[sdiy] How do contemporary velocity sensitive midi pads work?

nvawter nvawter at media.mit.edu
Thu Aug 6 21:44:44 CEST 2015

I've been working with Sensitronics' FSRs for about a year now.

Electrically, I use a divider circuit with (IIRC) a 3k resistor.  I've 
found I don't need to buffer
the electrical signal into the ADC on the STM32F4.  One really should, 
but really it just makes a
different curve.  :)  I am open to experimenting with different curves 
such as with a logarithmic amplifer, etc.,
if anyone has any suggestions.

I've been obsessively figuring out different ways to utilize the 
continuous values to emulate plucked strings, drum
heads and pressed keys.  My goal is to be able to press lightly with one 
finger while striking with the other
and have the tone come out a higher pitch/thinner and decay faster...  
sort like muting a drum head or strings
while striking it.  I've also been trying to emphasize the sound 
differences between a fingertip strike and
a fingernail strike!  I know it's possible, I've recorded the electrical 
signal and identified different
frequency content.  The challenge now is to transduce it into sound.  
There is lots of potential here, guaranteed
to keep you up late at night for years :)

One other interesting thing I found with them was feedback when I built 
a drum with them.  The speaker in the drum
was able to induce a detectable signal in the FSR.  Not sure if that was 
vibration or EMI, but it can happen :)

On 2015-08-05 22:25, Chris McDowell wrote:
> Howdy list,
> I'm wondering if anyone has some experience with the design of modern
> velocity sensitive pads (just found myself curious).
> Controllers like the akai LPD8 don't appear to have piezos, but do
> have concentric squares on the pcb underneath each pad. I can kind of
> guess that these may work like the staggered switches that provide
> velocity on a synth keybed, but it's not super clear. I've read a bit
> about "velostat" and FSRs also, and am wondering if they're perhaps
> doing the deed. I know older MPCs do indeed have piezos, and that
> seems pretty straightforward. Any info is appreciated!
> Cheers,
> Chris
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