others whose primary work is not clerical. Examples of remote data collection devices are machines for taking inventory, reading shipping labels, and recording job costs.

Voice Input
Does your computer have ears? Speaking to a computer, known as voice input or speech recognition, is another form of source input. Speech recognition devices accept the spoken word through a microphone and convert it into binary code (0s and 1s) that can be understood by the computer. Originally, typical users were those with "busy hands," or hands too dirty for the keyboard, or with no access to a keyboard. Such uses are changing radio frequencies in airplane cockpits, controlling inventory in an auto junkyard, reporting analysis of pathology slides viewed under a microscope, asking for stock-market quotations over the phone, inspecting items moving along an assembly line, and allowing physically disabled users to issue commands.

Most speech recognition systems are speaker-dependent; that is, they must be separately trained for each individual user. The speech recognition system "learns" the voice of the user, who speaks isolated words repeatedly. The voiced words the system "knows" are then recognizable in the future.

Speech recognition systems that are limited to isolated words are called discrete word systems, and users must pause between words. Experts have tagged speech recognition as one of the most difficult things for a computer to do. Eventually, continuous word systems will be able to interpret sustained speech, so users can speak normally; so far, such systems are limited by vocabulary to a single subject, such as insurance or the weather. A key advantage of delivering input to a computer in a normal speaking pattern is ease of use. Such systems may also be propelled by the explosion of hand and wrist ailments associated with extensive computer keying. Today, software is available to let computers take dictation from people who are willing to pause . . . briefly . . . between . . . words; the best systems are quite accurate and equivalent to typing 70 words per minute.

Touch Screens
One way of getting input directly from the source is to have a human simply point to a selection. The edges of the monitor of a touch screen emit horizontal and vertical beams of light that criss-cross the screen. When a finger touches the screen, the interrupted light beams can pinpoint the location selected on the screen. Kiosks in public places such as malls offer a variety of services via touch screens. An insurance company kiosk will let you select a policy or a government kiosk will let you order a copy of your birth certificate. Kiosks are also found in private stores. Wal-Mart, for example, uses a kiosk to let customers find needed auto parts. Many delicatessens let you point to salami on rye, among the other selections.

Delivering input to a computer by simply looking at the computer would seem to be the ultimate in capturing data at the source. The principles are reminiscent of making a screen selection by touching the screen with the finger. Electrodes attached to the skin around the eyes respond to movement of the eye muscles, which produce tiny electric signals when they contract. The signals are read by the computer system, which determines the location on the screen where the user is looking.

Such a system is not yet the mainstream. The first people to benefit would likely be those who, due to disabilities or busyness, cannot use their hands or voices for input.

Output: Information for the User

As we have seen, computer output takes the form of screen or printer output. Other forms of output include voice, microfilm, and various forms of graphics output.

A computer system often is designed to produce several kinds of output. An example is a travel agency that uses a computer system. If a customer asks about airline connections to Toronto, Calgary, and Vancouver, say, the travel agent will probably make a few queries to the system to receive on-screen output indicating availability on the various flights. After the reservations have been confirmed, the agent can ask for printed output that includes the tickets, the traveler's itinerary, and the invoice. The agency may also keep the customer records on microfilm. In addition, agency management may periodically receive printed reports and charts, such as monthly summaries of sales figures or pie charts of regional costs. We begin with the most common form of output, computer screens.

Computer Screen Technology
A user's first interaction with a computer screen may be the screen response to the user's input. When data is entered, it appears on the screen. Furthermore, the computer response to that data-the output-also appears on the screen. Computer screens come in many varieties, but the most common kind is the cathode ray tube (CRT). Most CRT screens use a technology called raster-scan technology. The backing of the screen display has a phosphorous coating, which will glow whenever it is hit by a beam of electrons. But the light does not stay lit very long, so the image must be refreshed often. If the screen is not refreshed often enough, the fading screen image appears to flicker. A scan rate-the number of times the screen is refreshed-of 60 times per second is usually adequate to retain a clear screen image. As the user, you tell the computer what image you want on the screen, by typing, say, the letter M, and the computer sends the appropriate image to be beamed on the screen. This is essentially the same process used to produce television images.

A computer display screen that can be used for graphics is divided into dots that are called addressable, because they can be addressed individually by the graphics software. Each dot can be illuminated individually on the screen. Each dot is potentially a picture element, or pixel. The resolution of the screen, its clarity, is directly related to the number of pixels on the screen: The more pixels, the higher the resolution. Some computers come with built-in graphics capability. Others need a device, called a graphics card or graphics adapter board, that has to be added.

There have been several color screen standards, relating particularly to resolution. The first color display was CGA (color graphics adapter), which had low resolution by today's standards (320x200 pixels). This was followed by the sharper EGA (enhanced graphics adapter), featuring 640x350 pixels. Today, VGA and SVGA are common standards. VGA (video graphics array) has 640x480 pixels. SVGA (super VGA) offers 800x600 pixels or 1024x768 pixels, by far the superior clarity.

