How the iPhone Works

In January 2007, Steve Jobs introduced the Apple iPhone during his keynote address at the Macworld Conference and Expo. In its first appearance onscreen and in Jobs's hand, the phone looked like a sleek but inanimate black rectangle.

Then, Jobs touched the screen. Suddenly, the featureless rectangle became an interactive surface. Jobs placed a fingertip on an on-screen arrow and slid it from left to right. When his finger moved, the arrow moved with it, unlocking the phone. To some people, this interaction between a human finger and an on-screen image -- and its effect on the iPhone's behavior -- was more amazing than all of its other features combined.

And those features are plentiful. In some ways, the iPhone is more like a palmtop computer than a cellular phone. As with many smartphones, you can use it to make and receive calls, watch movies, listen to music, browse the Web, and send and receive e-mail and text messaes. You can also take pictures with a built-in camera, import photos from your computer and organize them all using the iPhone's software. Although it's not a turn-by-turn GPS receiver, the iPhone also lets you view map and satellite data from Google Maps, including overlays of nearby businesses.

A modified version of the Macintosh OS X operating system, also used on Apple desktop and laptop computers, lets you interact with all of these applications. It displays icons for each application on the iPhone's screen. It also manages battery power and system security. The operating system synchs the phone with your computer, a process that requires a dock much like the one used to synch an iPod. It also lets you multitask and move through multiple open applications, just like you can on a laptop or desktop computer.

But instead of using a mouse or a physical keyboard, the iPhone uses virtual buttons and controls that appear on its screen. This isn't really a new phenomenon -- touch screens have been part of everything from self-checkout kiosks to smartphones for years. But the iPhone's touch-screen is a little different from many of the others currently on the market. When you touch the screen on a PDA or a Nintendo DS, you typically use a slender, pointed stylus. The iPhone, on the other hand, requires you to use your fingers. It can also detect multiple touch points simultaneously, which many existing touch-screens cannot do.

This article will explore exactly how the iPhone's touch-screen carries instructions from your fingertips to the phone's internal circuitry. We'll also look at the iPhone's features, its hardware and how it compares to smartphones and other electronic devices.

Electronic devices can use lots of different methods to detect a person's input on a touch-screen. Most of them use sensors and circuitry to monitor changes in a particular state. Many, including the iPhone, monitor changes in electrical current. Others monitor changes in the reflection of waves. These can be sound waves or beams of near-infrared light. A few systems use transducers to measure changes in vibration caused when your finger hits the screen's surface or camras to monitor changes in light and shadow.

The basic idea is pretty simple -- when you place your finger or a stylus on the screen, it changes the state that the device is monitoring. In screens that rely on sound or light waves, your finger physically blocks or reflects some of the waves. Capacitive touch-screens use a layer of capacitive material to hold an electrical charge; touching the screen changes the amount of charge at a specific point of contact. In resistive screens, the pressure from your finger causes conductive and resistive layers of circuitry to touch each other, changing the circuits' resistance.

Most of the time, these systems are good at detecting the location of exactly one touch. If you try to touch the screen in several places at once, the results can be erratic. Some screens simply disregard all touches after the first one. Others can detect simultaneous touches, but their software can't calculate the location of each one accurately. There are several reasons for this, including:

• Many systems detect changes along an axis or in a specific direction instead of at each point on the screen.
• Some screens rely on system-wide averages to determine touch locations.
• Some systems take measurements by first establishing a baseline. When you touch the screen, you create a new baseline. Adding another touch causes the system to take a measurement using the wrong baseline as a starting point.

The Apple iPhone is different -- many of the elements of its multi-touch user interface require you to touch multiple points on the screen simultaneously. For example, you can zoom in to Web pages or pictures by placing your thumb and finger on the screen and spreading them apart. To zoom back out, you can pinch your thumb and finger together. The iPhone's touch screen is able to respond to both touch points and their movements simultaneously.