Integrating a Yamaha keyboard into a homemade synth

[krismate]

Hi guys,

I have almost no prior experience with CPU's so I would like help with my project:).
I dissected the and intend to use it for my synth by translating the logic to a range of voltages and then control the synth by voltage per octave. Can someone explain to me what exactly is going on at the keyboard section? I have an idea but I can't really trust myself on this one. My questions are: How would I translate the keys to a range of voltages and if the 6 notes are grouped at the same pin at the decoder how is there polyphony?
https://www.manualslib.com/manual/1528785/Yamaha-Psr-E413.html?page=58#manual
Any help from more experienced people would be grand, be it a plan or some tips.

Cheers!