IIR filter pops when state changes (even with crossfading)

[kunz]

You maybe use a topology preserving filter (like the ones described by vladim) if you want to make fast modulations to the filter. For example when you modulate the filter with an oscillator.

For a normal use case a common IIR filter should be enough stable also when the user changes the value with a slider. Just make sure you smooth the filter cutoff value. I think JUCE already offers solutions for this. See here:

Smoothing IIR Filter Response

You create your SmoothedValue for a floating type (float, double) and a smoothing type. Linear smoothing makes ramps. Multiplicative smoothing is linear in log -that is, if you use it for frequency it will be linear in pitch, if you use it for amplitude it will be linear in level (dB). SmoothedValue<float /*, ValueSmoothingTypes::Linear */> yourValue; // linear by default SmoothedValue yourValue{ 0.5f }; // floating type deduced from initial value To set the length of your ramp, call reset. Y…

I never used the JUCE internal filter classes, so i can’t tell you how this works when you do block processing.