What does it take to design a 4th order band-pass filter with -24dB/Octave? I would like to implement in software the same isolators that the Pioneer EFX-500 features:



The manual says the following about the isolators:

High-performance 3-band Isolator Function
[Fourth-order filter (–24 dB/ oct.)]
Level control is possible between +6 dB to –inf. and a separation of up to –24 dB/oct. for each band ensure accurate sound reproduction.

The thing is, I can program in C# or C++ but I have no experience at all in audio processing so I am a bit at a loss on how to get started.

I have found the winfilter software which can generate C code I can reuse but I am not sure about what settings to choose:



Based on the specifications I've put above, can you suggest which settings should be used ?
After experimenting with winfilter, generating the following filters mimics the EFX isolators, amazingly :)


2 * Butterworth LP 2nd order @ 300Hz
2 * Butterworth BP 2nd order @ 300Hz/6000Hz
2 * Butterworth HP 2nd order @ 6000Hz


So each signal passes through two filters of the same type, for some reasons I don't know the cause of, a 4th order BP in winfilter simply doesn't work and produces garbage instead.

As a reference, here's what really helped me out in matching the filters properties, white noise recorded for each of the bands then guesses until it matches:



Surprisingly, I'd say the above filters match the EFX about 99%, the only differences are at the edges where the winfilter ones really trim down the frequencies much more.