LAC 2011 - Slides: A position sensing midi drum interface

2011·0.92 MB·English

by  Andrew Baxter

The synpad – a position sensing
midi drum interface
I will be talking today
about my attempts to
build a cheap, playable,
midi drum interface with
position sensing
capability.
Overview
Motivations – why build this?
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Research – previous designs.
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Physical design.
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Electronics.
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Driver software – firmware and position mapper.
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Synth software – using supercollider.
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Results.
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Similar work.
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Future directions.
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Conclusion.
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Motivations
Why build something like this?
I find drum machines limiting and awkward.
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I wanted something more immediate and
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responsive.
Drum triggers are good but you can't change
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the tone.
Wanted something more like a real drum.
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Research
Tried a couple of other designs.
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Voltage gradients in a conductive rubber sheet.
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Time of flight of pressure waves.
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Just sensing transferred pressure worked ok.
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Physical design
Playing surface is an
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aluminium sheet.
The sheet moves freely
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in a wooden frame.
Piezo sensors under
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each corner detect
differences in transferred
strike pressure.
The signals are brought
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out to an arduino board.
Electronics
Quite simple electronics.
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Provides a false ground level.
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Prevents voltage drift on the piezos.
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Circuit diagram
Driver software - firmware
Firmware turns analogue signals into 4
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velocities.
ADC sample rate: 76 kHz across 4 lines.
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Trigger level for detection.
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Absolute values summed for 16 samples = 0.84
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ms
Summed values are written to USB at 230400
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baud (0.34 ms)
1300 sample / 68ms release period.
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Position mapping software
The position mapping equation.
Python program maps
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sensor readings into x, y # s[n] = reading for sensor n
and velocity coordinates. # x = x or y return value.
# k[n] | f[n] = adjustable coefficients.
Pad calibrated by striking
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f1=s1
at known positions.
f2=s2*k2
Least squares curve fit
● f3=s3*k3
for position and velocity.
f4=s4*k4
x=(l1*f1+l2*f2+l3*f3+l4*f4)/
X,Y and V are converted
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(f1+f2+f3+f4)
to midi note and
return x
controller values.
Synth software
The pad is just an interface.
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Existing synths not suitable.
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I wrote my own in SuperCollider.
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Could have used CSound or PureData.
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Could also use a graphical modular synth like
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Ingen.
Learning to write synthdefs.
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'Synth Secrets' series from 'Sound on sound'.
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