[FieldTrip] [Fieldtrip] Line noise Removal

[Alessandro Orticoni]

Dear Jan-Mathijs,

Thanks a lot for your reply and your tips.
Actually I've already segmented the data in 30 seconds epochs, and the
reason why I did it is that these are sleep data: so I should keep this
length because of the night scoring.
I tried to enlarge the notch too, but it still doesn't work; the spectra
before and after the filtering are exactly the same.

Best,
Alessandro

Il giorno gio 28 giu 2018 alle ore 19:35 Schoffelen, J.M. (Jan Mathijs) <
jan.schoffelen at donders.ru.nl> ha scritto:

> Dear Alessandro,
>
> Given the code you provided it looks as if you have one single very long
> trial that you subject to dft-filtering. If the power line fluctuations
> actually do vary a bit in amplitude, which is likely (particularly given
> long sections of data), the pure 50 (or 60) Hz sinusoid will get some
> ‘bandwidth’, so the removal won’t be perfect. I’d recommend to call
> ft_redefinetrial first, and then do a second call to ft_preprocessing with
> the dftfilter switched on. Yet, I’d expect that with 30 s segments it still
> won’t work that well. In this case you could widen the notch a bit  by
> specifying cfg.dftfreq = 50+(-n:n)./30;, with n put to a small number, e.g.
> <=3. However, unless you have good reasons to have the data chopped in
> half-minute segments, I’d also consider to reduce this length, because you
> probably do not need to have a 1/30 Hz frequency resolution in your
> spectral decomposition.
>
> Best wishes,
> Jan-Mathijs
>
> J.M.Schoffelen, MD PhD
> Associate PI, VIDI-fellow - PI, language in interaction
> Telephone: +31-24-3614793
> Physical location: room 00.028
> Donders Centre for Cognitive Neuroimaging, Nijmegen, The Netherlands
>
>
>
>
> On 28 Jun 2018, at 16:22, Alessandro Orticoni <
> alessandro.orticoni at gmail.com> wrote:
>
> Dear experts,
>
> I'm basically a beginner in coding with Fieldtrip, and I'm having a
> problem with line noise filtering.
> I would like to remove it from the heart rate signal, and I tried just
> typing 'yes' at the option cfg.dftfilter.
> But is doesn't seem to work, since the power spectrum contains the 50Hz
> and her harmonics even after the filtering.
>
> You can find the code in the following lines:
>
> %% ===FREQUENCY Pre Filtering VERIFICATION===
> fs=data.fsample;
>
> cfg = [];
> cfg.output        = 'pow';
> cfg.channel       = 'ECG 2';
> cfg.method        = 'mtmfft';
> cfg.taper         = 'hanning';
> cfg.pad           = 'nextpow2';
> cfg.foilim        = [0.3 fs/2]; % teorema di Nyquist
> freq_epoched_Cz   = ft_freqanalysis(cfg, data_epoched_Cz);
>
> %% ===Plot Spectra===
> figure;
> plot(freq_epoched_Cz.freq,mag2db(freq_epoched_Cz.powspctrm));
>
> %% ===Filtering===
> cfg            = [];
> cfg.channel    = {'ECG 2'};
> cfg.dftfilter  = 'yes';
> ECG_Filtered   = ft_preprocessing(cfg,data_Cz);
>
> cfg          = [];
> cfg.length   = 30; % in seconds;
> cfg.overlap  = 0;
> data_epoched_ECG_Filtered = ft_redefinetrial(cfg, ECG_Filtered);
>
> cfg = [];
> cfg.output        = 'pow';
> cfg.method        = 'mtmfft';
> cfg.taper         = 'hanning';
> cfg.pad           = 'nextpow2';
> cfg.foilim        = [0.3 fs/2]; % teorema di Nyquist
> freq_epoched_Cz_Filtered = ft_freqanalysis(cfg, data_epoched_ECG_Filtered);
>
> %% ===Plot Spectra===
>
> plot(freq_epoched_Cz_Filtered.freq,mag2db(freq_epoched_Cz_Filtered.powspctrm));
>
> Thanks a lot.
>
> Kind regards,
> Alessandro Orticoni
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