[Mne_analysis] Low pass filtering questions

Tue Mar 26 15:16:19 EDT 2013

Thank you for continuing to try to address this question.

Please pardon my asking for clarification.

My question boils down to this: If I use the standalone mne_process_raw and use the standard filter settings of 40 Hz with 5 Hz dropoff, is everything above 42.5 Hz cut to zero?

Thanks,

Don

Don Krieger, Ph.D.
Department of Neurological Surgery
University of Pittsburgh
(412)648-9654 Office
(412)521-4431 Cell/Text

-----Original Message-----
From: mne_analysis-bounces at nmr.mgh.harvard.edu [mailto:mne_analysis-bounces at nmr.mgh.harvard.edu] On Behalf Of Hari Bharadwaj
Sent: Tuesday, March 26, 2013 2:31 PM
To: Martin Luessi
Cc: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Low pass filtering questions
Importance: High

Thanks! I should apologize for being too lazy to look at the C code.

Regards,
Hari

On Tue, March 26, 2013 9:36 am, Martin Luessi wrote:
> Hari,
>
> Please excuse the long silence. In the C code, method (1) is used, i.e.,
> the filter is designed in the freq. domain with smooth transitions at
> the corner frequencies (using a cosine). In MNE-Python on the other hand
> method (2) is used. It can use both IIR or FIR filters, the FIR filters
> are designed using
>
> http://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.firwin2.html
>
> the actual implementation uses an overlap-add FFT applied in forward and
> backward direction to get zero phase (like filtfilt, as you mentioned).
>
> I hope this helps,
>
> Martin
>
> On 03/14/13 14:08, Hari Bharadwaj wrote:
>> Hi Don, Alex and Matti,
>>      Not to introduce any more confusion but I have a clarification
>> question:
>> Which of the following is true about the C-code?
>> (1) The filter is realized in the frequency domain fully (i.e) the FFT
>> coefficients for blocks of 2048 time samples are tapered to have a
>> transition band of 5 Hz around the cutoff and then going back to time.
>> In
>> this case, the filter is a non-causal IIR filter with zero-group delay.
>>
>> (2) An FIR filter is designed first and then then implemented in the
>> frequency domain using the overlap-add method with FFT blocks of 2048
>> points each. In this case, the filter is FIR and non-causal with
>> zero-group delay but there are (small) sidebands extending upto the
>> Nyquist rate. This would be like MATLAB's fftfilt().
>>
>> Thanks,
>> Hari
>>
>>
>> On Thu, March 14, 2013 8:45 am, Matti Hamalainen wrote:
>>>
>>> On Mar 14, 2013, at 8:30 AM, Alexandre Gramfort wrote:
>>>
>>>> hi Don,
>>>>
>>>>> Please bear with a further question:
>>>>> If the low-pass is set at 40 Hz, then the cos^2 drop off is applied
>>>>> beginning at the 40 hz components and falling to zero at 45 ?  Or is
>>>>> it
>>>>> applied beginning at something like 38 hz so that the fourier
>>>>> coefficients are attenuated by a fact of 2 at 40 ?
>>>>
>>>> I am not 100% about the C code but I bet for 40 to 45 with 0 at 45 Hz
>>>>
>>>>> And pardon this stupid question: Does this produce a gain of 0.0 for
>>>>> all higher frequencies in the filtered signal above 45?  Or is there
>>>>> some kind of ringing which occurs?
>>>>
>>>> it does remove all frequencies about 45Hz up to some numerical errors
>>>> and if there is some ringing it will be in time. The less the stop
>>>> band (here 5) the more ringing in time domain.
>>>
>>> Hi Don,
>>>
>>> In the C code the default width of the lowpass transition is 5 Hz. For
>>> 40
>>> Hz lowpass this means that the falloff starts at 37.5 Hz and ends at
>>> 42.5
>>> Hz. You can adjust this value in the mne_browse_raw or with the
>>> --lowpassw
>>> option in mne_process_raw and mne_browse_raw.
>>>
>>> The manual tells this in sections 4.5.1 and 4.2.1.
>>>
>>> - Matti
>>>
>>>
>>>
>>>
>>> ---------
>>>
>>> Matti Hamalainen, Ph.D.
>>> Athinoula A. Martinos Center for Biomedical Imaging
>>> Massachusetts General Hospital
>>>
>>> msh at nmr.mgh.harvard.edu
>>> mhamalainen at partners.org
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> Mne_analysis mailing list
>>> Mne_analysis at nmr.mgh.harvard.edu
>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>
>>
>
>
> --
> Martin Luessi, Ph.D.
>
> Research Fellow
>
> Department of Radiology
> Athinoula A. Martinos Center for Biomedical Imaging
> Massachusetts General Hospital
> Harvard Medical School
> 149 13th Street
> Charlestown, MA 02129
>
> Fax: +1 617 726-7422
>
>
>

-- 
Hari Bharadwaj
PhD Candidate, Biomedical Engineering,
Boston University
677 Beacon St.,
Boston, MA 02215

Martinos Center for Biomedical Imaging,
Massachusetts General Hospital
149 Thirteenth Street,
Charlestown, MA 02129

hari at nmr.mgh.harvard.edu
Ph: 734-883-5954

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