[sdiy] AfterTouch - adding to Fatar synth Keybed ?

[sleepy_dog at gmx.de]

Hey,

> I recommend looking at the Ensoniq keyboard schematics from their polyphonic aftertouch models for ideas. Their design is inductive, rather than capacitive, but it shows a good approach.

Interesting! I like that thing, and I'm also a fan of the SID chip, 
which was designed by the same guy :-) Will have to look at that, and if 
only for higher level design of something like that. When I hear 
inductive, I hear "big & expensive". Maybe that's wrong, I don't know 
what could beat strips of copper tape, though.

> One thing is that they use analog multiplexor chips and a single ADC. So, you shouldn't really focus on processors with 32 ADC channels, or even processors with internal ADC. Quite often, custom designs like touch sensitivity might require an external ADC and mux. It's usually better to have the ADC chip on the same board as the sensors, so they share the exact same ground reference for lowest noise. Then use serial interconnects between boards to bring the data back to the main processor. Once you do that, it doesn't matter how many on-board ADC channels your MCU has.

well, it does not seem to be a restriction to use MCUs with 16..32 ADC 
channels, ST's ARM MCUs is throwing it at you for cheap. I could do all 
that - keep them physically close to the action. For non-mass producing, 
MCUs that cost 2...3,- EUR where I need maybe 2..4 of them - or spread 
it out further and have it even closer with a greater number of 
smaller/cheaper MCUs with fewer channels, it doesn't make a difference. 
At least external ADCs I know aren't cheaper than such an MCU, not 
significantly anyway (speaking non-mass-producing). So perhaps there is 
not really a point to have a dedicated ADC unless it has properties that 
would be really beneficial compared to using a bunch of MCUs? I guess if 
something like anti-alias filter would be necessary in this setup, for 
every ADC channel, this looks different - but perhaps fingers modulating 
voice volume is not nearly as sensitive as introducing high energy audio 
rate alias into an audio signal? But if the MCU has enough power to scan 
its channels at a much higher rate than the finger movements, alias may 
not be a problem, I guess.
I at least know that the switching of the internal ADCs works well 
enough, and that, if I do it myself with analog mux ICs, I might easily 
introduce switching noise / need to research more to properly guard 
against that, I guess.

Anyway, I'm saving these mails now and mark them as "come back to this", 
and my glorious MIDI controller has made a few steps closer to reality :-D

Steve

>   The Ensoniq design even has a slave processor on the keyboard itself, which isn't a bad idea considering the complexity of poly-aftertouch.
>
> I predict that the biggest challenge for polyphonic aftertouch would be calibration. Your hand-made solution will probably not be identical for every key, but if you can calibrate then that won't matter. The Ensoniq keyboards announce that the user should not touch the keyboard every time it is turned on, so that the system can calibrate the sensors.
Yeah I had that problem in the back of my head, but not thought about it 
further. I'd store calibration data in the internal flash of the MCU(s). 
But perhaps having to record some correcting curve for every single key 
(say, if I can't get sensor plate size & position exactly the same for 
every key) sounds like quite a pain.
I don't remember capacitor physics well enough, perhaps the function of 
proximity vs. capacitance has same curve *shape* even for differently 
sized & oriented plates, just the extreme values different? That would 
be easier then.


> You might be able to create a software mode that converts poly-aftertouch into mono-aftertouch for compatibility with MIDI sound generators that only support the latter and not the former. You could take the highest value (preferred) or the average value (more computation required) to map from poly to mono.
>
> Brian Willoughby
> Sound Consulting
>
>
> On Apr 6, 2016, at 1:20 PM, sleepy_dog at gmx.de wrote:
>> say I have a Fatar synth action keybed lying around. When I bought it, Doepfer wasn't offering the Aftertouch option anymore.
>>
>> Is it effortful / expensive to add such a feature in other ways than using the original aftertouch band (I assume its such a foamy thing?) from Fatar?
>> They won't sell to end customers, I already asked... (a couple years ago anyway)
>>
>> I had some insane ideas of adding a few cheap MCUs to the thing, and then a DIY hacked capacitive sensor (say, adhesive copper tape, one piece for the key, one for the base under the key) under each single key, manually... but I have no idea whether one can actually access the necessary places properly and attach those, without breaking anything.
>> (That would give polyphonic aftertouch, though, which sounds neat, although I have no idea how useful that actually is, or more an annoyance ;) )
>> Since the types of MCUs I use may have something like 32 mux'ed ADC channels and the needed rate of change for those finger modulations may not be super fast, I guess this could work, but it sounds like a bit of a PITA .
>>
>> Other ideas?
>>