Just on the off chance anyone has experience of this...
My newly prosounded gameboy is pretty quiet... but when it's got the link cable from my arduinoboy plugged in it suddenly picks up what sounds like slowly pulsing electromagnetic interference.
I should probably check if linking it to another DMG causes the same issue and eliminate "something I did to the link port" from the equation... But my current theory is that between the backlight and the (link cable powered) arduinoboy it's drawing too much power.
So... if that's the case is it likely adding a decoupling capacitor to the prosound will fix it?
Or would I be better off thinking about building an arduinoboy with it's own power?