Is bigger really better? Screen sizes are measured diagonally. Many personal computers come with a 15 inch screen. A 15 inch screen is fine for most single applications, but for applications with large graphics, or for having multiple windows open, it is sometimes inadequate. For a few hundred dollars more, 17 inch can be better. There are even bigger screens that cost substantially more. Bigger is usually better, but more expensive.
Types of Screens
Cathode ray tube monitors that display text and graphics are in common use today. Although most CRTs are color, some screens are monochrome, meaning only one color, usually green, appears on a dark background. Another type of screen technology is the liquid crystal display (LCD), a flat display often seen on watches and calculators. LCD screens are used on laptop computers. Some LCDs are monochrome, but color screens are popular. Some laptop screens are nearing CRTs in resolution quality.

A screen may be the monitor of a self-contained personal computer, or it may be part of a terminal that is one of many terminals attached to a large computer. A terminal consists of an input device, an output device, and a communications link to the main computer. Most commonly, a terminal has a keyboard for an input device and a screen for an output device, although there are many variations on this theme.

A printer is a device that produces printed paper output, known in the computer industry as hard copy because it is tangible and permanent (unlike soft copy, which is displayed on a screen). Some printers produce only letters and numbers, whereas others can also produce graphics.

Letters and numbers are formed by a printer either as solid characters or as dot-matrix characters. Dot-matrix printers create characters in the same way that individual lights in a pattern spell out words on a basketball scoreboard. Dot-matrix printers construct a character by activating a matrix of pins that produce the shape of the character. A traditional matrix is 5x7-that is, five dots wide and seven dots high. These printers are sometimes called 9-pin printers, because they have two extra vertical dots for descenders on the lowercase letters g, j, p, and y. The 24-pin dot-matrix printer, which uses a series of overlapping dots, dominates the dot-matrix market. The more dots, the better the quality of the character produced. Some dot-matrix printers can produce color images.

There are two ways of printing an image on paper: the impact method and the non-impact method. Let us take a closer look at the difference.

Impact Printers
The term impact refers to the fact that impact printers use some sort of physical contact with the paper to produce an image, physically striking paper, ribbon, and print hammer together. The impact may be produced by a print hammer character, like that of a typewriter key striking a ribbon against the paper, or by a print hammer hitting paper and ribbon against a character. A dot-matrix printer is one example of an impact printer. High- quality impact printers print only one character at a time.

However, users who are more concerned about high volume than high quality usually use line printers - impact printers that print an entire line at a time. Organizations that use mainframe and minicomputers usually have several line printers. Such organizations are likely to print hearty reports, perhaps relating to payroll or costs, for internal use. The volume of the report and the fact that it will not be seen by customers makes the speedy-and less expensive line printer appropriate. One final note about impact printers: An impact printer must be used if printing a multiple-copy report so that the duplicate copies will receive the imprint.

Non-impact Printers
A non-impact printer places an image on a page without physically touching the page. The major technologies competing in the non-impact market are laser and ink-jet. Laser printers use a light beam to help transfer images to paper, producing extremely high-quality results. Laser printers print a page at a time at impressive speeds. Large organizations use laser printers to produce high-volume customer-oriented reports. At the personal computer end, low-end black and white laser printers can now be purchased for a few hundred dollars. However, color laser jet printers are more expensive.

The rush to laser printers has been influenced by the trend toward desktop publishing-using a personal computer, a laser printer, and special software to make professional-looking publications, such as newsletters.

Ink-jet printers, by spraying ink from multiple jet nozzles, can print both black and white and in several different colors of ink to produce excellent graphics. As good as they are, color printers are not perfect. The color you see on your computer screen is not necessarily the color you will see on the printed output. Nor is it likely to be the color you would see on a four-color offset printing press. Nevertheless, with low-end printers now under $250, they may be a bargain for users who want their own color output capability.

There are many advantages to non-impact printers over impact ones, but there are two major reasons for their growing popularity: They are faster and quieter. Other advantages of non-impact printers over conventional mechanical printers are their ability to change typefaces automatically and their ability to produce high-quality graphics.

Voice Output
We have already examined voice input in some detail. As you will see in this section, however, computers are frequently like people in the sense that they find it easier to talk than to listen. Speech synthesis is the process of enabling machines to talk to people is much easier than speech recognition. "The key is in the ignition," your car says to you as you open the car door to get out. Machine voices are not real human voices. They are the product of voice synthesizers (also called voice-output devices or audio-response units), which convert data in main storage to vocalized sounds understandable to humans.

