Why do the HPFs explode?

Question

I haven't ever come across it anywhere else, but in Kyma if the signal coming into an HPF is a bit too loud, the filter seems to explode in a rather ugly way. Why does this happen? Is there a way to avoid it without reducing the signal headroom for all the rest of the signal flow after the HPF?

Scaling the HPF to 0.5 then gaining up to 2 could be a solution, although it still sometimes can explode, which is what I want to avoid in every case.
Also I looked at this chain on the Spectrum Analyzer scope and it seems to show that a noise floor comes up if you scale an HPF then gain it again later. Unless I'm seeing that wrong....

Answer 1

I usually set the Scale to 0.5 and put a Gain of 2 after the HPF.  Another contributing factor can be the Feedback value (try setting that lower if your filter is blowing up).

In answer to your question as to "why", an IIR filter is an infinite impulse response filter and there can be infinite gain at the poles.  There are internal delays and feedback within any IIR structure; it's the same principle as feedback between a live mic and speakers in your studio: for certain frequencies, the gain can become infinite.

Comments

ok, thanks for explaining I will follow your guidelines then