17. DPCM Sound

This one is a bit tricky. DMC files are huge. You might want to avoid them altogether. Most games don’t use them.

However, you might want to put in a sound effect that sounds like a real thing, a voice saying one word “fight”, or a dog barking, or maybe some more realistic drums, or cool bass line.

Try to keep them very short, like 0.2 seconds. You can / should edit them down with a sound editor like Audacity. I’m using some royalty free samples.

The faster the frequency of the DMC sample, the more bytes of ROM it uses. Faster is higher quality… below rate A, the sound quality is terrible. Maybe split the difference and use C or D. I actually used rate F for mine, but I might have lowered that if I ran out of ROM space.

Famitracker can import the sample, and map them to a note. With famitone2, you must put the samples on instrument zero, and use note range C-1..D-6. Also, the instrument zero must have a volume sequence, even if you don’t use it (just a quirk of famitone2). That’s under 2a03 setting, instrument settings. Put a check mark next to volume (this is true for all instruments).

19_FT2

Even if you only want to use the sample as a stand alone sound effect, and not in the song, it must be included in the song like this.

Now, when you export the text it will have the dmc file defined. and when you use text2data it will output a dmc file. Include that in crt0.s in a “dmc” or “sample” or whatever, segment. I called it “SAMPLES”.

This is the tricky part. DMC samples must go between $c000 and $ffc0. preferably as far to the end as possible. Subtract the byte size of the samples from ffc0 and round down to the next xx40. In the cfg file, define the SAMPLE segment to start there.

Now, also, go to the top of crt0.s and define FT_DPCM_OFF to start at the same value.

The .dmc file is 5,248 bytes long. That’s rather large for a NES game. It will be ok for a simple example code, though.

Now, if the samples are in the music, they should play with the music.

But, if you wanted them to be sound effects, you have to call them in your code, like

sample_play(sample#)

How do we know what the sample number is? It depends on what note you mapped them to in your famitracker file. If you look at the output file from text2data (mine is “DMCmusic.s”), you see a samples area. You can tell by the non-zero lengths at the right side, that our samples are at 25, 27, 29, 30 and 32. It counts up from C1, so if we had mapped it to C1, it would be sample_play(1). But the sample I want is at G3, so we need sample_play(32).

I put this with a new jump. It’s just my voice, saying “jump”, to be silly.

Hope this makes sense.

https://github.com/nesdoug/20_DPCM/blob/master/platformer4.c

https://github.com/nesdoug/20_DPCM