Humans can only control a limited number of dimensions simultaneously. You could say that humans have limited "bandwidth".[Cook(2001)] It is possible to provide too many dimensions of control, making the instrument difficult to play, hampering expressivity. If the instrument is too simple, then it will have a limited range. Existing instruments and playing techniques are a good model of how many dimensions of control are useful. One example is the bowing motion of the violin, which can easily be emulated using a mouse, or an arrangement of buttons, like a piano, which could come in many forms.
A good place to start for examples of the level of complexity that a human can manage in a physical interface are the tried-and-true traditional instruments. From violin to piano to any instrument that humans have mastered we can see examples of functional interfaces. The violin and the piano are good examples for this because both of them offer a wide range of control over the sound produced, but in quite different manners.
The violin has 4 dimensions of control: bowing velocity, bowing pressure, location of the bowing as compared to the bridge, and the finger position on the strings. Mostly, the violinist is playing one string with a maximum of two or maybe three strings at a time, but the violinist is basically always playing one line at any given moment.
The piano model differs from the violin; playing numerous simultaneous notes is standard. A good pianist can even play two separate lines on one hand, making four simultaneous lines possible. The trade-off comes in the amount of control that the pianist has over the string as compared to the violinist. The piano has 2 dimensions of control for each string: velocity of key press and application of the damper. The pedals provide another simple dimension of control with the soft pedal. The damper and sostenuto pedal both provide control over the same damper as the finger can control, but in a different way. The final dimension of control for the piano is which key you hit, giving a different pitch. This provides 4 common dimensions of control, with the soft pedal providing a rarely used and simple 5th dimension. So in comparison the violin, playing one line the piano is simpler, therefore there is bandwidth left to allow playing multiple lines simultaneously.
Hans-Christoph Steiner 2005-04-25