Trigger MIDI notes using simple piezo/capacitive sensors and Bela

nstalati

Hello everyone,

This is my first post here and I am extremely new to Bela so pardon me for any silly questions. I am wondering if it is possible to trigger MIDI signals from Bela by inputing analog signals through a vibration/motion sensor (could be some piezo element or maybe capacitive or anything else!) ?

So my goal here is to be able to have a setup where I can tap on a piezo element connected to Bela, which picks up the simple analog signal and converts it into a MIDI packet (with proper Velocity, pressure, etc. MIDI information) which can be probed to visually see on a oscilloscope or sent to a device that understands MIDI to output all sorts of sounds.
For example: One application could be as follows: a) Tap on sensor b) MIDI output from BELA (with sub ms latency) c) Connect to some synthesizer/computer with software (Garage Band?) to play based on the MIDI packet received.

Also, if there are other more suitable, < 1ms latency solutions to achieve my goal, I would really appreciate you guiding me towards it.

Thanks a lot for your help! 🙂

Neel

giuliomoro

The "new" image would allow you to do this: Bela shows up as a MIDI device on the computer it is connected to and therefore can send and receive MIDI messages to the host.

However, be advised that the MIDI - USB communication will go through the standard Linux driver priority on the Bela side and then through your operating system's USB and MIDI drivers, so it will not be "ultra-low latency". However, it's probably still fine for many applications. Keep an eye out for when we will release the image, it should take a week or so.

nstalati

Thanks for the response! Just to make sure I understand clearly, I have a couple of follow ups:

I would like to understand the functionality of the Analogue Inputs on the Bela cape board. Does Bela have any adjustable front-end circuitry which allows me to tweak the gain, filtering, or any other signal conditioning applied to my sensor signal before A/D conversion?

Also if it does have any such front-end processing, can I control any parameters using code?

Finally, you mentioned that outputting MIDI will go through standard Linux USB/MIDI but I was looking for a way to get very fast MIDI response to touch input actions. Is it possible to run the USB/MIDI thread within the Xenomai kernel, or is that beyond the scope of your project? If it’s out of scope, do you know how much latency (and timing jitter) is added in the Linux USB/MIDI driver? Can any parameters in the driver be tweaked to optimize latency?

Once again thanks a lot for your time and effort with this! 🙂

giuliomoro

There is no front-end circuitry to adjust gain or filtering. An expansion capelet is available which provides fixed high- and low-pass filtering, limiting the bandwidth to the audio range.

You would not be able to tweak the performance of the USB driver on the board, and whether you can do it on the host depends on your system.