[sdiy] Percussion MIDI controller

[Ingo Debus]

Am Donnerstag, 24.02.05 um 17:52 Uhr schrieb Magnus Danielson:

> I've done exactly this exercise and we had no problem what so ever. We 
> used a
> standard Tektronix desktop scope in single-trigger position. We had a 
> 450 us
> time difference between the two cases. Converting this time difference 
> into
> CV and Gate should be trivial, infact I intend to do the exercise very 
> soon
> anyway. We used two piezo-elements and a standard electric PVC tube.

PVC tube! That's a great idea. Anybody know the speed of sound in PVC?
How do I attach the piezo disks to the tube? I don't think piezos like 
to be bent. Is it better to attach them at the sides of the tube, or 
flat on some sort of "end cork"? In the former case I think I'd need 
some "adaptor" (perhaps made from aluminum) concave on one side, 
matching the tube's diameter, and flat on the other.


Fredrik Carlqvist wrote:

> If you use a (plastic) tube, you can put a piezo contact microphone 
> and a
> regular microphone inside the tube at the same end, and measure the 
> time
> difference between the two. Since the speed of sound in the tube is 
> higher
> than in the air inside the tube, you get a linear position dependent
> difference.
>
> At 1.5m distance, the air wave will take about 4ms and the plastic wave
> about 1ms.
>
> I guess you have to use a directed microphone so you don't pick up the 
> sound
> from the vibrating tube. But the sound wave should be stronger than the
> indirect vibrating plastic -> regular microphone signal.

Good idea too. I think the main problem here is to mount the regular 
mic so that it doesn't pick up the vibration of the tube via the 
mounting hardware.

> ... or perhaps you could even get away with only one regular mic... 
> Glued to
> the inside of the pipe.
>
> Add a peak detector. First comes a peak from the vibrating plastic 
> pipe,
> then the air wave as a second peak.
>

Hm, that sounds difficult. I found that the waveform (from the piezo) 
varies a lot, depending how and where the rod was struck. Electret mic 
capsules are cheap and piezo disks are even cheaper, so I can afford 
using more than one.


Paul Perry wrote:

> If you are using microphones at each end, I think the key
> would be to use tube (or rod) material that is as unresonant as 
> possible.
> Ideally, a cardboard tube surrounded by hard circular objects
> (like bangles) so you avoid problems of the structure resonating.

Wouldn't that defeat the linear relation between time difference and 
position? Something the processor could take care of, but it would be 
nicer if it hadn't to.

> If it is permitted to have a lead to the mallet, then you could put a
> microphone inside the mallet, and use the signal from the mallet
> mic to be one timing signal, and the front edge of the signal from a 
> mic
> at one end of the rod for the other.

Another great idea. When using two mallets it could be easily detected 
which one hit the rod. This way Magnus' duophonic idea could be 
realised too.


Thanks for all the feedback. I'm glad I asked here.

Ingo