[sdiy] Midi Drums

Quentin McDonald dqmcdonald at gmail.com
Thu Feb 8 20:16:33 CET 2018


I built this MFOS project which is kind of fun...


"Drums" are just piezo elements glued to blocks of wood. Kind of fun.


On Fri, Feb 9, 2018 at 7:30 AM, Tim Ressel <timr at circuitabbey.com> wrote:

> Thanks for the info!
> I assume these are piezo elements but who knows. Maybe I'll tear it apart
> at some point. I am looking to make a midi-cv device that is drum-centric.
> It is getting interesting as I figure out just what that means. A keyboard
> is easy: volt per octave out with gate and trigger, plus some analog outs
> for pitch, mod wheel, whatever. The drum thing is not so clear. I was going
> to have trigger out and analog velocity i.e. how hard it got hit (?)
> outputs. But then someone said have a gate out, which makes no sense to me.
> And then I thought there should be an output that mimics what a piezo
> element would put out when struck. that is to say an analog output that
> emits a signal with both pressure and duration info. This could go into a
> filter for using the filter pinging method of patching a drum.
> I meant using a scope was lame for trying to interpret the midi codes. I
> don't think they make a scope that can decode midi.
> --TimR
> On 2/7/2018 9:19 PM, rsdio at audiobanshee.com wrote:
>> Hi guys and dolls,
>> A great many MIDI drum sensors start out with an audio signal, typically
>> from a simple piezo mic. This signal is then rectified and smoothed, just
>> like any standard envelope detector that might feed a signal meter. For
>> pure analog systems, the envelope can be sent to a comparator with a
>> threshold to generate a Gate signal. The peak amplitude can be used for
>> Velocity. It’s even possible to feed the envelope directly to an output for
>> CV, e.g., to a VCA. These kinds of designs generally cannot detect pressure
>> or aftertouch, whether they’re pure analog or have an A/D converter. They
>> also cannot detect any sort of Note Off. A few MIDI drums simply never send
>> a Note Off, while others might use a preset time delay before the Note Off.
>> More advanced MIDI drums - like the drumKAT - have pressure sensitive
>> pads, and definitely have some digital components for MIDI and other uses.
>> These generate Note On with Velocity, and then also send Aftertouch
>> messages whenever the amount of pressure changes. These designs can send a
>> Note Off message at the exact moment you remove pressure from the pad,
>> since pressure sensing is continuous.
>> Velocity is the change in position over time, and Pressure almost always
>> corresponds to position (the amount of depression), therefore velocity can
>> be calculated by the difference between the first two pressure readings.
>> It’s a little tricky to use the first pressure reading alone, because
>> there’s no way to know the exact timing of the initial moment when the
>> pressure changed from zero to non-zero. Calculating on the first pressure
>> value alone would result in widely varying values for Velocity. However,
>> the first and second pressure readings will have a precise timing relative
>> to each other, based on the sample rate, so the difference between those
>> first two values can be used for velocity. The latter technique will be
>> more reliable because values will not vary so wildly. The faster the sample
>> rate for the A/D, the quicker the response.
>> I have never seen a MIDI drum with keybed style switches - but, of
>> course, that doesn’t mean it hasn’t been done. I have heard of people
>> making their own MIDI pads with nothing more complicated than a couple of
>> tin foil strips that conduct when you hit the surface. Off course, a bunch
>> of circuitry is still needed to make a MIDI drum from tin foil, but the
>> physical sensor pad can be super simple. I would say that the audio signal
>> technique is superior to the tin foil technique (or any single or dual
>> switch), while the pressure sensitive pads are the ultimate.
>> With most all designs, there is some crosstalk between MIDI drum pads
>> when there is more than one in the same box. The keybed style switches
>> would be the only design that probably wouldn’t have crosstalk. drumKAT
>> dealt with crosstalk by having a “learn” mode where the user would play
>> each pad, one at a time, and the system would learn how much “false” signal
>> is received on the pads that aren’t being played. The unit then made sure
>> to ignore those small amounts of position-dependent bleed when you’re
>> playing multiple pads in a kit.
>> By the way, I do not think it’s lame to take a look at the MIDI flow.
>> This is a good way to learn how music equipment works and will often allow
>> you to make the most of what you have. The only thing that I would consider
>> lame is not grabbing the MIDI specifications to study what messages are
>> available and what is supposed to happen. Unfortunately, there is some MIDI
>> gear out there in the thirty year history of the spec that doesn’t quite do
>> the right thing - like those rare MIDI drum pads that never send Note Off -
>> so it pays to look closely at what’s going on, especially when you get new
>> gear.
>> I assume you aren’t literally using an oscilloscope to look at the MIDI
>> flow. Unless you have a ‘scope that can decode asynchronous serial bits,
>> with 1 start bit, 8 data bits, no parity, 1 stop bit, and concert that to
>> (hexa)decimal numbers, then you’ll probably just confuse yourself staring
>> at the signals at that level. Instead, I would recommend any of the free
>> programs that display MIDI messages in a more human-readable form. This
>> way, you’ll know what those bursts are saying. No need to guess whether
>> it’s a Pressure update or a Note Off. I actually prefer old 8-bit computers
>> without USB-MIDI, because they can show the actual messages on the classic
>> MIDI port without the translation that occurs on a USB-MIDI interface. Then
>> again, it’s incredibly rare for USB-MIDI to make changes that would amount
>> to anything significant. I just like to know what’s really going on in case
>> something has a bug and might be generating abnormal MIDI messages. Since
>> I’ve created several embedded MIDI devices, I’ve managed to start out with
>> buggy code in the early stages, and having a good way to visualize the MIDI
>> helps a lot (even if it’s just print statements fed from a classic MIDI
>> interface).
>> Brian Willoughby
>> Sound Consulting
>> On Feb 7, 2018, at 8:10 PM, Mike HEQX <mike at heqx.com> wrote:
>>> How do they generate velocity? Is it done with an AD conversion? OR is
>>> it a timing thing between two events like a keybed?
>>> On 2/7/2018 7:38 PM, Quincas Moreira wrote:
>>>> Yeah, pads are basically just rubberized keys. They will generate the
>>>> usual info, note on, note off, velocity and note number. Sometimes
>>>> aftertouch too
>>>> On Wed, Feb 7, 2018 at 6:19 PM, Tim Ressel <timr at circuitabbey.com>
>>>> wrote:
>>>>> ayee,
>>>>> So like, how do midi drums work? I am playing with a Korg Nano Pad II
>>>>> and looking at the midi flow with my scope (i know, lame). Something I am
>>>>> curious about: I get a burst when I hit a pad ( expected) and another burst
>>>>> when I release it. Huh? Is that a note off command? Is it treated like a
>>>>> key being pressed it you hit and hold?
>>>>> Btw the MidiHost thingie from HobbyTronics is pretty cool. Eventually
>>>>> I will have to conquer doing USB Midi on an STM32, but for now I am having
>>>>> fun with this gizmo. I think I'll turn this one into a new-to-old Midi
>>>>> Convertor, and get a few more for the Washington of it.
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> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
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