#52

“Smart Fabric” is KMI’s piezoresistive sensor system, and it measures force, not location. To achieve a radial measure, multiple sensors are placed along the radius, and the location interpolated from the forces sensed. Since the top material is hard rubber, it is likely that in the middle of a sensor, not enough force is registered on adjacent sensors to accurately interpolate a value.

I have a Boppad and did some experiments with it. If you press with a stick and drag it radially, you’ll see that you can get pretty much all 127 values, but that there are “dead zones” where you get just 1, 32, 64, 95, or 127, just like you see with hits.

This tells me that there are probably only five sensors radially. This image, from a KMIvideo, shows ten bands, but I suspect they are grouped in pairs to make five sensors:

BopPad%20sensor
BopPad sensor.jpg1030×1172 117 KB

To achieve 127 discrete values would require sensor bands less than ½mm, and scanning 25x faster. Probably out of range for a $200 device.

14-bit resolution, or 16k values, is clearly out of the question: With the sensors it has, it can’t even interpolate to 7-bit resolution well. With discrete sensors, they’d need to be 0.6µ width - clearly not going to happen! This also, however, shows that 14-bit resolution over a 10cm surface is asking for micrometere accuracy, which is almost certainly out of the range of affordability in an instrument - not to mention playability.

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#53

You couldn’t drum on a Sensel morph: It can only take up to 5kg which under what a drum stick can generate. I don’t know if the Sensel surface breaks at 5kg, or just loses all precision.

The Sensel surface is a set of discrete force sensors - so again, you get position resolution (XY or radial) based on interpolating force sensed at adjacent sensors. They list 20k sensors over 33k sq mm. - So 1 sensor per 1.3 linear mm. Given the dimensions - there are fewer than 200 sensors along the longest edge - and hence at best 8 or 9 bit position accuracy (with interpolation). Mind you, that is still approx. mm. accuracy over 24cm, which for drum hits is probably the mostly you could ask…

EDIT: They claim 6502dpi tracking - which is 256 per mm. If calc.s above are right, and there is 1 sensor per mm-ish - then we can see they are estimating position based on interpolation down to 8 bits. The force sensors claim 32k (15 bits) resolution… so this somewhat checks out (but the surface material needs to be able to spread the force over more than one sensor to get this… but at one per mm, this is reasonable.)

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#54

That makes total sense, and probably explains the behavior I’m seeing. Makes me curious as to what’s inside the Mandala.

That was my concern. Not to mention the form factor is a bit weird for drumming on.

That would indeed be a dream.

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#55

I’ve had moderately decent results using drumsticks on the morph, and Peter Nyboer has confirmed to me a few times that the “Drums” overlay is intended to be usable with drum sticks.

I’m my experience drumming on it with sticks, the main problem is calibration. The morph app (last I checked) only lets you set “sensitivity”, which is basically a threshold value for how much pressure is required to trigger a note. Setting the sensitivity higher both a) loses you some dynamic range (or seems to), and b) actually introduces some latency, since there’s physically more time between when your stick hits the pad and when the pressure crosses that threshold.

It’s entirely a software / scaling problem, and something they should fix by switching from a threshold approach to a velocity curve approach.

The drum overlay also activates a different scan mode (low latency vs high dpi), but as mentioned, losing DPI resolution is opposite the goal of tracking position within the sensor.

(Also, really neat idea Rodrigo!)

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#56

I’ll have to see if I can find someone with a Morph to try it out in person.

Definitely don’t like the “Drum” overlay though, so I’d aim/hope for something that’s more unified across the entire surface, with mapping coming from X/Y/Z coordinates, rather than virtual little drum pads.

Definitely. Some kind of differential rather than an absolute threshold to exceed.

I know very little about the Morph, but I guess the different overlays can feedback information to the sensor itself? The crazy DPI is probably overkill for sticks, but the more accuracy, the better, without sacrificing latency of course.

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#57

I’ve written the Mandala people to ask some questions. I’ll report back what I hear.

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#58

really inspiring, thanks for sharing!

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#59

Would you happen to know of any DIY projects/solutions/approaches to this kind of thing?

I wouldn’t be looking for MPE or “pressure” really, just accurate-ish “velocity” and “position”, and it can be monophonic.

#60

what about 4 channel location with 4 mics being compared for loudness and delay - I think I’ve seen stuff like this at NIME, or along those lines. Now, i don’t know the speed of sound in material, but a dedicated small processor doing some sort of autocorrelation between the mics could tell you the distance, right?

