You may think of your computer as an object or a single thing, but it's not. It's more of an ecosystem or a collective, made up of multiple smaller devices. A lot of those devices are used to move information around, and they're described – logically enough – as input and output devices. No list of input and output examples could hope to be complete, but the common ones fall into a relatively small number of categories.
Input vs. Output in Computer Terms
Any computer, if you strip it down to its basic functions, is a tool for doing things with information. For it to work, that information has to get to the processor and then, once it has been processed, the computer has to do something with the results. That's a simple way to explain a complex process, but it explains the difference between input and output devices. If it provides information to the processor, it's an input device, and if it accepts information from the processor, it's an output device.
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Examples of Input Devices
The huge number of input devices used with computers can be broken down into categories in a number of ways, depending on how you classify them. One simple way is to separate the ones you use yourself, hands-on, from the ones that convert analog information to digital data for the computer's use.
Manual Input Devices
As a rule, anything you use your hands with is probably an input device. These manual input devices fall into a couple of broad categories:
- Keyboards: On a desktop or laptop computer, your keyboard is usually the most important input device. It's usually how you type information into your computer or do anything creative with words and numbers. Some keyboards connect with a cable to the USB port, and some are wireless. Some are designed to be ergonomic, meaning they're intended to keep your hands in a natural posture to reduce the risk of repetitive stress injuries, but other than that, they're all rather similar.
- Mouse and mouse-like pointing devices: The computer mouse has been in wide use since the 1980s for clicking or selecting items on your screen. You move the mouse, and the on-screen pointer moves; then, you click the buttons to do what you want. Manufacturers offer a range of alternative devices to do the same job, from trackballs – a sort of upside-down mouse that stays still while you roll a ball with your thumb to move the cursor – to the touchpads of most laptops and the TrackPoint device used on ThinkPads.
- Digitizing tablets: These are a variation on the theme of a touch screen or touchpad, but they're more sophisticated. Designed for graphic artists, they enable users to draw, sketch or paint on a surface at high resolutions with the use of a stylus.
- Game controllers: Some games work just fine with a keyboard and mouse, but others need something closer to the controllers on a game console. The computer versions might be anything from conventional thumb-pad controllers to elaborate joysticks. Specialized controllers such as steering wheels and pedals for driving games or control yokes for flight simulators are available for serious enthusiasts.
- Remote controls: A few entertainment-oriented computers even come with a TV-style remote control, which can be used to play DVDs or to select and stream media through a program like Kodi or Plex. This is also an input device, in its way.
Analog-to-Digital Input Devices
Computers can work with just about any kind of information, but it has to be in a digital format. Real-world objects and stimuli are just about always analog, so a whole other class of input devices converts those analog inputs into something the computer can work with:
- Microphones: Microphones capture sound waves, which are analog, and turn them into electrical signals. A digital-to-analog converter or DAC in the mic itself or on the computer converts it further into digital format. From there, the computer might record the sound or pass it through to the internet or your local network in real time.
- Scanners: Scanners do much the same thing but for visual input. Flatbed scanners and sheet-fed scanners do this with hard copy text or printed images, converting the image or the text into a data format the computer can use. The bar code scanners you see at stores are a variation on the same idea, reading codes from products and converting them to data.
- Cameras: Your computer's webcam or any digital camera you use to transfer images to your computer – even your phone – is another input device. It captures a digital representation of the analog world around you and then sends it to your computer through a wired or wireless connection.
Examples of Output Devices
A list of output devices will always be shorter than the list of input devices. Computers can receive information in any number of ways, as long as they're digital, but most output devices talk to humans, and we only have five senses. Taste and smell don't factor into mainstream computing at this point, so that leaves sight, hearing and touch as the ways a computer can send information back to us. All three are used, in differing ways.
- Your monitor: Older monitors used a glass cathode ray tube, or CRT, to display an image. Modern screens are usually LED panels, although the OLED and AMOLED technologies used on phones may be scaled up for computer use at some point. They translate the computer's output into images and text that humans can process. They're judged by their brightness, their resolution in dots per inch, their contrast and accuracy of color, and by how quickly they can refresh.
- Speakers and headphones: Speakers and headphones provide audible feedback to let you know what the computer's doing. Early computers used sound mostly as a way to alert users to a problem, but modern machines are more versatile. They can play music or the audio portion of a movie. They can read audiobooks to you or convert text to speech, and they provide the soundtrack and sound effects for games. The best computer speaker systems rival full-scale home theater audio, with subwoofers and multispeaker setups for immersive sound.
