[PD] Anti-aliasing filter

[julien.breval at tremplin-utc.net]

Selon Larry Troxler <lt at westnet.com>:

> On Wednesday 19 May 2004 08:32, julien.breval at tremplin-utc.net wrote:
> > Hello
> >
> > > I've got an additive synth which generates some very
> > > high frequency partials and hence aliasing distortion
> > > occurs when the fundamental is high enough.
> > >
> > > I tried to fix this by using a low pass (lop~) filter
> > > before the dac. This didn't fix the problem and I
> > > suspect that lop~ only filters up to about 22kHz. Is
> > > there a way of implementing a 'brick wall' filter
> > > which will stop ALL frequencies past a cutoff? (i.e.
> > > not just frequencies in the audible range)
> >
> > You can make a 'brick' filter using an FFT transform, but the sound will
> be
> > more or less altered (depending on the window size and the overlap you
> use
> > for the FFT). I programmed a dual similar filter for adding the low
> > partials of a sound to the high partials of another sound (the limit is
> as
> > vertical as possible). As there was nothing else running in the patch, I
> > could set the window size to 4096 (for accurate frequency resolution) and
> > the overlap to 16 (for accurate "time" resolution). The problem of this
> > method is that it takes lots of CPU.
> >
> 
> No, actually, the problem of this method is that it doesn't work in this
> case.
> Once aliased frequencies are introduced, the game is over, because the 
> frequencies have already been aliased.

you are right

It only works if the additive synthesis itself is 100% FFT-based (which is not 
the regular solution)