[music-dsp] crossover filters for compressor

[Didier Dambrin]

I was thinking of what could work as a good crossover filter for a 3-band 
multiband compressor, without using FIR's.

I can imagine:


-a simple lowpass & highpass (or 2 lowpass), subtracted from the original 
signal to give the third band. But due to phasing problems, this will work 
only with simple 6dB IIRs, right?


-a higher-order low & highpass, and live with the fact that the mid band 
would be more the residual than the mid band.
I'm only worried that each band won't have the same compression settings, so 
they should be separated enough. But maybe it's not important.


-a low & highpass, but involving 2 allpass to correct the phase problem so 
that the residual will really be the mid. I have no idea how to compute that 
allpass, and if it's even doable, though.


-a low, band & highpass, living with the fact they won't sum up like the 
original signal (or are there IIR's designed for this?)


-using minimum-phase IIR's? Are there IIR's really better than others for 
this kind of stuff? I don't know much about this either.


I was wondering if one of those methods was more common than the others.

thanks