Designing a moving average filter using DESIGNFILT

Dear ML-community,

I am trying to implement a moving average filter using the DESIGNFILT method to get an according DIGITALFILTER object. I use the following code:

MAFilterSizeInSamps = 18;
MACutOffFrequencyInHz = 24.601; %Compute from transfer function
Filter = designfilt('lowpassfir',...
                    'DesignMethod','window',...
                    'FilterOrder',MAFilterSizeInSamps-1,...
                    'CutoffFrequency',MACutOffFrequencyInHz,...
                    'Window','rectwin',...
                     'SampleRate',1000);

b = ones(1,MAFilterSizeInSamps)/MAFilterSizeInSamps;
a = 1;

[b;Filter.Coefficients]
fvtool(Filter,b,a)

As I understand it, this should yield the same filter coefficients as specified in b (all-0.055556). Instead I get the following coefficients:

    Columns 1 through 6
       0.045501     0.048867     0.051924      0.05463     0.056946      0.05884
    Columns 7 through 12
       0.060284     0.061259      0.06175      0.06175     0.061259     0.060284
    Columns 13 through 18
        0.05884     0.056946      0.05463     0.051924     0.048867     0.045501

So I am stuck here, since I think these should be the same filters! But the filter transfer functions say otherwise!

I am grateful for any clues!

Greetings, David

Answer 1

[Josselin Gautier]

Hi, if you are trying to get a digital filter object to use with filtfilt() I would rather recommend to specify directly the numerator and denominator coefficients b and a, and call directly filtfilt() with them. Otherwise, you might be able to export digital filter object with filter designer or filter builder from Signal Processing toolbox