On 10/23/13 2:19 AM, Ross Bencina wrote:
The idea is to isolate each vocal tract filtered glottal pulse in its
own grain (i.e. glottal pulse convolved with the impulse response of
the vocal tract). Thus changing the grain rate is more or less
equivalent to changing the glottal pulse rate leaving the vocal tract
IR remains unchanged (except you're also convolving with a window).
i like to call this "Lent's algorithm" after Keith Lent from a 1989
CMJ. it has also been attributed to Hamon. it needs a very good pitch
detector to pull it off. and i wrote an AES paper about it in 1995, i
think.
If the IR length is longer than the fundamental period you won't be
able to isolate the pulses exactly. But if the IR is shorter than the
period then you would expect lowering the frequency to add gaps.
Similarly, raising the frequency would increase overlap of each
filtered glottal pulse.
What I'd like to know is what's the best way of centering the windows
on the pulses? and is it better to use asymmetrical windows?
dunno how to answer the 2nd question... perhaps an asymmetrical window
is better.
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.
--
r b-j r...@audioimagination.com
"Imagination is more important than knowledge."
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