mvcf~ - Well, it breaks when the Q is high and freq is moderately high, or at
high freq and Q, but it seems to be that the higher you get with either
frequency or Q it has a greater probability of breaking (all dac~ outputs at 1
(or possibly 1e+07), no more audio can be played by the patch.
What I mean is that with the noise~ object as the input, the filter breaks
after about 1-3 seconds when Q=0.70-0.8, f=0.7-0.8, and there are variations on
this depending on the settings. Something is profoundly un-robust about this
filter, but it's cpu load is very low. I want to have a stable version for not
much more CPU.
I will look at the RJDJ filters mentioned also. I would like not to have to
worry about filters when programming libpd audio engines. It would be nice to
get a really efficient IIR with a musical response that uses low CPU into the
core - I am not an expert at using poles and zeroes however (but I'm working on
my higher math often these days, with the Khan academy :) Some of this is
subjective, perhaps the "musical" nature of filters, but certain types of
filter are assumed by many (musicians, DJs etc) in this part of the world to be
normally available. The 24dB lowpass resonant filter a la Moog is one of them,
and a fundamental analogue processor as well. I understand the analogue better
than the discrete, so I'm anxious to know what limit is reached in the
coefficients of the filter that causes the undefined result (NaN).
But I'll try the other rjlib filters first!
Ed
Gemnotes-0.1alpha: Live music notation for Pure Data
http://sharktracks.co.uk/
________________________________
From: Joe White <j...@rjdj.me>
To: Ed Kelly <morph_2...@yahoo.co.uk>
Cc: PD List <pd-l...@iem.at>; pddev <pd-dev@iem.at>
Sent: Tuesday, 20 March 2012, 17:50
Subject: Re: [PD-dev] Filter design for iPhone
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 As Rich suggested maybe you're using an outdated version of libpd?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_2...@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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>Pd-dev mailing list
>Pd-dev@iem.at
>http://lists.puredata.info/listinfo/pd-dev
>
>
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