Re: [chuck-users] AutoCorr behavior

26 Jul 2020

      I noticed in the source code that there's a UAna that just does the domain
change (Flip). That's exactly what was needed; no implementation change
necessary. :)

Corrected example:
SinOsc s => Flip f =^ AutoCorr c => blackhole;
440 => s.freq;
1024 => f.size;
f.size()::samp => now;
c.upchuck();
Plot plot;
"autocorrelation of 440 hz, 1024 samples" => plot.title;
plot.plot(c.fvals());
100::ms => now;

On Thu, Jul 23, 2020 at 11:14 PM Curtis Ullerich 
wrote:
...
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 
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