bugGNU Octave - Bugs: bug #56851, [octave forge] (signal) butter...

 
 

bug #56851: [octave forge] (signal) butter returns incorrect value

Submitted by:  None
Submitted on:  Mon 02 Sep 2019 06:08:21 PM UTC  
 
Category:  Octave Forge Package Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Incorrect Result
Status:  Need Info Assigned to:  None
Originator Name:  hjborsje Originator Email:  -email is unavailable-
Open/Closed:  Open Release:  5.1.0
Operating System:  Microsoft Windows

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( Jump to the original submission)

Tue 10 Sep 2019 03:38:20 PM UTC, comment #10: 

Ok, it looks to me like filtfilt is returning an output regardless of the warning message. If I disable the warning, there doesn't seem to be any problem here. The warning is removed permanently in Octave version 6.

Is the presence of the divide-by-zero warning the only problem you are reporting here? If so, that's already fixed in Octave 6, and easy to silence in any previous version of Octave.

Mike Miller <mtmiller>
Project Administrator
Tue 10 Sep 2019 08:22:00 AM UTC, comment #9: 

Sorry for not being clear in this matter.  I use the function filtfilt.  Line 66 uses sum(b)/sum(a).  This is where the division by zero occurs.

Henk B

Henk Borsje <hjborsje>
Mon 09 Sep 2019 08:49:06 PM UTC, comment #8: 

We can show that it's not an error for the butter function to return filter coefficients that aren't practically useful when the filter spec is unreasonable. IOW, garbage in == garbage out.

For compatibility with Matlab, the butter function should not enforce some kind of constraints on the input or output and report an error, it should just return what it can compute.

Can you show specifically what gives you a division by zero? Not the 'filter' function and not the 'freqz' function. Maybe some other function can report an improved error message in this case?

Mike Miller <mtmiller>
Project Administrator
Thu 05 Sep 2019 06:53:05 AM UTC, comment #7: 

You can close it as a bug, but is there room for improvement?  It currently results in division by zero.

Henk Borsje <hjborsje>
Wed 04 Sep 2019 07:12:49 PM UTC, comment #6: 

> Apparently butter(5,1/28800) is not a feasible filter


Does this mean you think this bug report should be closed as invalid?

Mike Miller <mtmiller>
Project Administrator
Wed 04 Sep 2019 05:45:00 PM UTC, comment #5: 

Apparently butter(5,1/28800) is not a feasible filter, given the reported differences in returned coefficients.  My expectation is based on the MATLAB idfilt function in the system identification toolbox.
idfilt(z,[0 1/28800],'FilterOrder',5) does seem to work correctly. According to the documentation idfilt uses a Butterworth filter.

idfilt does not seem to exist in Octave

Henk Borsje <hjborsje>
Tue 03 Sep 2019 04:49:35 PM UTC, comment #4: 

Different versions of Matlab return different results from what is suggested in comment #0. With Matlab's online version R2019a, sum(a) returns -eps and sum(b) returns 0.

Mike Miller <mtmiller>
Project Administrator
Tue 03 Sep 2019 07:12:32 AM UTC, comment #3: 

It sure does look like a round-off error, but one with grave consequences.  This filter spec is not unreasonable.  Changing the spec to e.g. butter(4,1/28800) works correctly, as well as butter (5,1/28801).  The resulting sum(a) = 0 causes a division by 0 when one tries to use the filter.   MATLAB handles this case correctly, so it sounds like a bug to me.

Henk Borsje <hjborsje>
Mon 02 Sep 2019 09:57:24 PM UTC, comment #2: 

I think you are seeing roundoff errors from floating point arithmetic.

Doug Stewart <dastew>
Mon 02 Sep 2019 06:38:05 PM UTC, comment #1: 

are you wanting an analogue or digital filter?

Doug Stewart <dastew>
Mon 02 Sep 2019 06:08:21 PM UTC, original submission:  

>> [b,a] = butter (5,1/28800)

b =

 Columns 1 through 4:

   4.825699732353868e-22   2.412849866176934e-21   4.825699732353867e-21   4.825699732353867e-21

 Columns 5 and 6:

   2.412849866176934e-21   4.825699732353868e-22

a =

 Columns 1 through 4:

   1.000000000000000e+00  -4.999646999743641e+00   9.998588061278351e+00  -9.997882185366413e+00

 Columns 5 and 6:

   4.998588185872336e+00  -9.996470620406335e-01>>

>> [sum(a) sum(b)]

ans =

   0.000000000000000e+00   1.544223914353238e-20

MATLAB returns the value 5.5511e-16 for sum(a) and 8.88178e-16 for sum(b)

Anonymous

 

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    Follow 4 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2019-09-09 mtmiller StatusNone => Need Info
    2019-09-03 rik5 Carbon-CopyRemoved 72865 => -
    2019-09-03 mtmiller Summarybutter returns incorrect value => [octave forge] (signal) butter returns incorrect value
    2019-09-03 rik5 CategoryOctave Function => Octave Forge Package

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