This might be the way to support game with stereo sounds, even though most games will use mono, and is probably possible to implement in hardware, works with cartridges which include their own audio, doesn't cause problems on mono systems, etc
Make the unused $4009 register to control stereo:
Therefore, the default can be $0F. (Even if the cartridge has no pins for the audio, it can be treated as a direct connection instead.)
Make the unused $4009 register to control stereo:
- bit7 = output internal square waves to left channel.
- bit6 = output internal triangle/noise/DPCM to left channel.
- bit5 = output internal square waves to right channel.
- bit4 = output internal triangle/noise/DPCM to right channel.
- bit3 = output internal square waves to cartridge.
- bit2 = output internal triangle/noise/DPCM to cartridge.
- bit1 = output cartridge audio to left channel.
- bit0 = output cartridge audio to right channel.
Therefore, the default can be $0F. (Even if the cartridge has no pins for the audio, it can be treated as a direct connection instead.)