# [PD] Anti-aliasing filter

matthew jones m.jones at signal.qinetiq.com
Wed May 19 16:39:28 CEST 2004

```yeah this stuff had me confused for a long while....!
it's straightforward in maths to express the sum of two frequencies as the
mid freq modulated by a low frequency (half the separation freq) but it's
not so straightforward explaining why we can actually HEAR this low
frequency even though a fourier transform wouldn't show anything there.....

weird non-linear ears....

Matt

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----- Original Message -----
From: "Martin Peach" <martinrp at vax2.concordia.ca>
To: <julien.breval at tremplin-utc.net>
Cc: "Al Riley" <alrileyuk at yahoo.co.uk>; <pd-list at iem.at>
Sent: Wednesday, May 19, 2004 3:24 PM
Subject: Re: [PD] Anti-aliasing filter

> julien.breval at tremplin-utc.net wrote:
>
> > Another solution is to calculate the highest partial that won't produce
> > aliasing, in function of the fundamental.
> > If we call F the fundamental, the k-th partial will have a frequency of
F + k*F
> > (if you chose another harmonicity system, you may adapt this formula).
> > Therefore the maximum number of computable partials of the F fundamental
is the
> > integer part of (22050-F)/F.
> > For F = 10 Hz, you can compute 2204 partials
> > For F = 1000 Hz, you can only compute 21 partials
> > The idea, here, is to limit the number of partials to the maximum (you
can
> > evaluate this maximum in realtime, in function of the fundamental). You
can
> > either mute the partials that outpass the limit (the most simple
solution) or
> > not even calculate them.
>
> The effect of ultrasonic partials can be very audible. The sound seems
> to come from intermodulation distortion in the ear sensors themselves,
> which is causing lower frequency artefacts. Any calculation to simulate
> this effect (e.g. multiply together the ultrasonic partials while adding
> the audible ones) would perhaps add realism to the sound of higher
> frequency complex tones.
>
> Martin
>
>
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```