9

The NES/famicon has always used the Ricoh 2A03, a modified MOS 6502 processor to generate the memorable music and sound effects of classic Nintendo games and continues to define the newer 8-bit / chiptune music to this day.

Several games included additional chips (known as Memory Management Controllers or MMC) to increase processing power of games. I have always understood that the presence of additional chips can increase memory through methods like ROM bank switching, but I have never understood how certain MMCs like the VRC6, the VRC7 and even the Famicon Disk System were able to add additional sound channels.

Are these MMCs simply memory storage or are they actually just additional CPU's? And how are the additional channels mixed with the 2A03? Do the MMCs route audio through itself and back to the famicon?

13

In the case of the Famicom, the extra chip(s) on the cartridge would contain some additional sound generators. The cartridge slot contained a pin that would accept audio from the game cartridge and mix it with the other audio coming from the console and send it all to the TV.

Pinouts: https://wiki.nesdev.com/w/index.php/Cartridge_connector

Notice that the audio pin is missing from the NES cartridge slot; it's not possible for NES cartridges to add additional sound channels without hardware modifications. The audio-input pin is present on the expansion connector underneath the NES. Some flash carts, like the Everdrive N8, will emulate the extra audio chips and send the sound to an unused pin on the cartridge. This unused pin is also available on the expansion connector, and bridging it to the audio pin (usually with a resistor or two to correct the volume) will allow Japanese games run from the flash cart to produce expansion audio when played on an NES.

http://www.retrofixes.com/2014/05/open-hidden-sound-channels-in-nes.html

  • This is a great first answer! – cbmeeks Feb 9 '17 at 21:53
  • @cbmeeks Long time listener, first time caller. ;) Had the answer right on the tip of my tongue (fingers?) since I had to resistor-mod my NES when I got my Everdrive. Mostly I loiter around the DBA site, though. – db2 Feb 10 '17 at 14:42
5

The MMCs are neither memory storage nor additional CPUs. They're integrated circuits -- basically they're chips containing lots of circuitry. The CPU and PPU in the NES are also ICs, and so is the processor in your computer. The circuitry in the MMCs commonly performed functions like bankswitching and scanline counting

The Famicom's cartridge connector contained an "audio in" pin which allowed circuitry on the cartridge to output audio, so some of the more advanced MMCs had sound generation circuitry, which behaved similarly to the 2A03's sound generation. The audio pin is missing on the NES; see my answer here.

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