This is one of the four SIDFB module boards that will be in MIDIbox ASIDITY. It contains two SIDs and their shift registers; a CD4052-based digitally-controlled filter capacitor switching circuit, 4 pairs capacitors per SID; LM1973-based digitally-controlled audio attenuators controlling feedback to self, feedback to other SID on the board, feedback to SID on next board, external audio output, and master L and R outputs; and the mixer circuit for all the feedback inputs (and external input) to e