What do you mean by zero-phase? Zero group delay in the passband maybe? Try a Butterworth design, it will have a small frequency-dependent group delay in the passband (ad a large delay in the transition band). A linear phase FIR design has a group delay about half of the filter length, so if delay is important you must use a low-order filter.
"Hans " <firstname.lastname@example.org> wrote in message <email@example.com>... > I'm looking for how to build a zero phase low pass filte. For example: > h = fdesign.lowpass(Fpass, Fstop, Apass, Astop, Fs); > Hd = design(h, 'ifir'); > do a good job to easily create a filter operator, that works fine with filter. But there is a big offset when I use the "filter" command. > y = filter(Hd,time-series-data); > > "filtfilt" .... > would avoid this offset, but doesn't like this filter operator. > So my question is how, create as easily such a low pass filter operator, accepted by "filtfilt" or how to create otherwise a zero phase low pass filter ? > > hans