[PD-dev] Filter design for iPhone

Rich E reakinator at gmail.com
Tue Mar 20 17:37:57 CET 2012


What do you mean by break, crashes?  If yes, what version of libpd are you
using?  Up until about a month ago, floating point exceptions (such as NAN)
would crash libpd, whereas pd-vanilla ignores them.  Peter B. recently
added a commit that changed this:

https://github.com/libpd/libpd/commit/27c848a56d3d27e62231f8b0d9ef17c7c56cd9f9

Anywho, you'll probably want to take care of those NAN's anyway. :)  Not
sure why pd vanilla ignores SIGFPE.

cheers,
Rich

On Tue, Mar 20, 2012 at 4:57 PM, Ed Kelly <morph_2016 at yahoo.co.uk> wrote:

> Hi,
>
> I'm trying to design or find a resonant lowpass filter for use with libPd
> for Openframeworks running on an iPhone 4.
>
> 1. I've tried the rjlib filter attached (filtertest_list.tar.gz) and it
> works fine in Pd, but instantly breaks when I run it on the iPhone - all
> other playing audio cuts out, and it is impossible to reset. I suspect that
> the buffers fb1 fb2 ff1 ff2 ff3 are hitting NAN values and staying there,
> but I haven't tested this.
>
> 2. I've had a go at implementing the filter from
> http://www.musicdsp.org/archive.php?classid=3#25 as a Pd external
> (mvcf~.tar.gz) and it is very efficient CPU-wise. However, higher values of
> f or q cause the internal filter buffers to hit NAN and stay there (tested
> in Pd - I haven't tried this on the iPhone yet).
>
> I would ideally like to find out why the second attempt breaks at high q
> and/or f, as what I want is a 24dB/Octave moog-ish filter. I'm sure those
> of you with higher math skills will intuitively know why, but I'm just
> learning calculus now (my early education was a bit chaotic). I'm also
> limited by the fact that this is for a commercial project (although I'll be
> happy to share the code :) so I can't just take a GPL external and
> recompiled libpd with it.
>
> Ed
>
> Gemnotes-0.1alpha: Live music notation for Pure Data
> http://sharktracks.co.uk/
>
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>
>
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