I was taught:

for(int sample = 0; sample < 128; sample++) {}

…because the actual number of iterations is right there in the test…

Since you seem to have a great setup with a reference mic already - mind recording how this off-by-one actually sounds like? As in, how it distorts the ~300Hz range. Spectra are great but I'd love to hear it (mostly to figure out whether I'd have noticed it or not)

If it takes too long, I guess I can call this an excuse to learn more about audio processing and try to replicate this myself

<Bored Marvin the depressed robot voice>
File a bug report. Complaining on social media doesn’t work.
</Bored Marvin>

…or as we cynically used to say inside Apple:

Radar # or it didn’t happen.

I don't understand those images or your explanation of the bug (other than the fact that it's an off-by-one error), but that audio demo is about as subtle as being run over by a goddamn freight train.

Jesus Christ

Yeah pretty much. This DSP needs to be implemented somewhere and PipeWire ended up being the best option. And it does supersede most other frameworks/use cases (PA, JACK, etc.) so there's very little reason not to use it.

Though we switched our users over earlier last year anyway, because PA was broken with headphone jack hotplug and PW worked fine, and it wasn't worth spending time debugging that when we already planned to require PW for the upcoming speaker support.

The speaker safety stuff, though, will be a stand-alone daemon working together with the kernel driver. So if you really want to use PA or anything else, it'll sound (very) bad, but you won't blow up your speakers. If the safety daemon dies or is not present, the kernel will fall back to a very low but safe volume cap.

...really?

Get real speakers.
Jeez.

It doesn't sound like "higher volume", though, it's a very distinct, harsh, dissonant buzz.

What you're describing as "higher volume" is saturation/distortion. Distortion is always harmonic, so although it can sound harsh, it is not fundamentally changing the pitch of the sound.

Here are three tones. The first one is pure, the second one is clipped (distorted), and the third one is what happens with this weird kind of artifact (the actual M2 Air output is actually more broken than this, it's not just this bug, but it's part of it). The third one has a distinct "buzz" that sounds out of tune, like interference from another tone. That's what you get on the Mac. It's a very unnatural, unmusical kind of artifact that sounds very harsh because it cannot be produced through physical processes in real life (no instrument sounds like that).

@argv_minus_one You can use PA/JACK/ALSA/etc without PW, we just aren't building the DSP processing tooling for those. So if you want your speakers to sound good with something that isn't PW, you're on your own. I actually considered doing the DSP as an ALSA plug-in to make it more universal at one point, but it turned out to be a bad idea for various reasons.

PipeWire provides PulseAudio, JACK, and ALSA compatible interfaces, so apps that use any of those will work with PipeWire and can use our DSP. But you need to actually run PipeWire.

edit:
Not fine broken too!
I first used 300 HZ like in your text.
If I use the 270 Hz like you did in the video it’s very noticeable distorted too.

plays fine on my M2 MacBook Air 13“ with Sonoma 14.0
Found the very same website tone generator you used.
MBAir plays the same like iPad and not as distorted like in your video - maybe it is an “not all Models” Problem if some hw parts?

Edit:#2
Use 270 HZ not 300 Hz to test.