The basic process behind detecting touch

The iPod touch Screen

The thing that sets the iPod touch apart from other iPod models -- other than the latest nano -- is its touch-screen interface. When you touch the screen, the iPod's circuitry detects the presence of your finger. It keeps track of how many fingers you have on the screen and where you move them. It also gives the iPod touch the capability of running apps -- something even the newest iPod nano can't do.

The iPod touch does this using a layer of capacitive material under a protective covering. You can read How Capacitors Work to learn more about them, but the basic idea involves taking advantage of the electrical properties of the human body. When you touch a capacitive surface, the amount of charge it holds changes. This is why devices like the iPod touch require you to touch them with your bare skin -- insulating materials like gloves, pens and styluses don't cause the same changes in the capacitive circuitry.

There are two possible methods the iPod touch can use to measure changes in electrical states:

  • Self capacitance: Circuitry monitors changes in an array of electrodes.
  • Mutual capacitance: A layer of driving lines carries current. A separate layer of sensing lines detects changes in the electrical charge when you place your finger on the screen.

Sensing circuitry in a self-capacitance screen measures changes in electrical charges.

In mutual capacitance, a grid of driving lines and grid of sensing lines work together to detect electrical changes.

Regardless of which method the screen uses, you change the electrical properties of the screen every time you touch it. The iPod records this change as data, and it uses mathematical algorithms to translate the data into an understanding of where your fingers are. In the next section, we'll explore what the iPod touch does with this data and how to navigate through its features.