This article describes the Apple Trackpad technology.

Key Points

• A conductive surface (such as a finger) is needed to move the cursor. Pointing devices such as pens, a Newton stylus or even a finger nail won't work. Humidity can also impair contact.
• There is a direct correspondence between the rectangular trackpad and rectangular screen.
• Solid state mechanism allows space to be allocated to larger, longer lasting batteries.
• More reliable than a mouse or a trackball.
• Provides clicking and dragging capability on some PowerBook models.

The Apple trackpad is a pointing device that provides precise control over cursor movement. Based on user testing and feedback, Apple engineers have tuned the interface to make the trackpad a very efficient pointing device for interacting with the computer.

More recent PowerBook models provide the ability to use the trackpad to select and drag icons or text blocks rather than having to use the trackpad button. Check the Trackpad control panel, shown in Figure 1, to see if clicking and dragging are supported on your particular PowerBook model.

Figure 1 Trackpad Control Panel

Technology Overview

The technology works on a principal called coupling capacitance. As your finger moves over the surface, the trackpad evaluates the change in capacitance between two layers of measurement electrodes built into the surface of the trackpad. The electrodes are arranged in a checkerboard configuration, which creates a series of intersections, or couplings, where the capacitance is measured by the trackpad. The horizontal electrodes send out a test pattern signal which the vertical electrodes sense. As a finger approaches the trackpad surface, its presence lowers the capacitance (coupling coefficient) at the closest electrode intersections and modifies the signal received by the sense electrodes.

By monitoring this change in capacitance, the trackpad is able to first locate and then compare the current location of low capacitance (where the finger is touching) to the previous location, and then moves the cursor accordingly. This cycle of sending out a test pattern, sensing the change in capacitance, and moving the cursor is constantly repeated. The high resolution of the trackpad (387 dots per inch in the horizontal and vertical directions) allows precise control over cursor movement down to the pixel level. The Trackpad control panel provides a adjustments over a full range of acceleration to ensure precise control.

Trackpad Speed Adjustments

The faster your finger moves across the pad, the faster and farther the cursor will move across the display. If the finger moves quickly, the cursor will traverse the display completely. If the finger moves slowly, the cursor can be controlled quite precisely. In fact, just rocking the finger tip slightly on the pad gives pixel level accuracy! New trackpad users can experiment with the control settings in the Trackpad control panel to see which acceleration setting works best.

The acceleration curve settings offered range from slow to fast. The higher the curve chosen, the farther the user will be able to traverse the screen with a shorter finger movement on the pad. The setting that is most preferred among users is the fourth setting up from the slowest setting.