How the iPhone works

Screen dimming, the tilt feature, shake and more

Going 'tilt'

The tilt feature, or accelerometer, changes orientation in e-mail, the browser and other apps, and comes in handy in games such as Heavy Mach., where you can tilt the iPhone to move a tank left or right.

Jon Peddie, a consumer tech analyst, explains how the accelerometer works. "The accelerometer knows where the center of the earth is," Peddie says. "The trick is to sense a change."

Peddie explains that the first accelerometers had tiny magnetic cylinders that could slide from inside one very tightly wound transformer to another nearby, causing the signal in one to go down while the signal in the other went up. These were called linear voltage displacement transducers.

Today, like everything else electronic, the iPhone employs micro-electromechanical systems (MEMS). These devices have tiny (3 microns thick and 125 to 150 microns long) polysilicon arms with small hammer-like blocks on the end. They act like springs and hold the MEMS structure above a substrate. Acceleration causes the arms to deflect from their center position. And just like in the old electro-mechanical devices, the movement of that tiny mass is detected, by capacitors in this case, and a signal is generated.

iPhone's tilt feature

The tilt feature in some games uses the iPhone accelerometer, which changes direction in relation to the center of the earth.

When you shake the iPhone while listening to music, you will hear a random song -- either from the artist who's currently playing or from the playlist. Some apps, such as the Daylite Touch business productivity manager, use the shake feature to sync data. But you have to give the iPhone a good, firm shake to convince the application that you really want to sync up.

What we know from hands-on testing is that the iPhone shake feature uses the accelerometer. When it senses the phone has moved (in this case, from side to side), the OS triggers an API call for syncing or a random song. The accelerometer is sensitive enough, says Peddie, to know the difference between a shake and just turning the phone to its side to signal landscape mode.

Global positioning system finds your current location

As long as you are outside and not stuck in a tunnel or inside an office complex, your iPhone can find your exact location using an internal GPS. Like most devices and smartphones with built-in GPS, the receiver in the iPhone can read a signal from a series of orbiting satellites -- about three dozen of them -- that transmit a near-constant signal.

The iPhone receiver reads data from the satellites and figures out -- based on how long the transmission takes -- the distance to the satellites, and then calculates your location. When the GPS receives another GPS signal, it more accurately determines your location. Once the iPhone acquires three satellite signals, the receiver triangulates your position.

Apps that use the iPhone microphone

Leaf Trombone, Vocoder and similar apps can sense when you sing, talk or blow into the microphone.

According to the developers of both apps, Leaf Trombone and Vocoder can interpret the intensity of a sound wave -- essentially, a vibration that moves through the molecules in air -- and then trigger a related sound. With Vocoder, for example, as you blow softly you can play a note on a piano that is equally soft, and as you blow harder the sound becomes louder. Leaf Trombone reads the sound waves and creates matching pitches depending on where you move the leaf shown on the screen. Both apps use the iPhone microphone to read the sound waves.

Leaf Trombone reads the intensity of the sound wave over the microphone.

"We are leveraging audio signal-processing algorithms to analyze the audio stream from the microphone," says Ge Wang, Leaf Trombones creator and assistant professor at the Center for Computer Research in Music and Acoustics at Stanford University. "The algorithm tracks the energy level of the audio input and conditions that signal into something that can be readily used" for blowing into the microphone.

Augmented reality apps show pop-up data

Augmented reality apps, such as Nearest Tube and TwittARound, can read an area around your iPhone -- using the built-in camera -- and show pop-ups that augment what you see. For example, TwittARound can show the status of nearby Twitter users, so tweeters can see which person is sending a status update.

These tools depend on several iPhone functions. According to the developer, the Nearest Tube app reads the GPS location of your phone, finds nearby train stations in London and then reads compass data and the accelerometer to find out which direction you are pointing the phone so it can then show you which way the station is. Open GL technology -- essentially a graphics engine for displaying pixels -- shows the Tube data in real-time. The app shows the overlays on top of the camera view so they look like they are pop-ups.

John Brandon is a veteran of the computing industry, having worked as an IT manager for 10 years and a tech journalist for another 10. He has written more than 2,500 feature articles and is a regular contributor to Computerworld.

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