Every time I look at the pinout of the C64 cartridge port, it pisses me off a little that it doesn't have a pin that feeds into the SID's audio input. There's three fucking ground pins on it, there was room to spare. Hell for some reason it has the dot clock off the VIC-II, but audio? Like fuck.
@elfi I would have traded the top address bit for an audio pin if it came to it.