[PD] Filter design for iPhone

[Ed Kelly]

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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