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Moving Average filter window size automatically up-scaled by 1

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Varun Gupta
Varun Gupta on 22 Jul 2020
Commented: Deepak Gupta on 23 Jul 2020
I implemented a moving average filter using the 'FilterDesigner' with the following features (image attached)
I Specified the window size to be 15. (also created other instances of this filter with window sizes 5,10 and 13)
The group delay for MA is calculated by the formula (N-1)/2 but my group delay curve is plotting N/2 instead. (image attached)
where 'N' is the window size.
Does Matlab automatically upscales the window length by +1? i.e
[(15+1)-1] / 2 = 7.5 instead of 15-1/2 = 7


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Accepted Answer

Deepak Gupta
Deepak Gupta on 22 Jul 2020
Hi Varun,
In filterDesigner, you have mentioned filter order as 15 and filter taps or length of filter is order+1. The N generally mentioned in literature is number of taps(filter length), hence N = order+1 = 16.
I hope this clarifies your doubt.


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Deepak Gupta
Deepak Gupta on 22 Jul 2020
For a window size of 15, you need a filter of order 14.
window size = order+1 as was mentioned earlier.
Order: In simple terms, Order of a filter is defined by the maximum number of delay elements used to design the filter.
Varun Gupta
Varun Gupta on 22 Jul 2020
If you can, please help me with one more conecern.
I desinged a Moving average filter with all the above properties except,
filter order = 6
and 'scale passband' selected.
My data's last 5 fields are -> 34.2700, 34.2700, 34.2700, 34.2800, 34.2800
Simple Moving Average output SHOULD BE = 34.274 by (adding all above/5)
But, I'm getting = 34.2716
Do you know, why?
Deepak Gupta
Deepak Gupta on 23 Jul 2020
As your filter order is 6 hence number of filter taps = 6+1 = 7. You need to look at last 7 samples.

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