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 <curtuller...@gmail.com> 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 <curtuller...@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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