What's going on with LSP's Spectrum Analyzer's mid-side information here? Bug? [solved]

[elcalen]

Hiya. I've recently been noticing something a little weird with the LSP Spectrum Analyzer in mid-side mode. There appears to be content shown that is not present in the normal stereo mode and I don't think should be part of the audio signal; what looks kinda like a noise floor or something. It's particularly visible in the high frequencies, but that might be partly because I have it set to a 4.5dB slope.

I wonder if sadko4u or anyone else can explain what's going on here, and whether it's a bug?

Below are two screenshots of the same sound source (a low-passed kick sample, so there shouldn't be much high content). The first is the one with mid-side enabled, the second without. Note the difference in the 2-20kHz region. (This is with LSP 1.2.15, clap version.)

[sadko4u]

Hello! Yes, this can be the problem. What's your audio settings? I mean, sample rate and buffer size.

[elcalen]

I'm using REAPER, with pipewire-alsa as the output device, running at 48kHz, with a 2048 sample buffer size. I doubt my distro would have much effect on something like this, but I'm on Kubuntu 23.10, if relevant. And like I said, this is the latest LSP build and the CLAP version (the plugin's settings can be seen in the screenshots). I haven't tested other audio settings or plugin formats.

[sadko4u]

OK, I'll try to reproduce the problem.

[sadko4u]

Reproduced the problem.
The reason was in invalid implementation of AVX and AVX-512 Left/Right -> Mid/Side conversion function in the lsp-dsp-lib library.
The fix will come in the next release.

[elcalen]

Awesome, thank you!