Matt Bradshaw designed and built PolyMod, a very cool, open source, polyphonic, modular digital synthesizer.
The PolyMod can operate in polyphonic (poly) mode making each module able to play multiple sounds, generating chords and harmonies.
This synth is entirely digital using a Teensy 3.6 with a Teensy Audio Shield to replicate traditional analog functions in software. This approach allows users to easily create polyphonic patches, something that can be more challenging in analog systems. This project also has the advantage of being a fairly low cost way to get into modular synthesis.
When the synth is powered up, the Teensy scans each of the module sockets and for each one detected, creates a virtual equivalent in software. When a key on the keyboard is pressed, the Teensy coverts the data to a control voltage (CV) signal, which can be connected to any of the modules to generate sound.
Current modules include an oscillator, filter, envelope, low frequency oscillator (LFO), amplifier, and noise generator.
More details on the PolyMod can be found on the HackaDay project page.
Code can be found in this GitHub repository.