The idea is simple, make a state-variable (fancy name for a filter that can be many things, such as LP, HP or BP) analogue filter that is digitally controlled.

I plan to design the actual filter around the LM13700 chip, both because it sounds great and because the cutoff voltage can be controlled with an input current.

The digital part will be realized using some kind of Atmel chip which converts the incoming diital MIDI signal to analogue control voltage that controls various parameters in the filter such as cutoff frequency, blend, LFO etc.