- Printers and plotters: Printers and plotters take a computer's output and translate it into hard copy form, printing on paper, transparencies, labels and other media. Most modern printers use either laser technology, which is much the same as a photocopier's output, or inkjet technology, which uses fine drops of ink. Plotters are a specialized form of printer designed to create large-format engineering drawings and similarly oversized graphics.
- 3-D printers: A newer output device is the 3-D printer, which straddles the line between consumer/hobbyist devices and professional engineering tools. It turns the computer's output into physical objects, usually built up from plastic or resin that can be extruded while hot and then becomes rigid when it cools.
- Haptic feedback devices: You may not recognize this one by name, but you've probably used one. These are the small vibrating devices used by phones, tablets and other small devices to let you know when something happens. It might be the silent ring of your phone, or your Fitbit's hourly reminder to get up and move around. They're not used much in conventional computers but are worth mentioning because they're so common in the mobile computing world.
Some Devices Perform Input and Output
Although the line between input and output devices seems clear enough, that's not always the case in the real world. Some devices have both input and output functions, so they don't fit neatly into one category or the other. They can be divided into two broad groups: the ones that you use and the ones the computer uses.
Two-Way Consumer Devices
Most of the consumer devices you work with yourself combine an input function and an output function in one unified package. A few examples include:
- Audio headsets: Gamers and video chat-centric users are familiar with these convenient devices, which combine a microphone with an in-ear or over-ear headphone and often a band or clip to keep it in place. They're available in wired or wireless versions, so you can sit or pace while you use them. They're useful for video calls, voice recognition programs and networked gaming with your friends.
- Multifunction printers: These combine a conventional inkjet or laser printer with a scanner and sometimes other features as well. They're convenient for anyone with limited space or with only an occasional need to use the second function. It saves the space and cost of having separate devices for each job.
- Touch screens: Essential on a mobile device and handy on computers with touch-aware operating systems, touch screens display information like any other monitor but also accept input directly on-screen.
- MIDI interfaces and instruments: MIDI stands for Musical Instrument Digital Interface, which is self-explanatory. With MIDI equipment, musicians can play their instruments and have the sound converted into digital data, as opposed to sound recordings, which can then be manipulated and played back through the MIDI device itself. In practice, they're mostly used as input devices, but they work both ways.
Computer I/O Devices
Other devices with two-way functionality are built into a computer, and they play an important role in how it works. These are referred to formally as input/output devices, or I/O devices. The ones you can find on almost any computer include:
- Network interface cards: Most computers come out of the box with two kinds of networking built in, a wireless interface for Wi-Fi and a wired connection you can use to connect directly to your router or other devices. Older computers often had a modem built in as well, which is also an I/O device, but now, you usually connect your whole home network to a single external modem and router.
- Sound cards: Sound cards were an optional extra decades ago, but they're routinely built into modern computers. If your computer has a headphone jack and microphone jack, it has a sound card built in. The mic jack is an input, and the headphone jack is an output; therefore they're I/O devices. Demanding users can add a higher-end sound card to a desktop or tower through one of its PCIe expansion cards referred to as the computer's I/O bus.
- Storage devices: The drives you use in your computer are another set of I/O devices. They store information from your computer and load information to your computer constantly while it's in use. These include the hard drive, a DVD-ROM drive if you have one, and any USB thumb drives or memory cards you use with the computer.
Specialized Input and Output Devices
The number of specialized input and output devices is nearly limitless because there are so many niche markets, but they're all fairly similar to the main categories listed here. You could think of the tiny fiber-optic camera a surgeon threads through your veins as a refined webcam, for example, and the big CNC machine in a factory is similar in function to a 3-D printer.
Others, such as the assistive input and output devices designed for users with special needs, show a real genius at combining technologies in useful ways. Visually impaired computer users might rely on a screen reader that converts text to speech or Braille on a specialized touchable pad. Voice recognition software turns a microphone into an alternative keyboard. Those who can't use their hands can use sip-and-puff controls to navigate the screen or specialized devices that detect head movement or brain waves. For those with some mobility, joysticks, trackballs, and wands and sticks using various wireless technologies can take the place of a conventional mouse or touchpad.