could you tell me step by step the algorithm to design an arbitrary phase response IIR allpass filter design ?
I know Matlab uses a "brute force" algorithm to find an arbitrary group delay response but how do they do that?
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Depends a bit on what exactly you are trying to do. Generally, strict all-pass filters have reciprocal zeros and poles. This will always result in 180 phase shift per filter order so an arbitrary phase response that's strictly allpass cannot be designed. Example: let's say you want something that's in phase at DC, sits at -90 degrees in the band and then goes slowly back to zero at Nyquist: this can't be implemented using strict all-passes.
Of course you can always approximate but the best method tends to depend on the specifics of your design problems and what types of errors you can tolerate.
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$\begingroup$ thank you for your answer. Do you know the function from Matlab iirgrpdelay ? It designs an arbitrary allpass filter with iterations. I want to match the phase of an IIR to make it almost linear. I know it's possible because i've seen it in some implementations. I can tolerate some errors but i want to have the maximum linearity in the phase response. thanks. Jeff $\endgroup$ – ion_one Aug 28 '13 at 14:18
