Biosemi data format/Reading .bdf into Matlab
Posted: Tue Sep 23, 2014 10:40 am
Hi everyone,
I am new to Biosemi and I have a couple of question regarding the Biosemi data format and reading in .bdf files into Matlab.
I would like to use Fieldtrip, a Matlab toolbox for EEG analysis (see http://fieldtrip.fcdonders.nl/), to analyze my data, but I am encountering some problems when trying to read in raw biosemi data (.bdf).
The main thing is that the data looks odd to me as the vertial scale is very large [ -178000 178000 ], but also filtering and trial selection does not work properly.
From what I have read so far about Biosemi, I suspect that this might have to do with the DC-offset and that I might have to re-reference the data. I tried this using one of the EXG channels ('EXG5'), but this did not help.
I am afraid that I dont fully understand Biosemi data and it would be great if someone could give me some more insights into this matter.
- Is this scaling problem indeed caused by the offset/Do I indeed need to re-reference the data before I can use it in Matlab?
- If so, what should be used as a reference?
- In addition, I was wondering if someone could explain to me what these EXG channels (there are 8 of those in my data) are exactly?
Since I wasn't able to work with the .bdf files, I tried to convert them to .edf (using the converter from http://www.biosemi.com/download/).
This .edf file looks much more normal in Fieldtrip, but the data is still very noisy and I still encounter some problems, especially when trying to filter the data. When applying a high-pass filter (0.5Hz) on the data for example, Matlab gives an error reporting unstable poles.
- I was not sure whether I need to re-reference this .edf file as well (which leads again to my question about re-referencing and the EXG channels)
- Any additional suggestions on why I might have problems with filtering are of course also very welcome!
In addition, I am curious what exactly happens to the data when it is converted from .bdf to .edf, so additional information on this would also be very much appreciated.
Many Thanks in advance!
Best,
Ricarda
I am new to Biosemi and I have a couple of question regarding the Biosemi data format and reading in .bdf files into Matlab.
I would like to use Fieldtrip, a Matlab toolbox for EEG analysis (see http://fieldtrip.fcdonders.nl/), to analyze my data, but I am encountering some problems when trying to read in raw biosemi data (.bdf).
The main thing is that the data looks odd to me as the vertial scale is very large [ -178000 178000 ], but also filtering and trial selection does not work properly.
From what I have read so far about Biosemi, I suspect that this might have to do with the DC-offset and that I might have to re-reference the data. I tried this using one of the EXG channels ('EXG5'), but this did not help.
I am afraid that I dont fully understand Biosemi data and it would be great if someone could give me some more insights into this matter.
- Is this scaling problem indeed caused by the offset/Do I indeed need to re-reference the data before I can use it in Matlab?
- If so, what should be used as a reference?
- In addition, I was wondering if someone could explain to me what these EXG channels (there are 8 of those in my data) are exactly?
Since I wasn't able to work with the .bdf files, I tried to convert them to .edf (using the converter from http://www.biosemi.com/download/).
This .edf file looks much more normal in Fieldtrip, but the data is still very noisy and I still encounter some problems, especially when trying to filter the data. When applying a high-pass filter (0.5Hz) on the data for example, Matlab gives an error reporting unstable poles.
- I was not sure whether I need to re-reference this .edf file as well (which leads again to my question about re-referencing and the EXG channels)
- Any additional suggestions on why I might have problems with filtering are of course also very welcome!
In addition, I am curious what exactly happens to the data when it is converted from .bdf to .edf, so additional information on this would also be very much appreciated.
Many Thanks in advance!
Best,
Ricarda