[sdiy] MIDI velocity

Simon Brouwer simon.o at brousant.nl
Wed Apr 6 08:46:16 CEST 2016


Hi, 

That could only work up to 40 keys, since at most 80 of that processor's 144
pins can be used for I/O, and there are two contacts per pin to scan. 

The keyboard matrix with a diode per contact seems to me the more simple
approach.

Best regards
Simon



> On 05 April 2016 at 20:02 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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