[sdiy] MIDI velocity

[Roman Sowa]

Connecting all 122 key contacts to a micro will give the quietest 
solution, from EMI point of view, and possibly the fastest also, but 
unless it's older keyboard, or you make contacts yourself, it's not 
possible to get a keybed not wired in diode matrix.
Not to mention tedious wiring, or you could make Pa CB with bubble 
contacts and the micro, that is 85cm wide. Not possible to split into 2 
boards like regular matrix, has to be in one piece.

As for scanning only when any key is pressed, that's what I do in my 
smallest scanner, set all drivers down, and look for 8 sense pins until 
they are anything different than 0xFF (pull-ups there). When that 
happens, regular scanning starts and is repeated until all keys are 
detected released and debounced.

Roman

W dniu 2016-04-05 o 20:02, sleepy_dog at gmx.de pisze:
>
> 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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