Filter help

[joshb]

Hey guys,

First, let me say, I'm not that well versed in the math of filters but I'm working on it.

I've been using the IIRFilter inside JUCE for a few things which I believe are 2nd order and have a 6db/octave rolloff. I'm looking to create a similar low pass filter that has a steeper rolloff...12, 18 or 24 db/octave. Would that be regarded as higher order filters?

I notice that JUCE's makeLowPass() returns 6 filter coefficients, I'm guessing they're a0, a1, a2, b0, b1, b2. For higher order filters do I need to create more coefficients?

Or is it as simple as stacking multiple 2nd order filters in series?

Any help would be appreciated. Thanks.

[mtytel]

Yeah just put it in series. 2nd order filters usually have a 12dB rolloff so if you put two in series you'll have a 24dB filter.

You'll have to fool around with resonance of each one to get the behavior you want though. If you control the resonance of both you'll get much more (probably too much) resonance. Even if you compensate the resonance range, the frequency response around the cutoff will look different than in the 12dB version.

If you only use the resonance of one, you'll actually diminish the overall resonance because a biquad with no resonance diminishes the cutoff frequency (-6dB for RBJ biquad I believe). A solution is to make the frequency response flat for one of the filters (Q=1/sqrt(2) for RBJ?) and then just use the resonance of the other filter.

[juha_p]

+ https://www.dsprelated.com/showthread/c ... 4598-1.php (Butterworth type LP/HP)

http://www.kvraudio.com/forum/viewtopic ... 3&t=470183

[juha_p]

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