## What's the fastest rate of repeated IRQ possible?

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems.

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lidnariq
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### Re: What's the fastest rate of repeated IRQ possible?

Pretty easy to simulate that situation, right?

Say we're aiming for 10kHz audio, 180cy per DAC update. OAMDMA would knock out 2 samples every 16ms.

A little bit of trivial perl:

Code: Select all

``````use Math::Trig;
for (\$cy = \$sa = 0; \$cy < 297805; \$cy += 180, \$sa++) {
\$newsample = 63.5*(1+sin(\$sa*pi/5)); # sine wave at exactly sample rate / 10, so 994Hz
if (\$cy*2 % 59561 >= 1028) {
\$oldsample = \$newsample;
}
print chr(\$oldsample);
}
``````
yields this sound:
sim.ogg
(3.7 KiB) Downloaded 109 times
Of course, this is roughly the worst possible situation; a sine wave will make this distortion maximally audible.

Memblers
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### Re: What's the fastest rate of repeated IRQ possible?

Neat, that does have a familiar ring to it. Thinking about the old samples I had recorded, IIRC, 001.WAV and 002.WAV are the only ones recorded while I purposely turned the OAM DMA off (they are sine waves also). Aliasing seems especially noticeable in 007.mp3 (those detuning glitches were in the music driver, I know it sounds awful).
http://membler-industries.com/squeedo/2 ... ps/007.mp3
http://membler-industries.com/squeedo/2 ... ion/samps/

tokumaru
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### Re: What's the fastest rate of repeated IRQ possible?

rainwarrior wrote:I thought as long as you set OAMADDR back to 0 before vblank ends things should be fine? (At least, I've used it before and not seen anything wrong...)

Is there some additional corruption you're talking about?
This whole OAM corruption thing confuses me to no end, but I think you're right. Setting OAMADDR back to 0 before sprite evaluation starts should work just fine.

It's just that for years we knew that using \$2003/4 was unreliable somehow, but AFAIK it was only recently that the exact behavior was properly analyzed and documented. I guess I just put OAM manipulation through \$2003/4 in my "list of things to never do" and it remains there to this day.

Anyway, I understand that the purpose is to be able to have PCM audio during actual gameplay, but 50% of the CPU time, slow ass OAM updates, vblank time being spent on audio... those are pretty severe restrictions, making the technique still prohibitive for most kinds of games.

lidnariq
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### Re: What's the fastest rate of repeated IRQ possible?

I know I've had situations where just writing to \$2003 caused corruption, even if we write 0 to \$2003 before rendering starts.

The last time this came up, Quietust said that in his tests, it works for some CPU-PPU alignments and not for others.

tokumaru
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### Re: What's the fastest rate of repeated IRQ possible?

Now that you mentioned it, I think I remember someone saying that the only safe way to use \$2003/4 is to write all 256 bytes and let OAMADDR wrap back to 0 (which's exactly what an OAM DMA does, only faster). That'll eat nearly your entire vblank time, even if you use unrolled code.

lidnariq
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### Re: What's the fastest rate of repeated IRQ possible?

last time

That was me.

You can also safely write the first 7 bytes by hand, but ... being only able to update sprite 0 and everything but X in sprite 1 isn't very useful.

tokumaru
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### Re: What's the fastest rate of repeated IRQ possible?

Oh, I completely forgot about that topic! Hahaha Yeah, that was certainly the most recent conversation that contributed to my "only fill the OAM using DMAs" stance.

rainwarrior
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### Re: What's the fastest rate of repeated IRQ possible?

Hunh, well that's new to me. I'm going to have to do some tests laster, ha ha. :S

za909
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### Re: What's the fastest rate of repeated IRQ possible?

I've tested the effect of the OAM DMA distortion with my FME-7 test ROM before. This exemplifies the worst possible distortion though where the output is delayed, since the sample is fetched from ROM inside the IRQ handler. But this could be solved as well by manually adjusting the vector after each OAM DMA to get the "missed samples" effect instead.
Attachments
Warped with OAM DMA.nes
(520.02 KiB) Downloaded 103 times
Warped.nes
No DMA
(520.02 KiB) Downloaded 102 times

dougeff
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### Re: What's the fastest rate of repeated IRQ possible?

Sounds good. Those file sizes, though. Hmm.
nesdoug.com -- blog/tutorial on programming for the NES

lidnariq
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### Re: What's the fastest rate of repeated IRQ possible?

11.6kHz audio consumes an awful lot of ROM awfully fast. Sometimes compression helps, but ...

On the other hand, it is pretty valid to say that, right now, the economies of ROMs are such that you may as well use a 512KiB ROM if you have any PRG bankswitching at all.

tepples
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### Re: What's the fastest rate of repeated IRQ possible?

lidnariq wrote:right now, the economies of ROMs are such that you may as well use a 512KiB ROM if you have any PRG bankswitching at all.
Unless you're limited by CPLD macrocells.