[FieldTrip] about detrend

[Alexander Nakhnikian]

It looks as though the take home message is that in theory high pass 
filters remove DC but you can't be sure how they will interact with 
empirical data since there is no perfect filter in the real world.  If 
other people report substantial artifacts after filtering data without 
removing the DC component I suggest getting rid of it.  To answer your 
original question, assuming you're detrending each epoch individually it 
should remove the DC component or at least reduce it substantially.

I hope that helps.



On 12/5/14, 6:20 PM, Yun Wen wrote:
>
> Hi, Alexander & Rodrigo. Thank you very much for your help. I 
> collected my data with DC recordings (BioSemi system). I have seen 
> some discussion on the DC offset:
>
> http://sccn.ucsd.edu/pipermail/eeglablist/2014/008068.html
>
> http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html
>
> http://sccn.ucsd.edu/pipermail/eeglablist/2010/003315.html
>
> http://erpinfo.org/erplab/erplab-documentation/manual/Filtering.html
>
> It seems to me that I’d better remove DC offset of continuous data 
> before high-pass filter. Am I right?
>
> Best,
>
> Yun
>
> *From:*fieldtrip-bounces at science.ru.nl 
> [mailto:fieldtrip-bounces at science.ru.nl] *On Behalf Of *Alexander 
> Nakhnikian
> *Sent:* Friday, December 05, 2014 10:19 PM
> *To:* FieldTrip discussion list
> *Subject:* Re: [FieldTrip] about detrend
>
> Hello Again,
>
> I think there's an error in my response. I think removing the linear 
> trend should also set the mean to zero already since the best fit line 
> incorporates vertical offset.  Sorry about that.
>
> A
>
>   
>
> On 12/5/14, 4:20 PM, Yun Wen wrote:
>
>     Hi all, I want to remove DC offset before applying the high-pass
>     filter. I  was wondering if I can achieve that by putting
>     cfg.detrend = 'yes' before cfg.hpfilter = 'yes', something like:
>
>     cfg = [];
>
>     cfg.dataset = filename_raweeg;
>
>     cfg.detrend = 'yes';
>
>     cfg.hpfilter = 'yes';
>
>     cfg.hpfiltord = 2; % Order of the filter = 2.
>
>     cfg.hpfreq = 0.1; % High-pass cutoff at 0.1 Hz;
>
>     [data] = ft_preprocessing (cfg);
>
>     Thank you in advance.
>
>     Best,
>
>     Yun
>
>       
>
>       
>
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