A dead simple implementation of the Frequency Selected Sound Energy Bead Detection B in Game Dev's Beat Detection article. Less than 70 lines including visualizations! Woot!
This is a quadrature encoder based on the WW-12 Wheel Watcher encoders built for robotics. They cost nearly 50 bucks for two of them, so we're designing our own. This is a virtual encoder that allows you to see the signal and test different encoder disk styles and sensor arrangements.