[sdiy] fatar keyboards

[Carsten Tönsmann]

Hi Tom,

yes, up to now I've been interested only in the key information, as I use my 
keyboard for composing and MIDI editor programming purposes only.  So I've 
not tried to get those dynamic information yet.

And no, I don't think it is easy to get those dynamic informations. It 
sounds easy, but it is not. As I know WERSI for instance offers a 
microcontroller based MIDI keyboard interface providing additional dynamic 
information. These interfaces are black boxes and even WERSI could not 
provide detailed information about the scanning and interpreting process. 
Please consider the matrix diode interface on the keyboard hardware bottom I 
described on my pages and in my interface circuit. You have to implement a 
scanning process knowing which COMBINATION of input and output lines are 
active as all lines are used more than once. This is the minimum to be done 
to get the key information and I did this with my microcontroller interface 
described on my page. To get the key velocitiy you have to expand the 
interface and the scan procedure by a time measurement between the output 
line pair of the matrix, where I just had to controll one line of the pair 
binary.

And the pressure mesurement is even more tricky. Especially in my keyboard 
version I can't imagine where to place additional pressure sensores as the 
rubber contact boards inhibit direct access to the moving key. To get the 
pressure information from the rubber contacts you have to measure the 
current passing the contact. That can be done only outside the matrix. But 
consider again the matrix structure of the interface. In my keyboard the C#0 
and the B#1 key use the same matrix output lines, so the rubber contact 
currents add if you don't synchronize the current measurement with your key 
scanning pulses. Perhaps you could analyse the matrix input side 
additionally, but I think there are a lot of problems to solve to get 
precise informations.

On the other hand why not trying it. I've already made first concepts to get 
some more dynamic information from my keyboard as I want to understand the 
whole process. Of course buying a MIDI keyboard would be a solution, but 
this means I don't understand and I don't control the processes. We'll see.

Regards

Carsten

----- Original Message ----- 
From: "Tom Wiltshire" <tom at electricdruid.net>
To: "Carsten Tönsmann" <carsten at analog-monster.de>
Cc: "David Moylan" <dave at westphila.net>; <Synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, November 14, 2007 7:19 PM
Subject: Re: [sdiy] fatar keyboards


Carsten,

On 14 Nov 2007, at 09:22, Carsten Tönsmann wrote:

> Hi Dave,
>
> I built a MIDI keyboard interface for a Fatar keyboard.
>
> These keyboards (as mine) usually have two so called rubber  contacts per 
> key. These contacts have a dynamic resistance  depending on key pressure, 
> so if you analyse the resistance you get  two informations: A key is 
> pressed and how much is the pressure.  This enables you to create control 
> voltages depending on the key  pressed AND depending on your dynamic play.

This sounds very exciting! Have you tried it? On your website you say
that you were only interested in the note information.

I'd previously understood that the Fatar keyboards needed pressure
sensors to be added if you wanted any kind of aftertouch response.
However, if you can use the rubber contacts to also determine key
pressure, then it would be possible to build a velocity and
polyphonic aftertouch sensitive keyboard using an off-the-shelf Fatar
keyboard.

One thing though, if it's so easy, why don't more keyboards have
polyphonic aftertouch?

Regards,
Tom