IRR peak filter unstable in lower bands

[danMatarov]

Hello
I am fairly new to JUCE and i’m trying to get my head around using juce’s IRR filters.
The plugin I am making relies on using an array of peak filters across most of the spectrum, and the filters need to be very precise with a narrow bandwidth. I have looked into making higher order filters by stacking multiple IRR peak filters at the same frequency, and staging their gain, and using high Q values(over 40). However, in the lower frequency spectrum below 200hz, I am getting unstable behaviour

the picture should demonstrate what is happening.
does anyone have any suggestions that might help?

[zsliu98]

stacking multiple IRR peak filters at the same frequency, and staging their gain, and using high Q values(over 40)

AFAICT, this is an unusual way to create high order peak filters.

However, in the lower frequency spectrum below 200hz, I am getting unstable behaviour

First, change all filters to double precision (and cast incoming audio samples if needed).

Next, PluginDoctor uses FFT to analysis magnitude response, of which low frequency precision is limited. It is highly possible that your filter is actually stable.

[kerfuffle]

@zsliu98

Next, PluginDoctor uses FFT to analysis magnitude response, of which low frequency precision is limited. It is highly possible that your filter is actually stable.

To add to this, try setting the Quality option in PluginDoctor to Highest. This will show more details.

[danMatarov]

thanks for the suggestions. After changing the quality option it turns out that my filter was indeed stable. Inaccurate measurements, there’s definitely a lesson here