[chuck-users] AutoCorr behavior

[Curtis Ullerich]

I believe I found the bug. I sent a pull request
<https://github.com/ccrma/chuck/pull/151> with the fix and a lengthy
description. xcorr_fft is taking the FFT of the input, but the upstream
unit of the XCorr/AutoCorr UAnae is already required to be a UAna
<https://github.com/ccrma/chuck/blob/main/src/core/uana_extract.cpp#L1210> (so,
it's already an FFT).

On Tue, Jul 21, 2020 at 1:07 AM Curtis Ullerich <curtullerich at gmail.com>
wrote:

> As a preamble, I'll note that I posted a question on chuck-dev about
> AutoCorr/XCorr always crashing for me. This patch
> <https://github.com/ccrma/chuck/pull/150> seems to fix that.
>
> That said, I don't understand the results I get from AutoCorr. For
> periodic inputs, I expect to see periodicity in the output. For small FFT
> sample sizes, I see the expected peak at 0, and at large sample sizes I see
> a second peak at the end of the window. See example plots at 128
> <https://i.ibb.co/BBZXgBB/autocorr-128.png> and 4096
> <https://i.ibb.co/Mn5fty5/autocorr-4096.png>. That second peak is
> correlated with the window size, not the input frequency.
>
> Thanks to Mario for his gnuplot wrapper
> <https://github.com/mariobuoninfante/ChucK_various> that captured those
> plots.
>
> Here's the plotting code if anyone would like to repro:
> SinOsc s => FFT fft =^ AutoCorr c => blackhole;
> 4400 => s.freq;
> 128 => fft.size;
> 300::ms => now;
> c.upchuck();
> Plot plot;
> "autocorrelation of 4400 hz, 128 samples" => plot.title;
> plot.plot(c.fvals());
> 200::ms => now;
>
> Should I be using AutoCorr differently? Am I looking at the power spectrum
> or something, and not the correlation vector like I think I am? My current
> understanding comes from reading uana_extract.cpp many times with
> references like this <http://paulbourke.net/miscellaneous/correlate/>.
>
> Thanks,
> Curtis
>
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