[FieldTrip] Low-pass frequency when downsampling using ft_resampledata (Nieuwenhuijzen, M.E. van de (Marieke))

["Jens Klinzing, Universität Tübingen"]

Hi Marieke,
I had the impression that you are interested in the automatic low-pass 
filtering performed by ft_resampledata.

If you don't specify cfg.resamplemethod = 'downsample', ft_resampledata 
will call the built-in matlab function 'resample' to do the job. This 
function automatically applies a low-pass filter before resampling.

mathworks.com/help/signal/ref/resample.html

"resample applies an anti-aliasing (lowpass) FIR filter to x during the 
resampling process. It designs the filter using firls with a Kaiser window."

I couldn't find an explicit statement what the cutoff frequency of that 
filter would be, though. There are some forum entries on the web, but 
they don't give the answer either.

Can someone help?

All the best,
Jens

Am 23.01.2015 um 17:56 schrieb Schoffelen, J.M. (Jan Mathijs):
> Marieke,
> Have you considered to generate a spectrum of your downsampled signal (up to Nyquist frequency) and check the low-pass cutoff there? I guess it will be easily visible.
>
> JM
>
>
> On Jan 23, 2015, at 2:39 PM, Eleanor Harding <harding at cbs.mpg.de> wrote:
>
>> Hi Marieke,
>>
>> A rule of thumb for downsampling is to low-pass filter at 1/3 of your desired sampling rate, so, for you that would be 100 Hz. But of course you shouldn't make the filter cutoff too close to what you are looking for in your data. Below is a helpful publication,
>>
>> Widmann, A., Schröger, E., & Maess, B. (in press). Digital filter design for electrophysiological data - a practical approach. Journal of Neuroscience Methods.
>>
>> Good luck,
>> Ellie Harding
>>
>>
>>
>> Message: 5
>> Date: Thu, 22 Jan 2015 16:50:26 +0000
>> From: "Nieuwenhuijzen, M.E. van de (Marieke)"
>> 	<m.vandenieuwenhuijzen at donders.ru.nl>
>> To: "fieldtrip at science.ru.nl" <fieldtrip at science.ru.nl>
>> Subject: [FieldTrip] Low-pass frequency when downsampling using
>> 	ft_resampledata
>> Message-ID:
>> 	<D270731257905445A56ECB1A4502301E395CEE at exprd04.hosting.ru.nl>
>> Content-Type: text/plain; charset="iso-8859-1"
>>
>> Hi Fieldtrippers,
>>
>> I have a small question about ft_resampledata. I have ECoG data that was measured with a sampling frequency of 1000 Hz, which I downsample to 300 Hz. >From what I understand, to avoid aliasing, this function applies a low-pass filter to the data before downsampling. How can I determine what the low-pass frequency of that filter would be?
>>
>> Best,
>> Marieke
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>> -- 
>> ------------------------------------------------------------------
>> Eleanor Harding
>> PhD Student
>> Max Planck Institute for Human Cognitive and Brain Sciences
>> Stephanstraße 1A, 04103 Leipzig, Germany
>> Phone: +49 341 9940-2268
>> Fax: +49 341 9940 2260
>> http://www.cbs.mpg.de/~harding
>>
>>
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