Take the 2-minute tour ×
Mathematica Stack Exchange is a question and answer site for users of Mathematica. It's 100% free, no registration required.

I want to filter data using Butterworth filter. I am currently using Matlab and I want to know how to do it in Mathematica.

I have the following data:

data1 = Table[
   PDF[NormalDistribution[3.5, .8], i] + 
    PDF[NormalDistribution[6, 1], i], {i, -5, 15, .01}];
noise = RandomReal /@ RandomReal[{-0.2, .2}, Length[data1]];
data2 = data1 + noise;

I filtered this data using Matlab as follows:

[b,a] = butter(8,0.04);

filtfilt(b,a,data2 );

In the previous question's answer, bill s suggested to do it as follows in Mathematica:

RecurrenceFilter[
 ToDiscreteTimeModel[ButterworthFilterModel[{2, 0.04}], 1], data2
 ]

The result is as follows: enter image description here

The problem I am facing is whatever I change the filter order and frequency, I am not able to produce the same filtered data as I am getting from Matlab.

Can someone suggest for me what to do and what values to use to get the same filtered data as I am getting from Matlab?

Thanks

share|improve this question
1  
filtfilt does a forward-backward filtering. Have you tried filtering, reversing the output, filtering again and finally reversing again in Mathematica? –  rm -rf Apr 21 at 5:24

1 Answer 1

up vote 2 down vote accepted

filtfilt does a forwards and then a backwards filtering, as rm -rf suggests. To filter data in this way, you might try

forwards = RecurrenceFilter[ToDiscreteTimeModel[ButterworthFilterModel[5], 1], data];
backwards = RecurrenceFilter[ToDiscreteTimeModel[ButterworthFilterModel[5], 1], Reverse[data]];
forwards + Reverse[backwards]

The nice thing about filtfilt (or the forwards/backwards filter above) is that the filtering has no phase shift/delay when compared with the original data.

share|improve this answer
    
Good. Great Thanks. –  Algohi Apr 21 at 22:07
    
it is nice I upvote this answer and you edited it. this was my second question in this forum and that time I didn't know there is upvote. sorry for waiting all this time to upvote your answer :) –  Algohi Oct 15 at 19:09

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.