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Peter Bindels
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Peter Bindels

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

1 Oct 2023 at 16:08 | Open on mastodon.social
Hector Martin

@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).

@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.

1 Oct 2023 at 16:18 | Open on social.treehouse.systems
Peter Bindels

@marcan Yes.

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

1 Oct 2023 at 16:44 | Open on mastodon.social
Peter Bindels

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

1 Oct 2023 at 16:45 | Open on mastodon.social
Hector Martin

@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.

1 Oct 2023 at 16:47 | Open on social.treehouse.systems
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