The Rhythm Tree is a collection of circa 300 percussion sensors. A simple microprocessor on each pad analyzes the signal coming from a piezoelectric (PVDF) strip, which picks up the strike. A set of simple parameters are extracted from a 5 ms sample of the PVDF signal after a significant peak has been detected, indicating a valid hit. These parameters include the polarity of the initial PVDF signal peak (indicating a top vs. side impact), the number of zero crossings detected (indicating a sharp elastic hit vs. a dull slap with the hand remaining on the pad), and the net integrated signal amplitude (producing 14 bits of velocity information). After a fast, bit-slice poll from the bus host, the struck pads send their data across a shared RS-485 serial network to a host processor, which formats the data into MIDI and passes it to the main computer running ROGUS music generation software. In order to simplify cabling, up to 32 pads can be daisy-chained (like a string of Christmas lights) onto a single host and bus line. We have 10 such strings running in the Brain Opera Lobby. Each pad also houses a bright LED, which can be illuminated with a dynamically variable intensity. The pads are completely programmable via MIDI system-exclusive commands. We have written a Visual Basic application to adjust the parameters (i.e. trigger sensitivity, light flash options, trigger rejection flags, integration time,...) for individual pads or groups of pads, in order to rapidly configure the rhythm tree into a working configuration. In addition to these capabilities, the rhythm pad string used with the performance instruments is augmented with Gesture Wall sensors, enabling the hand motion above the pads to be tracked before the pads are struck. All electronics, sensors, and the LED are potted in a compliant urethane, which is struck by the hand when the pad is played.
Link here for more pictures and a video clip.
Link below for a paper with a detailed description of the Rhythm Tree system:
The Brain Opera Technology: New Instruments and Gestural Sensors for Musical Interaction and Performance Joseph Paradiso. Journal of New Music Research, Vol. 28, No. 2, 1999, pp. 130-149.
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