Just curious - what are you using to provide the equivalent of vibrating lips to the brass horns? They need more than just air blowing in don't they?
I'm working on a midi trumpet, which will mainly focus on simple buttons, a pressure sensor (that only measures blowing pressure, not actual lip vibrating) and a midi connection to a simple general midi synth. But the idea of connecting stuff to real brass instruments got me really interested!
The Hornblasters or train horns look really interesting. They seem to work with a vibrating membrane, the frequency of which is "locked" by the length of the horn, just like with real brass instruments. In real brass instruments you have to blow the somewhat right note with your lips, and the fingering, which sets the length of the tube from the mouth piece to the bell, just confirms the tone. So these Hornblasters might have some preset tension in the membrane for each desired tone. And then the length of the horn just confirms the right tone or frequency. If a trumpet player's lip tension doesn't meet exactly the length of the horn, it's still the length of the horn that sets the right note. This might be true with the Hornblasters, too (my guess). So what I'd like to test is cutting the Hornblaster right at the root of the horn-bell part, leaving a hole just at the membrane cup. Then I'd attach a horn instrument without mouth piece to the hole.
First I'd test the membrane (diaphragm) without the attached horn instrument. I'd run air from a compressor through the thing. I'd try to vary the pressure to see if only changing the air pressure would change the produced frequency in the membrane. If that doesn't change the frequency (pitch), I'd need something to change the tension of the membrane. If that starts to work, I'd be ready to attach the horn instrument. What it would need is something to control the pressure of the supplied air, maybe something to control the membrane tension, and three sollenoids pushing the valves of the horn instrument. Everything disguised in a paper mache mannequin.