Rockband 2 electronic drum adapter

So, I know this has been done before (not with the arduino) but I have some spare arduino's sitting around and always like creating myself.

I have a roland td-9 drum set which outputs midi so the plan was to input the midi into the arduino using the method someone posted on the forum here turn the midi note into a analog value and forward it to the the rockband controller.

What I'm not sure about is how to trigger the rock band controller properly which will be wired to the arduino.

From what I can tell the current solutions around the net seem to use opto-isolators but I don't think I understand what they do or there real purpose. I read wikipedia but I still don't think I understand their usage.

I'm also not sure if there usable in my case because of some differences from rockband to rockband2. Largely the velocity sensitivity that theys included. True its only 2-3 levels and not true velocity sensitivity but i'd still like to have to work properly.

So I guess to make a long question short what would I need to turn the midi note into an analog value fit for the rockband controller? i'm assuming I need some sort of electronic component but I'm can't figure it out.

Any help would be greatly appreciated.

I believe I've actually figured out what an opto-isolator does but please someone correct me if I'm wrong.

Based on a definition I found an re-read 100 times a opto-isolator will have an electrical output waveform that is identical to the electrical input waveform.

That part makes sense to me. It isolates the signal by breaking the circuit and sending it by light. I follow.

Also, could you use PWM outputs to turn the midi note into an analog value? obviously it would take some tweaking to find the right values but in principle it should work right?

I have a bunch of the TLC5940NT sitting around so I was thinking I could use 1 of them.