15 Jan
2006
15 Jan
'06
9:58 a.m.
Tim Goetze <tim(a)quitte.de> writes:
[Hans Fugal]
Probably libSoundTouch?
I'm about to write a DSSI/LADSPA plugin that
among other things, detunes
the signal by up to 15 cents. My understanding is that detuning is
accomplished by resampling. If that's the case, what do you do
with the time difference? Do you pad/truncate to get the same number of
samples you started out with? Wouldn't that introduce undesirables?
It would. As far as I know, there are two common ways to do this:
You can periodically window and granulate the signal, resample the
grains and resynthesize -- that's the faster way, doing it all in the
time domain. You'll get some comb filtering from the time-stretched
phase cancelling out during grain overlap, but if you're only doing 15
cent, it might turn out just fine. There's a library out there that
does this, I can't seem to remember its name though. Anyone? The other way I know of is to use a phase vocoder.
Basically, you
periodically window and granulate and then FFT the signal, do a bit of
math on the frequency content and resynthesize. The phase vocoder
keeps track of signal phase so the comb filtering effect is no issue.
The downside is the FFT and the math required to compute phase and
amplitude; it's quite a bit heavier on the CPU. Some code that does it
this way is at quitte.de/dsp/pvoc.html but for your intended use I'd
first look at the simpler solution mentioned above.
I'd like to suggest this article:
http://www.dspdimension.com/data/html/pshiftstft.html
It is wonderfully detailed, includes all the necessary C source
code to take the basic idea and make it fly. I used this
method to write my instrument tuner by ripping out the last phase
of the algorithm (resynthesis) and only using the exact
frequency peak info. Its pretty useful, and I learned a lot
from this article. Actually, I think thats the single most useful
DSP tutorial I've ever read. It made me really grok FFT, which
is something by its own right :-)
Oh, and it is pretty simple to make this stuff work with libfftw3, just
read tuneit.c (http://delysid.org/tuneit.html).
--
CYa,
Mario