[FieldTrip] edf signal amplitude different using ft_preprocessing and edfread

[Roy Cox]

hi all,

We've encountered some unexpected signal values when reading edf files
using ft_preprocessing.

It appears that, for certain signals, ft_preprocessing returns data with an
undesired change in offset. The first figure shows the same signal read in
using ft_preprocessing and Matlab's edfread. The next one shows the signal
in the EDFbrowser app. I also compared to eeglab's biosig plugin (not
shown). All signals are/appear identical, except for the one produced by
FieldTrip.

Of note, this behavior so far seems to occur only for some non-EEG signals
(e.g., accelerometer from zmax device), but I didn't test extensively.

Any thoughts as to what makes ft_preprocessing decide to change the offset
(and/or scaling), when it will do so, and how to not make it do that? We'd
like to import the raw data "as is" without alterations.

Best,
Roy
[image: ft_preprocessing_vs_edfread.PNG]
[image: edfbrowser.PNG]
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