Triggering algorithm for the Boie radio drum

The Boie radio drum is used to play electronic music synthesizers. The drum is a sensitive surface that tracks the position and velocity of two or more drum sticks in a shallow volume above the surface in three dimensions. Positions and velocities are determined by measuring the strength and distribution of electrical capacitance between the tips of the drum sticks and the surface. The drum is made force sensitive in the surface normal direction by coating either the sticks or surface with an elastic foam layer. Position information of a stick in contact then corresponds to two-dimensional position on the surface and force into the surface. One of the drums functions is to generate triggers that start notes. The trigger algorithm must be very reliable in that one and only one trigger is produced no matter how gently or how hard the drum is struck. An algorithm, patterned after the action of a piano key has been implemented. A trigger plane and a reset plane are defined above the drum surface. Motion through the trigger plane produces a trigger. Motion back through the higher reset plane enables the next trigger. Loudness is controlled by the stick velocity as it crosses the trigger plane. The algorithm gives very reliable triggers over a bigger dynamic range than existing synthesizer keyboards.