Zero phase filter design using 'sptool'

Hi, I want to build a zero phase high pass filter (offline) using Butterworth approximation. Even though 'filtfilt' function allows designing zero phase filters, I wish to do this using 'sptool', for it is more user-friendly. Anyway, I couldn't find a way to design a zero phase filter using 'sptool' and am wondering whether 'sptool' doesn't support this particular function. Please kindly let me know if you know how to do this. Thank you!

Answer 1

[Star Strider]

You have to use filtfilt (or write your own version of it) to do maximally-flat-phase-response filtering with a discrete filter.

I don’t know what you intend by ‘offline’, but the only hardware filter realisation that has a maximally-flat phase response is the Bessel design. All other filter designs — including the Butterworth design — will introduce phase distortion.

It is not possible to realise a Bessel filter as a discrete filter. The resulting design will not have the desired flat phase response, and the Signal Processing Toolbox will not design a discrete version of the Bessel filter for reasons discussed in the documentation for the besself function.

[Dilini Darmawardana]

Thank you! What I meant by offline is that all data is collected prior to filtering. So that one can design a non causal filter with zero phase shift.(https://au.mathworks.com/help/signal/ug/iir-filter-design.html) I have no intention of hardware implementation of this filter.So, Bessel filters cannot be used anyway. Thanks again for your kind support!

[Star Strider]

My pleasure!

Since you collect your data prior to filtering it, just design your filter and use filtfilt to filter it in MATLAB. That is what I would do.