[PD] [PD-dev] Filter design for iPhone

[Joe White]

Hi Ed,

I just tested your filtertest_list patch in RjDj and it worked exactly as
expected. As Rich suggested maybe you're using an outdated version of libpd?

If Frank's around I'm sure he would know for certain but if I remember
rightly the [u_lowpass] / [u_beequad] combination is not really designed
for dynamically changing parameters. Maybe take a look at [e_reslop],
[e_lop2] and [e_lop4] for some other filter implementations.

Regarding your second attempt, at what frequency does it 'break' and what's
your sampling rate?

Cheers,
Joe

On 20 March 2012 05:57, 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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