[sdiy] Midi Drums

Michael E Caloroso mec.forumreader at gmail.com
Thu Feb 8 07:23:51 CET 2018


Simmons pads are very simple.  The transducer is just a piezo element
which can generate velocity signals.  The problem is they are
sensitive enough to suffer crosstalk from adjacent pads, you have to
have accurate aim with a drum strike, and they don't with on a
traditional drum head.

MC

On 2/8/18, rsdio at audiobanshee.com <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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