Making a butterworth filter

Hi,

Any suggestions on how to make a filter in Matlab using the following settings:

Butterworth Zero Phase Filters: Low Cutoff: 1.0000 Hz, Time constant 0.1592s, 24 dB/oct High Cutoff: 30.0000 Hz, 24 dB/oct

Thanks, Mark

Answer 1

[Wayne King]

Hi, You won't be able to make a zero-phase Butterworth filter, but you can implement zerophase filtering with filtfilt. You can use fdesign.lowpass and use the design method 'butter' to design your filter and then use filtfilt().

[Jan]

And if FILTFILT feels slow: http://www.mathworks.com/matlabcentral/fileexchange/32261-filterm

[Wayne King]

Jan, Just out of curiousity have you looked at the performance of filtfilt since it was overhauled in R2011a? The major change was that it now accepts IIR filter objects in biquad form, but there should be some performance enhancements as well.

[Jan]

@Wayne: Unfortuanely I do not have a modern version and therefore I've not tested it, but I'd be very glad if somebody could run the included test and send me a copy of the output...

FILTER is parallelized now, but my Mex is not currently. The old FILTFILT suffers from creating 5 temporary copies of the input signal, while the replacement needs just 2 copies. I'm still surprised, that this is 5 to 20 times faster - compared to 2009a.

Answer 2

[Michael Leung]

I believe that the butter function is able to do 0th order filters. http://www.mathworks.com/help/toolbox/signal/ref/butter.html

[Wayne King]

Hi Michael, That is not correct. The order must be positive. A 0th order IIR filter would mean that the output is equal to the input.