Re: [fluid-dev] A change to the IIR filter

[David Henningsson]

On 2011-04-04 17:52, Krzysztof Foltman wrote:
> On 03/04/11 10:38, David Henningsson wrote:
>
> > 3) I'm not an IIR filter expert, and so I'm a little out on deep water
> > here and I'm not completely sure this is the right way to fix it. If
> > you happen to know any digital filter algorithm expert, feel free to
> > ask him/her to review the code :-)
>
> The whole problem is a royal PITA for everyone.
>
> www.dafx.ca/proceedings/papers/p_057.pdf
> http://www.acoustics.hut.fi/~vpv/publications/icassp98-trel.pdf
>
> And this thread on KVR:
> http://www.kvraudio.com/forum/viewtopic.php?p=4134891
>
> I had some success interpolating filter coefficients for a number of
> frames (say, 64), but this is just a partial solution. The problem is,
> depending on filter implementation there may be a risk of coefficients
> getting out of the unit circle, causing temporary instability.
>
> There are some clever solutions to try. For example, interpolating
> log(magnitude) and phase of the poles/zeros separately, so that the
> poles never get outside of the unit circle and the amplitude changes are
> "even". But it sounds like a rather CPU-consuming approach and I never
> tried that myself.
>
> K.

Thanks for the input - seems like you know much more about the 
background and math behind this formula than I do. Have you had a look 
at the code? The old code would linearly interpolate a and b 
coefficients over FLUID_BUFSIZE = 64 samples. What I came up with was to 
modify w(n) based on changes of b, if b changes rapidly. Might be kind 
of hacky, so I'm happy for some input for a more correct and/or faster 
approach. I understand that there isn't something that's both optimal 
and fast, so do you have any suggestion on a best compromise that would 
work for FluidSynth?

// David