[linux-audio-dev] detune

[mlang]

Tim Goetze <tim at quitte.de> writes:

> [Hans Fugal]
>
>>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?

Probably libSoundTouch?

> 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