[chuck-users] AutoCorr behavior

[Curtis Ullerich]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.cs.princeton.edu/pipermail/chuck-users/attachments/20200721/09584a30/attachment.html>