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

[Ed Kelly]

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 <joe at rjdj.me>
To: Ed Kelly <morph_2016 at yahoo.co.uk> 
Cc: PD List <pd-list at iem.at>; pddev <pd-dev at 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 As Rich suggested maybe you're using an outdated version of libpd?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_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/
>_______________________________________________
>Pd-dev mailing list
>Pd-dev at iem.at
>http://lists.puredata.info/listinfo/pd-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20120322/44e71168/attachment-0001.htm>