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Expand the comment on spectrum frequency filtering

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Přemysl Eric Janouch 2021-07-05 23:42:51 +02:00
parent 0646cea126
commit 66bc3f1c2c
Signed by: p
GPG Key ID: A0420B94F92B9493
1 changed files with 11 additions and 2 deletions
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nncmpp.c
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@ -769,9 +769,18 @@ spectrum_init (struct spectrum *s, char *format, int bars, struct error **e)
// Discard frequencies above 20 kHz, which take up a constant ratio
// of all bins, given by the sampling rate. A more practical/efficient
// solution would be to just handle 96/192/... kHz as bitshifts.
// solution would be to just handle 96/192/... kHz rates as bitshifts.
//
// Trying to filter out sub-20 Hz frequencies would be even more wasteful.
// Filtering out sub-20 Hz frequencies would be even more wasteful than
// this wild DFT size, so we don't even try. While we may just shift
// the lowest used bin easily within the extra range provided by this
// extension (the Nyquist is usually above 22 kHz, and it hardly matters
// if we go a bit beyond 20 kHz in the last bin), for a small number of bars
// the first bin already includes audible frequencies, and even for larger
// numbers it wouldn't be too accurate. An exact solution would require
// having the amount of bins be strictly a factor of Nyquist / 20 (stemming
// from the equation 20 = Nyquist / bins). Since log2(44100 / 2 / 20) > 10,
// it would be fairly expensive, and somewhat slowly updating. Always.
double audible_ratio = s->sampling_rate / 2. / 20000;
s->bins = ceil (necessary_bins * MAX (audible_ratio, 1));
s->useful_bins = s->bins / 2;