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0 pointszeta01343y ago0 comments

Very carefully scheduling NMI and IRQ updates to achieve a virtual sound channel on an old NES, resulting in a sawtooth bassline with volume control. This steals about 13% of the CPU time. The biggest hack in the arrangement is the scheduling of OAM DMA to sneakily just barely fit between timed audio updates, since it pauses the CPU and would otherwise cause an audible glitch. Among other things, if NMI notices that it interrupted IRQ, it apologizes profusely and exits immediately, which is a rather unusual workaround on 6502 systems.

I've open sourced the technique with a more detailed writeup [1] and I like to think the game I created with it is pretty fun, [2] but of course that's subjective. You can try it in your browser here. [3]

[1] https://github.com/zeta0134/z-saw

[2] https://zeta0134.itch.io/tactus

[3] https://rusticnes.reploid.cafe/wasm/?cartridge=tactus.nes

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superdisk3y ago

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without having to use crazy timing stuff?

zeta0134OP3y ago

Yes, it comes down to how the DPCM channel encodes sound data. A sawtooth gets most it's energy and harmonics from the sudden jump at one end. This impulse is impossible to encode in 1-bit delta encoding, the closest you can get is a slightly steeper slope. The audible effect is similar to a low pass, with the strength dependent on the intended amplitude, so instead of a nice sharp saw, you get a muddy not quite triangle.

A few commercial games did use the DPCM channel melodically, but the main drawback is the large size of the samples relative to the commonly available ROM chips of the era. Price is a big factor when you need to put one chip in every game. Here's how that sounded in practice:

https://www.youtube.com/watch?v=LEgoYUzwabI

superdisk3y ago

Ah sure, but how about just generating a triangle using a small block of code in RAM? Then you're not wasting any storage.

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0 pointszeta01343y ago0 comments

[1] https://github.com/zeta0134/z-saw

[2] https://zeta0134.itch.io/tactus

[3] https://rusticnes.reploid.cafe/wasm/?cartridge=tactus.nes

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superdisk3y ago

Cool. Kind of unrelated, but is there anything stopping someone from just generating a sawtooth DPCM sample at runtime and using the channel as normal without having to use crazy timing stuff?

zeta0134OP3y ago

https://www.youtube.com/watch?v=LEgoYUzwabI

superdisk3y ago

Ah sure, but how about just generating a triangle using a small block of code in RAM? Then you're not wasting any storage.

1 more reply

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