[sdiy] Percussion MIDI controller - again

[Ingo Debus]

Am Sonntag, 27.03.05 um 21:35 Uhr schrieb Ian Fritz:

> Let's see ... the main peaks at one transducer should be separated by 
> the time it takes a sound wave to travel to the other end of the tube 
> and back, no?  Sound in solids travels about 5mm/us, so a 2m tube 
> should ring at about 0.8ms. Sound roughly like what you are seeing?

I don't have a storage scope here. Now I recorded the signals into my 
computer at 96 kHz sample rate. This reveils more than I could see on 
the analog scope. First, the waveform does *not* look like a dampened 
oscillation. The peaks don't have the same distance in one impulse. It 
looks more like a bass drum sample (remotely), with different time 
scale of course.
Anyway, the peaks are more like 0.4ms apart. I don't think this ringing 
is caused by the refections at the ends of the tube. There are also 
visible echoes, but these are about 10ms...20ms apart. I think these 
are caused by bouncing. I used a small screwdriver as a beater.
I suspend the tube very loosely from two rubber bands hanging off two 
microphone stands (already fell down several times ;-)). Perhaps that's 
enough to dampen the reflections?

> Your best bet for detection might be to simply look for the first 
> arrival of the excitation signal: amplify the signal by a large 
> amount, set a threshold somewhat above the noise and detect the first 
> crossing of the threshold using a comparitor followed by a pulse 
> stretcher (to keep the final output from further switching during the 
> time the excitation rings down).
>

Yes, I think I'll get a real comparitor like a LM393, this should work 
better. The 74HCT14 has a very large hysteresis.
The signal from the piezo at the far end builds up slowly. So setting 
the threshold will become tricky.
Perhaps I'll try more than two piezos. Thus the two piezos used for 
detection aren't that far away from the beater, and the impulse doesn't 
get that much distorted. The microprocessor I'm using has five 
"Programmable Counter Array" modules, so five piezos could be handled.
Viewing the recorded signals I also notice that the relationship of the 
amplitudes of the impulses could be used for position detection. This 
was hard to see on the non-storage scope.

> Don't give up!  You should be able to get this idea to work.

Thanks for the encouragement! I was indeed almost about to give up.


If anyone is interested in the AIFs I recorded I can send them.

Ingo