[sdiy] MIDI velocity

Mikko Helin maohelin at gmail.com
Tue Apr 5 21:37:23 CEST 2016

Instead of the usual double-throw switch timing analysis it would be
better to use optical sensors together with physical modelling to
sense the velocity:


On Tue, Apr 5, 2016 at 9:02 PM,  <sleepy_dog at gmx.de> wrote:
> I wondered about this "scanning all the keys all the time".
> I'm only familar with stm32 platform (Also cheap! you could even get an
> stmf100 in a 144pin LQFP package for about 4,- EUR and do this even without
> any muxing matrix stuff or whatever way this is done with such keyboards :-D
> ),
> and there, I'd have, say, 8 ports of 16 pins each which are enough for 2x61
> switches.
> So 4 ports are for all first-switches on all keys, 4 ports are for the
> second ones.
> So you might do 8 ports reads, taking 2 cycles each I think, @ 24MHz. Doing
> this in a timer interrupt at 8kHz seems feasible.
> Each of the 61 keys has a maybe 16bit counter.
> 24 MHz /  8000 Hz = 3000 cycles for all this stuff, I think 2 cycles x 8
> ports, + some checking logic which ports are not all zero and then looking
> in more detail at the bits of those which aren't,  + (worst case) 61
> conditional increments fit well in those 3000 cycles. You might even be able
> to display stuff on an LCD or so with that same processor. If that 144pin
> beast is unwieldy, 3x 48pin LQFP stm32f030 for 0.8 € each, each scanning 1/3
> of the keys, one being master and joining all the MIDI event streams to one,
> might do?
> Or maybe I'm talking nonsense because one can't access all the keyboard
> lines that way and one really has to use some kind of muxing logic?
> I faintly remember somethig like that with the Fatar synth action keybed I
> have lying around... Alas I did not have time yet to build the monstrous
> MIDI controller I envision, but am glad to have bought this before Doepfer
> apparently stopped selling them... SO who says buying stuff you don't need
> right now has to be bad... :-D
> Oh, on a tangent again...
> Steve
> Am 05.04.2016 um 18:47 schrieb Ingo Debus:
>>> Am 05.04.2016 um 13:43 schrieb Richie Burnett
>>> <rburnett at richieburnett.co.uk>:
>>>   Is it reasonable to expect that I could use a low-end micro (e.g. PIC)
>>> to
>>> scan a 61-key Fatar velocity sensitive keyboard with sufficient velocity
>>> resolution to work well?
>> Maybe there’s a smarter way than scanning the whole keyboard all the time?
>> For instance, in wait-for-key mode all the keys are in parallel, so when one
>> key is pressed, the processor can react very quickly (thus measure the time
>> very exactly), without knowing at first which key it was. Then, while the
>> key is travelling, the keyboard is scanned to find the key. Then the
>> processor waits until the key hits its final position and measures the time
>> again.
>> It would also be less noisy, because the keyboard is only scanned when a
>> key actually is pressed.
>> Some problems might arise, what happens when a key is pressed while
>> another one is on its way?
>> Ingo
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