numerator and denominator coefficients from designfilt function
33 views (last 30 days)
Show older comments
Rohit Mallampati
on 13 Jul 2020
Answered: Peter Bode
on 1 Mar 2021
I am designing an lowpass IIR filter using designfilt function. I found coefficients for the filter which is 9x6 Double. Now, how can I find numerator and denominator from this?
0 Comments
Accepted Answer
Nipun Katyal
on 15 Jul 2020
The type of coefficients you encounter are called Second order section coefficients. Second-order section coefficients, specified as a matrix. sos is a K-by-6 matrix, where the number of sections, K, must be greater than or equal to 2. If the number of sections is less than 2, the function treats the input as a numerator vector. Each row of sos corresponds to the coefficients of a second-order (biquad) filter. The ith row of sos corresponds to [bi(1) bi(2) bi(3) ai(1) ai(2) ai(3)].
%Create a filter object using designfilt
filterObj = designfilt('lowpassiir', 'PassbandFrequency', .45, 'StopbandFrequency', .55, 'PassbandRipple', 1, 'StopbandAttenuation', 60);
%Convert the second order section coefficients to tranfer function format
[b,a] = sos2tf(filterObj.Coefficients);
%Here b is the numerator and a is the denominator
2 Comments
John Gauci
on 1 Feb 2021
This does not always work. For example:
filterObj2 = designfilt('lowpassiir', 'PassbandFrequency', 0.02, 'StopbandFrequency', 0.04, 'PassbandRipple', 1, 'StopbandAttenuation', 60);
fvtool(filterObj2);
[b,a] = sos2tf(filterObj2.Coefficients);
fvtool(b,a);
shows two very different filter reponses, the second being drastically wrong and which gets worse the closer the stop band gets to the pass band.
Is there another way of getting the coefficients from designfilt?
More Answers (1)
Peter Bode
on 1 Mar 2021
Do Mathworks stiil support the legacy filter design functions which have been proven useful for decades?
0 Comments
See Also
Categories
Find more on Digital Filter Design in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!