MIDI is a venerable protocol (dating back to 1983) that is used to communicate between synthesizers, controllers, sequencers, sample players, computers, mobile devices, drum machines, and other electronic music making devices.
Electronic music gear is often made up of multiple discreet components that each have their specialized task to perform. They can communicate with each other through data messages, control voltages, gate and trigger signals, and/or audio signals in order to come together as a whole, united system capable of being performed, recorded, played back, and ultimately, producing audible music!
A simple and very common use case is to have a controller, such as a piano-style keyboard, send Note On and Note Off data to a music synthesizer (including software synths on your computer or mobile device).
Press a key and a message is sent telling the synth to play a specific musical note. Release the key and a message is sent to the synthesizer telling it to stop playing that note.
Getting a bit fancier than simple on/off messages, MIDI can also be used to send "continuous controller" CC messages, typically the result of turning a knob or pushing a slider on the keyboard controller. These can be used to sweep through the cuttoff frequency of a low pass filter, or modulate a tremolo, and many, many other parameters.
A similar scheme is also used to send pitch bend info -- often represented by a pitch bend wheel on the left side of a MIDI keyboard.
We can use the Trellis M4 to send any kind of MIDI message we like, right over USB. In this project, we've mapped the 32 buttons to play the "keys" on your synth, from a low C (MIDI Note 32) up to a high G (MIDI note 64). You can hold many buttons at once to create chords if you're using polyphonic synth software.
But that's not all! We can also play very expressive pitch bend and modulation with the Trellis M4. This is all thanks to the Analog Device ADXL343 accelerometer built right onto the board! Tilt left and right to pitch bend down and up, and tilt forward and backward to gradually adjust the filter or tremolo or whatever you like on MIDI CC 1.