Mechanical vs. digital: a GUI isn't always the answer
Moving from a set of mechanical controls, such as dials, sliders, and buttons, to a graphical touchscreen is often a developer's dream come true. The developer is no longer limited to a fixed family of settings limited by the number of dials on the front panel.
Menus allow infinite user-configurable options. Scrolling means that you can provide endless lists of data. The GUI provides a window into the internals of the device that was previously unthinkable.
Many designers get carried away with the new toy and foist upon users far more complexity and data than they want or need. Making some of the controls mechanical rather than graphical can have major usability advantages, which we will explore in this article.
While a GUI has many advantages over custom displays, it's important to note a couple of the disadvantages. A GUI allows a number of different controls on the screen, but they all have the same tactile feel when making an input. If the input is via a touchscreen, they all feel flat. If the input is via a trackball, the same roll-and-click motions are used to manipulate any of them. It's possible to build some tactile feedback into a touchscreen, but it's limited.
In contrast, a throttle controlling the speed of an aircraft will be physically larger and have a heavier feel than the volume dial for the aircraft radio. These differences communicate the significance of the action to the user. Imagine trying to drive your car with a mouse and screen as the only inputs and you'll get an idea of how the feeling of control can be lost. The car is quite an extreme example. In a car, it's important to be able to perform one action with each hand at the same time, such as steering and changing gears.
Operating two controls simultaneously on a single GUI is generally not possible. Even when the technical challenges are overcome with hardware capable of interpreting multiple touches, you still face the trickier problem that if two independent controls are on the screen, you're likely to cover information with your hand while performing one action, but that information needs to be visible to perform the second action.
The iPhone allows multiple touches, but they're generally connected to the same action--for example two fingers are used to shrink a single picture. The gesture is multitouch, but it's still a single gesture.