one paradigm of asymmetric windows is to convolve a symmetric one (like the
Hann) with a filter. So what they actually do is 1) inverse-filter the
sound; 2) PSOLA it with Hann window; 3) filter the outcome. Seems there are
plenty of papers discussing using the linear-predictive filter in 1) and 3).
But I've been wondering 'bout this: why centre the window with an energy
bump? and what if there's no bump?
Xue
-----Original Message-----
From: robert bristow-johnson
Sent: Sunday, October 27, 2013 9:29 AM
To: music-dsp@music.columbia.edu
Subject: Re: [music-dsp] PSOLA pitch shifting - resample or not?
about the first, i would square the incoming audio and filter it with a
LPF but with a high cutoff frequency. look for maximum bumps in that
smoothed squared waveform (maximum energy), record the latest bump
location, and *nudge* the window location so that the center of the
windows (assuming a symmetrical Hann-like window) eventually gets
centered around the maximum energy pulses. in other words, 99% of the
location of the window should be 1 period later (as determined by the
pitch detector) than the previous window location. and 1% or 2% should
be nudging it either a little earlier or a little later toward the
nearest maximum energy pulse.
if you're doing an asymmetrical window (which i haven't done), perhaps
center the maximum of the window around the maximum energy pulse. the
problem i have with the non-symmetrical window is making it sufficiently
complementary. if you have complementary windows (upslope + downslope =
1), then if there is zero pitch shifting, what comes out is an exact
(but delayed) replica of what goes in.
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