There are two basic approaches to getting a computer to talk. The first is synthesis by analysis, in which the device analyzes the input of an actual human voice speaking words, stores and processes the spoken sounds, and reproduces them as needed. The process of storing words is similar to the digitizing process we discussed earlier when considering voice input. In essence, synthesis by analysis uses the computer as a digital tape recorder.

The second approach to synthesizing speech is synthesis by rule, in which the device applies a complex set of linguistic rules to create artificial speech. Synthesis based on the human voice has the advantage of sounding more natural, but it is limited to the number of words stored in the computer.

Voice output has become common in such places as airline and bus terminals, banks, and brokerage houses. It is typically used when an inquiry is followed by a short reply (such as a bank balance or flight time). Many businesses have found other creative uses for voice output as it applies to the telephone. Automatic telephone voices ("Hello, this is a computer speaking. . . " ) take surveys, inform customers that catalog orders are ready to be picked up, and, perhaps, remind consumers that they have not paid their bills.

Music Output
Personal computer users have occasionally sent primitive musical messages, feeble tones that wheezed from the tiny internal speaker. Many users remain at this level, but a significant change is in progress.

Professional musicians lead the way, using special sound chips that simulate different instruments. A sound card, installed internally in the computer, and attached speakers complete the output environment. Now, using appropriate software, the computer can produce the sound of an orchestra or a rock band. Those of us who simply enjoy music can have a full sight/sound experience using multimedia, which we will explore in detail in the next chapter.

Computer Graphics

Let us take a moment to glimpse everyone's favorite, computer graphics. Just about everyone has seen TV commercials or movies that use computer-produced animated graphics. Computer graphics can also be found in education, computer art, science, sports, and more. But perhaps their most prevalent use today is in business.

Business Graphics
It might seem wasteful to use color graphics to display what could more inexpensively be shown to managers as numbers in standard computer printouts. However, colorful graphics, maps, and charts can help managers compare data more easily, spot trends, and make decisions more quickly. Also, the use of color helps people get the picture-literally. Finally, although color graphs and charts have been used in business for years-usually to make presentations to higher management or outside clients-the computer allows them to be rendered quickly, before information becomes outdated. One user refers to business graphics as "computer- assisted insight."

Video Graphics
Video graphics can be as creative as an animated cartoon. Although they operate on the same principle as a moving picture or cartoon-one frame at a time in quick succession video graphics are produced by computers. Video graphics have made their biggest splash on television, but many people do not realize they are watching a computer at work. The next time you watch television, skip the trip to the kitchen and pay special attention to the commercials. Unless there is a live human in the advertisement, there is a good chance that the moving objects you see, such as floating cars and bobbing electric razors, are computer output. Another fertile ground for video graphics is a television network's logo and theme. Accompanied by music and swooshing sounds, the network symbol spins and cavorts and turns itself inside out, all with the finesse that only a computer could supply.

Computer-Aided Design/Computer-Aided Manufacturing
For more than a decade, computer graphics have also been part and parcel of a field known by the abbreviation CAD/CAM-short for computer- aided design/computer-aided manufacturing. In this area computers are used to create two- and three-dimensional pictures of everything from hand tools to tractors. CAD/CAM provides a bridge between design (planning what a product will be) and manufacturing (actually making the planned product). As a manager at Chrysler said, "Many companies have design data and manufacturing data, and the two are never the same. At Chrysler, we have only one set of data that everyone dips into." Keeping data in one place, of course, makes changes easier and encourages consistency.

Graphics Input Devices
There are many ways to produce and interact with screen graphics. We have already described the mouse; the following are some other common devices that allow the user to interact with screen graphics. A digitizing tablet lets you create your own images. This device has a special stylus that you can use to draw or trace images, which are then converted to digital data that can be processed by the computer.

For direct interaction with your computer screen, the light pen is ideal. It is versatile enough to modify screen graphics or make a menu selection-that is, to choose from a list of activity choices on the screen. A light pen has a light-sensitive cell at one end. When you place the light pen against the screen, it closes a photoelectric circuit that pinpoints the spot the pen is touching. This tells the computer where to enter or modify pictures or data on the screen.

Finally, a well-known graphics input device is the joystick, dear to the hearts of video game fans. This device allows fingertip control of figures on a CRT screen.

Graphics Output Devices
Just as there are many different ways to input graphics to the computer, there are many different ways to output graphics. Graphics are most commonly output on a screen or printed paper, as previously discussed. Another popular graphics output device is the plotter, which can draw hard-copy graphics output in the form of maps, bar charts, engineering drawings, and even two- or three-dimensional illustrations. Plotters often come with a set of four pens in four different colors. Most plotters also offer shading features.

New forms of computer input and output are announced regularly, often with promises of multiple benefits and new ease of use. Part of the excitement of the computer world is that these promises are usually kept, and users reap the benefits directly. Input and output just keep getting better.