# filtfilt fails with B=1, A=1 on matrix

3 views (last 30 days)
Alain on 20 Sep 2021
Commented: Walter Roberson on 20 Sep 2021
filtfilt(1,1,ones(4000,1)); % works OK
filter(1,1,ones(4000,2)); % works OK
filtfilt(1,1,ones(4000,2)); % worked in R2019b Update 6, fails in 2021a Update 5
Error using filter
Initial conditions must be a vector of length max(length(a),length(b))-1, or an array with the leading dimension of size max(length(a),length(b))-1 and with remaining dimensions matching those of x.

Error in filtfilt>ffMultiChan (line 382)
[~,zo] = filter(b(:,ii),a(:,ii),xt,zi(:,ii)*xt(1,:)); % outer product

Error in filtfilt>efiltfilt (line 140)
yCol = ffMultiChan(b2,a2,xCol,zi,nfact,L);

Error in filtfilt (line 89)
y=efiltfilt(b,a,x);

Walter Roberson on 20 Sep 2021
It appears to be a bug.
In context: filtfilt() calls filter() to do some of the work.
When a and b are scalars, filter is going to be called as
filter(a, b, xt, EMPTY )
where xt is going to have one row and number of columns equal to the number of columns in x. xt is initial conditions of the filters, and the initial values are calculated from the first two rows of the input x.
In this case of scalar a and scalar b, EMPTY is calculated as zeros(0,1) * xt . With xt being size 1 x ncol, the result of the * operation is going to be zeros(0, ncol)
Now...
• when you call filter() with scalar a, scalar b, array X, and EMPTY, then if if EMPTY is 0 x ncol, then filter will succeed for any 2D array size X that is not a row vector (and has ncol columns)
• when you call filter() with scalar a, scalar b, array X, and EMPTY, then if EMPTY is 0 x 1, then filter will succeed for any 2D array size X including row vector X
If you read this carefully, you will see that if X were not a row vector, then having a 0 x ncols initial conditions works. The error message is telling you that your initial conditions must be this size, 0 x ncols. But the error message is wrong for the case where X is a row vector, in which case the initial conditions must be 0 x 1 not 0 x ncols.
If I were to guess, I would guess that X as a row vector is being automatically transposed to X as a column vector, size ncols x 1, and it is wanting the 0 x ncols to match that number of columns (1) .
If I am correct, then the problem would be fixed if filtfilt() were to pass in the dimension to filter along.
In the meantime, all you can do is filtfilt() one column at a time of your signal.
Walter Roberson on 20 Sep 2021
Mathworks mostly doesn't read these postings. Some individual Mathworks employees volunteer to answer questions that interest them... if they happen to notice the posting.
The way to get things fixed is to open a Support case about it. Which I have done for this already, and pointed them to this topic for backing information.

R2021a

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