From the video notes:
This is my friend Jay-Boi playing with the Music Maker Deluxe in a very basic but effective musical setup.
The Commodore controller is connected directly on the keyboard matrix of a Commodore 64c (SID 8580). The sound used is a 3 voice sawtooth waveform with adjustable LFO amount and filter amount using the 2 potentiometers on the side.
The audio out of the C64c is going inside a DIY MS-20 like filter box that receives it’s modulation from the a Korg MS-20. The output of the filter box goes to a first track. The Korg MS-20 is connected to a second track.