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

[Krzysztof Foltman]

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.