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#61

I’ve not done that kind of (euclidian?) distance stuff before, but I think it’s a fairly solved problem (??).

I would imagine a bela, or even one of those new super fast Teensy 4s would be up to handling the maths involved.

The stuff you remember, was it air/condenser mics, or contact mics? I guess there are pros/cons to each, just mainly thinking of having a minimal footprint so it doesn’t impact the playability of the drum (too much).

#62

I vaguely remember in was rubber.
Here are speeds: https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
So you need a pretty quick sampling rate. Could be fun, but seems a hard problem for me for now…

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#63

i guess autocorrelation could estimate time-of-arrival at different points.

then you are doing TOA sonar/radar stuff, which is well studied indeed
https://aip.scitation.org/doi/10.1063/1.4947001

#64

Hi everyone,
thank you Rodrigo for you Boppad video.
I was thinking about buying one too to control some synths. I was wondering how i could use it with a e-drum hihat trigger to have more expressivity ?
I guess I would have also to also get a drum module and plug my hihat trigger jack in it and then send midi from the drum module to my synth ? I don’t really know how e-drums work ans if they can send midi to other pieces of gear.

#65

I’ve not worked with hihat triggers at all, but presumably you could use an Arduino (or Teensy) and convert whatever it’s output is to regular MIDI?

My understanding of hihat sensors is they are analog “pressure” sensors.

You can also go a fully DIY route:

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#66

Yes I could do that but if possible I want to use solutions that already exists.
Because the Boppad has a midi expander, I was thinking about using a e-drum module(like the Roland TMC6) to convert hihat pedal in midi signal and pack it with the boppad to then send it to a hardware synth.
But it seems very unlikely that a drum module would mix both type of signal as if it was a simple e-drum hi-hat, maybe you’re right I’ll have to convert audio to midi from the hihat trigger and mix it with the boppad signal.
This could be a good start :
https://forum.pjrc.com/threads/50114-FSR-pedal/page5

#67

So… My Boppad is… well… slowly being beaten to death: The USB connector can be flaky, and the black outer bezel is detaching. AND, as K-M never ever fixes firmware bugs, it still has the bug where it decides to issue totally nonsensical radius values.

(Yeah, I submitted a bug, they acknowledged that they could reproduce it - and their solution was to tell me to unplug and replug the unit, as it resets back to normal… like I’m going to do that in the middle of a performance!!! And yeah, they kinda outright said they’ll never ever fix it or rev. the firmware.)

SO - Anyone have any good alternatives? The feature set is right. I don’t want a $1k+ machine with it’s own sound generator. I don’t want “an electronic drum set” - I want a MIDI control that I can drum on with sticks. I want multiple zones (four is fine) - and I’d love radius or x/y as well as velocity.

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#68

Might not be in your price range but a friend of mine who tours with some electronic percussion uses a MalletKat by Alternate Mode and she enjoys it. They have some other beatable things as well. Maybe something like this?

#69

it’s not a matter of price… (though that “merely” $500 DrumKAT requires a $600 DITI controller to actually do anything…) – Just the KAT line seems to be very awkward - lots of cables, heavy parts… and yet with a feature set that doesn’t look like the kind of flexible thing folks in this thread are looking for. The sensors have no position sensing, and it isn’t clear if they do anything post-hit. The DITI unit is aimed at setting up a traditional style drum layout - including sensors on standard drums.

What I want is a well made Boppad, or a Sensel Morph made for beating with sticks.

#70

Mine is still structurally sound (though I did get a replacement sent out because mine was getting “polka dots” which was an issue with the first run I guess).

I have been pretty disappointed with the radius data, and after sending them some videos a while back, it seems like the radial (?) resolution is primarily 5 steps which it interpolates between, and that’s most aparent when you play softly as you get these crude jumps in position that have little bearing on where you’re playing. I’ve made a workaround abstraction where if the velocity value is repeated (unlikely in the “real world”) I +/- a random amount. Not great, but it sounds a bit better as it’s less machine-gun-y.

I’ve been talking with a friend about working out some kind of DIY alternative as I’ve not seen anything that does this well. I did find the Mandala drum but prior to this they weren’t available, and checking now the webpage has been taken over by spam…

So all of that is to say, that I too am keenly interested in alternatives.

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#71

20 characters of good luck!