[sdiy] Percussion MIDI controller - again

[Ian Fritz]

Ingo: (inline)

At 06:06 AM 3/28/2005, Ingo Debus wrote:

>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.

Well, you are looking at the bending modes of of a fairly rigid tube, so 
the mode spectrum might be complicated.  At least it's comparable with my 
order-of-magnitude estimate.

>  I don't think this ringing is caused by the refections at the ends of 
> the tube.

I don't see what else it could be.  Maybe something in your geometry I missed?

>There are also visible echoes, but these are about 10ms...20ms apart.

That would correspond to the time for the round-trip transit in 
*air*:  4m/0.34m/ms.
(Two series of bounces, one in the tube and one in the air inside it.)

>  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?

No, probably not.


>>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.

Yes, that will be the tricky part.

>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.

So you would take the ratio of two amplitudes?  Could work.

   Ian