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OH MY FUCKING GOD.

Pictured: Apple's M2 MacBook Air 13" speaker response (measured with a mic), and the response you get when you zero out every 128th sample of a sine sweep.

They have a stupid off-by-one bug in the middle of their bass enhancer AND NOBODY NOTICED NOR FIXED IT IN OVER A YEAR.

So instead of this (for a 128-sample block size):

for (int sample = 0; sample <= 127; sample++)
// process sample

They did this:

for (int sample = 0; sample < 127; sample++)
// process sample

Legendary audio engineering there Apple.

We can now, very confidently say the audio quality of Asahi Linux will be better than Apple's. Because we don't have blatant, in your face off-by-one bugs in our DSP and we actually check the output to make sure it's good 😂

FFS, and people praise them for audio quality. I get it, the bar is so low it's buried underground for just about every other laptop, but come on...

Edit: replaced gif with video because Mastodon is choking on the animation duration...

Edit 2: Update below; I can repro this across a large range of versions on this machine but none of the other models I've tried so far. It is definitely a bug, very very obvious to the ear, and seems unique to this machine model.

Edit 3: Still there in Sonoma, this is a current bug.

@marcan What causes the parallel lines to the main frequency?

@dascandy42 It is aliasing at intervals of 1/128th of the sampling frequency, which is what happens with this kind of bug due to the mathematics involved. Basically you can think of it as downsampling (folding and truncating the spectrum) to 1/128th the sampling frequency (375Hz in this case, which gives you 187.5Hz worth of spectrum), and then copying and pasting and mirroring the resulting spectrum 128 times to fill in the entire original frequency range.

The increasingly angled lines in the real spectrum are the intended result of the filter (harmonics).

@marcan Yes.

I mean the two lines around the first harmonic that are parallel to it.

Peter Bindels

@marcan Look to be 50Hz above and below the frequency. Interference from mains?

@dascandy42 Ah yes, that's another aliasing-type artifact (and possibly a more audible one). I don't have a simple explanation for that one, but it might be related to the 1/128 bug in some way (maybe a larger block size is involved?).

It's not mains intermodulation, that would affect everything.