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Butterworths are made up of second order sections (e.g., modfilter, modiirq) and in the case of odd order Butterworths only, and additional first order section.   

On the following Wikipedia page, you will find a table labelled: "Factors of Polynomial Bn(s)" that you can use to derive the needed Q values.  So, for example, for a 5th  order Butterworth with the factors (s + 1)(s2 + 0.6180s + 1)(s2 + 1.6180s + 1) we get the following information:

(s + 1) — this is a first order section, so use something like modhicut or hicut.  

(s2 + 0.6180s + 1) — this is a second order section with a Q of 1/0.6180 (~1.6181)

(s2 + 1.6180s + 1) — this is a second order section with a Q of 1/1.6181 (~0.6180)


All of these filter sections will share the same cutoff frequency.  Butterworths don't have as steep a rolloff as some other filter shapes, but it's fairly easy to build higher-order filters with them once you know how to read the table.


macgee wrote:
Thanks pmcculloch… This was at the default Q of 0.5 so shouldn't have been that? I should try reduce the gain and see if that makes a difference In case. I'll try the modiirq filter as well. I'm not familiar with Butterworth filter design. Are you able to share a short description of how to achieve that in Vsig land? Everything I found was about the maths or electrical.. Sadly it's nearly impossible to find Vsig style designs…

pmcculloch wrote:

modiirq is probably going to work better for high frequency sweeps, though you can't control the Q at audio rate.

One thing that's important to remember is that when you stack up filters you're also stacking up any gain that those filters introduce.  (e.g. if you have a Q of 1, you're going to introduce some gain.  More sections = more gain)  If you want a higher order filter that doesn't introduce gain, you might look at something like a Butterworth filter (you would need to cascade multiple filters for that)