Optical Digital Output Mod

I am looking for some good info about how to add digital sound output to consoles (genesis, pc engine, and super famicom + mabye DC and PSX) to eliminate humming and various other undesirible audio artifacts. Does anyone have any links or general info for what I am trying to do? I have some techinal knowledge and proper equipment to do these mods but I just need more Info about where to pull signals from and such.

For SNES/SFC you can go to www.alpha-ii.com
You can find information on how to do it for Gamecube online, I just can't remember any sites at the moment.

EDIT-For PS1 I think you can just use the PS2's digital output...