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The D-Box: a Hackable Digital Musical Instrument

2014 - musical instrument

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The D-Box is a quite mischievous instrument that made it to the finals of the 2015 Guthman Competition. It was designed and developed by Andrew McPherson and myself. Its main purpose is to trigger our imagination when we play a digital device, giving the possibility to break the predefined interaction schemes that generally characterize DMIs, as opposed to acoustic and electronic instruments.

It is apparently very simple, maybe too simple…a box with only a couple of touch sensors, to trigger samples, modify pitch and apply a filter. Despite its impressive responsiveness, playing this instrument might feel like a bit limiting at the beginning and would likely push you to wonder what they were thinking when they designed such a simple device! And to start listing all the obvious modifications that would improve it. At that point it is already too late. You find yourself hacking the circuitry that is contained in the box, pulling wires and components, probing in real-time how your modifications destroy your samples, affect the controls and your whole perception of the instrument. The process generally ends with the embrace of very unconventional ways of playing the instruments, that could have hardly been planned or predicted.

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The D-Box is a self contained instrument, shaped as a 15cm side wooden cube. On top there are 2 multitouch capacitive sensors, mounted on a pressure pad. The case also embeds a speaker, a mini jack audio and a battery, and carries a BeagleBone Black [BBB] single chip computer. A set of samples can be loaded onto the SD card the board is equipped and played back through additive synthesis; the first sensor triggers the current sample, with pressure controlling volume, and acts as a continuous control control for pitch. The second sensor is mapped on a filter, that can be used to shape up to 5 independent bands, one with each finger.

However, the core of the D-Box is the matrix, a breadboard carrying wires and components directly connected to a custom cape mounted of the BBB. The matrix is organised in columns, each composed of a circuit whose electronic configuration defines synthesis and control parameters of the software processes running on the BBB. These parameters include clock speed, oscillators’ waveform, playback position and sample selection.

By opening one of the side plates of the cube, it is possible to access the matrix and hack the D-Box, even while the instrument is playing. Its solid design allows the most reckless operations, like playing with floating wires and short circuits, adding new sensors and components, and patching together unrelated parts of the circuit. All in line with the design motto “No Silence, No Smoke”.

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The platform used to develop the D-Box is Bela, which allows the design of ultra-low latency self contained instruments. Bela is composed of a software environment running on the BBB and of a custom cape, with audio in/out capabilities and a set of analog and digital inputs and outputs. The D-Box software uses the Bela API and was written in C++. The analog channels of the cape are used to design the circuits on the matrix, as direct voltage controls or hysteresis oscillators working as feedback loops between hardware and software.

The two multitouch sensors come from the TouchKeys project and were originally designed as overlays for piano keys.

The loaded sample files are exported using SPEAR, a free software that uses spectral analysis to split a single audio file into several sinusoidal components, that the D-Box plays back using a highly optimized internal oscillator bank, written in C++ and Assembly.

Several musicians around the world have a D-Box, thanks to the artistic collaborations, the user studies and the workshops we ran in Europe and North America.

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Meet the D-Box [and some performers]:

More performances, by Madcoil [Bogdan Vera] and myself:

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  • Zappi V., McPherson A., “The D-Box: How to Rethink a Digital Musical Instrument”, Proceedings of the 21st International Symposium on Electronic Art, Vancouver, BC, 2015.
  • McPherson A., Zappi V., “Exposing the Scaffolding of Digital Instruments with Hardware-Software Feedback Loops”, Proceedings of the International Conference on New Interfaces for Musical Expression, Baton Rouge, Louisiana 2015.
  • Zappi V., McPherson A., “On Hacking Instruments”, Almanac on Contemporary Instruments, online collection, 2015.
  • Zappi V., McPherson A., “Design and Use of a Hackable Digital Instrument”, Proceedings of the International Conference on Live Interfaces, Lisbon, Portugal, 2014.
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