In hardware, the audio from the 2A03 APU pins go through an inverted
preamp using one of the 74HCU04 inverter chips in the motherboard.
Though this may be cost-effective on Nintendo's part, the signals on the
hex inverter chip can interfere with one another, especially on the
audio. This can be heard as a buzz or a high pitched whine.
The only known signals to interfere with the audio so far is:
- /A13, the inverted signal of PPU A13 going into the cartridge
connector.
- /OE1, Output enable for controller reads, though this is only found
present on the RF Famicom.
