[sdiy] AfterTouch - adding to Fatar synth Keybed ?

[rsdio at audiobanshee.com]

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.

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. 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.

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?
>