The Radio Drum
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The Radio Drum is an analyzer of gestural data.  It is a three- dimensional controller, a sort of computerized musical drum.  Invented in the late 1980’s, the Radio Drum uses the “jeu de jacquet” or “Backgammon” form of capacitive sensing to interpret the gestures of a performer.  The drumsticks or mallets transmit a radio frequency that is detected by the drum surface.  Each drumstick operates at a different frequency.  There are four analog signals for each of the drumsticks generated by antennas situated at the four corners of the drum. 

Figure from ATT Bell Laboratory Radio Drum Manual

By using a displacement algorithm, the four signals can be transformed into three data channels representing the x,y,z displacement versus time. The amplitudes of the four signals coming from each antenna determine the values of x and y.  As a drumstick moves towards an antenna at the corner of the drum, the signal transmited by the antenna increases.  The sum of the four antenna signals gives the z value.  The largest value of z occurs when the drumsticks are touching the drum itself; at infinity z equals zero.

Various drumming motions can be characterized by the signals generated by the drumsticks versus antennas displacements.  To process these signals, the gestures must be mapped to control a musical event and all the motions associated with the gesture must be captured.  For instance, the start of a strike is recognized when the velocity exceeds a threshold value.  At the time of minimum z of a strike, the digitized measure of the four corner signals is reported to the host computer.  The mapping may also trigger different sounds, change the volume, pitch, tone or timbre depending on the x,y position at strike time.