[PD] Anti-aliasing filter

[Larry Troxler]

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.

> Another solution is to calculate the highest partial that won't produce
> aliasing, in function of the fundamental.

This is the only solution, short of techniques like resampling ahead of time 
at a much higher rate.

Larry Troxler