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Bug 1571 - dftfilter does not work when not segmenting trials?

Reported 2012-06-28 16:47:00 +0200
Modified 2014-03-12 12:21:33 +0100
Product: FieldTrip
Component: core
Version: unspecified
Hardware: PC
Operating System: Windows
Importance: P3 normal
Assigned to: Jörn M. Horschig
Depends on:
See also:

Jörn M. Horschig - 2012-06-28 16:47:29 +0200

On 6/28/2012 3:13 PM, Philipp Hintze wrote: The issue with the continuous data is the following: I tried using cfg = []; = {'EEG'}; cfg.datafile = dataset; cfg.headerfile = dataset; cfg.dataset = dataset; cfg.continuous = 'yes'; (cfg.padding = 10;) cfg.dftfilter = 'yes'; filteredContinuousData = ft_preprocessing(cfg); but looking at averaged timelocked data for the subject, the data looks identical to the result I get using no filter at all, i.e., contaminated by strong line noise. Including padding makes no difference either, therefore the brackets. ---------------- First step: check whether this is a bug

Robert Oostenveld - 2012-06-28 16:55:13 +0200

I suspect the data segment is so long that the dftfilter is too sharp and that the actual frequency that is filtered out is slightly off from the line-noise. It is not uncommon for an EEG system to report that it samples at 512 Hz (in the specs and in the header), but where the actual sampling is at 512.01 Hz If your data is long enough (say 100 seconds), the spectral resolution of the DFT is too high (0.01 Hz) and the mismatch between the specified and true sampling rates becomes apparent.

Jörn M. Horschig - 2012-06-28 17:04:54 +0200

ah, very good to know ;) I'm gonna check first with some synthetic signals, and there I can also model such cases.

Jörn M. Horschig - 2012-07-04 15:31:50 +0200

step 1: verify with my own data >> sum(filteredContinuousData.trial{1(:)-unfilteredContinuousData.trial{1}(:)) ans = -4.4985e-008 meaning, dftfilter is doing something I verified by doing an fft, and can see that the line noise is reduced by a factor of 8. The remaining (still huge) 50Hz is a 'feature' of the dftfilter (see FAQ) step 2: build some synthetic signal and play around with sampling frequency: I built a signal with 512.001Hz but setting fsample hardcoded to 512Hz. I also couldn't observe anything 'strange'/unexpected there.

Jörn M. Horschig - 2012-08-23 14:02:00 +0200

bug closing time (

Robert Oostenveld - 2012-11-29 10:09:57 +0100

in the automated regression testing this one often fails because PBS Job Id: Job Name: test_bug1571 Exec host: dccn-c014/3 job deleted Job deleted at request of job violates resource utilization policies Either it is taking too much memory, or too much time. Preferably the test script is trimmed down a bit in the memory or time requirement. Or perhaps there is an interactive figure on which it waits? @Jorn, can you have a look?

Jörn M. Horschig - 2012-12-05 10:10:33 +0100

It's probably memory issues, at least it's no interactive figure and the function needs about 4 minutes of time. I changed a strange assert statement to using isequal, maybe that helps. The test script passed successfully the last times though. Let's see what happens.

Robert Oostenveld - 2012-12-05 12:26:59 +0100

Here is a conceptual answer: the dft filter will be too sharp if the segment is very long, consequently it is likely that it will just miss the line noise frequency. I.e. removing the 50.000 Hz component whereas the linenoise is at 50.010 would not result in the desired effect, although technically the filter performed well. Also slow fluctuations in the line noise amplitude (more likely with long segments) will cause the dft filter to be less efficient. See The question remains why the test script is using too much memory or running for too long. The data file is inly 77MB large, so it should fit comfortably in memory. Perhaps do some memtic/memtoc and tic/toc, or use the profiler.

Jörn M. Horschig - 2012-12-05 14:19:40 +0100

oops, found it: data.time{1} = 0:1/real_fsample:100*fsample; should be data.time{1} = 0:1/real_fsample:100; svn ci test_bug1571.m -m "enhancement - reduced synthetic data size and changed the other assert statement as well" Sending test_bug1571.m Transmitting file data . Committed revision 7097.