[sdiy] Keyboard trigger...

[Rob B]

Well, you need to send a 32khz or so square wave to an opamp. The
plate in the key and the plate underneath are in the feedback path of
an opamp iirc. Then, you check the original signal to the output
signal, and look at the phase. This will tell you how close the plates
are.

You set a software threshold whereby phase change more than X amount =
note on. Then, you set 2 thresholds, one early and one late, and run a
timer from the time phase change = X min to X max. Then, use the area
beyond X max to generate aftertouch.

Problem with this is it takes recalibration, but it does work.

Use silicone rubber bumpers to have the keyboard have some *give* so
that you can dig into it for aftertouch.

I did this as a project (using one key) in a Heathkit 6805
instructional kit, instead of using the A/D. My instructors though it
was the coolest thing for that app that I got an A immediately, even
though I didn't use the A/D like it said to.

I told them the A/D was for wimps. ;)  I had to code a lot less too. I
had a calibration algo and everything, and it was still less coding
than using the A/D. I was only looking at the timing difference of the
output vs. the input, and the rate of change of the timing difference.

Rob

----- Original Message -----
From: "matti" <matti at devo.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, February 16, 2002 8:03 PM
Subject: Re: [sdiy] Keyboard trigger...


> On Sat, 16 Feb 2002, Rob B wrote:
>
> *snip*
>
> > Personally, I would do the capacitive phase change route.
> >
> > Rob
> >
>
> *snip*
>
> Could you explain this, or provide a link?
>
> enjoy
>
> -
> you stupid vulgar greedy american death-sucker
> some trillions of years ago a sloppy dirty giant flicked grease
> from his fingers
> one of those gobs of grease is our universe on the way to the floor
> *splat*
>