# [Eeglablist] filters

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Wed Jun 6 14:29:14 PDT 2012

```Thanks Clemens!

Makoto

2012/6/6 Clemens Brunner <clbrunner at ucsd.edu>:
> Makoto, it's the other way round for me -- I'm not an expert in analog
> filter design, but I do know a little bit about signal processing :-). Yes,
> your intuition is correct. You can still apply this to digital filters --
> IIR filters are typically designed using analog design technique and then
> transferring that to the digital domain. FIR filters are usually
> digital-only, so I'm not sure if these properties apply (of course, if you
> increase the filter order of an FIR filter, the cutoff will be steeper, but
> I cannot tell you if there is a way to determine the slope).
>
> There is a good introductory paper by the Mathworks, which talks about
> filters and has MATLAB examples:
> http://www.mathworks.com/tagteam/55876_digfilt.pdf
>
> HTH
> Clemens
>
>
>
> On 06/06/2012 11:28 AM, Makoto Miyakoshi wrote:
>>
>> Dear Tim and Clemens,
>>
>> Thanks for feedback.
>> Since I don't know much about signal processing, my understanding of
>> filter is the analog one used in an electric circuit with capacitors
>> and inductors (as used in speakers and amplifiers). Sor it is
>> intuitive for me, as Tim said, that slope and filter order are such
>> that 6dB for the first order (1 capacitor/inductor), 12dB for the
>> second order (capacitor + inductor), 18dB for the third order (3
>> elements), etc. But it seems filter can be 'designed' pretty freely in
>> signal processing.
>>
>> Clemens, I would appreciate if you could show us an introductory
>> website or paper to learn what is digital filter.
>>
>> Makoto
>>
>>
>>
>> 2012/6/6 Tim Schoof<t.schoof at ucl.ac.uk>:
>>>
>>> Hi Makoto,
>>>
>>> I got the following response from Clemens Brunner:
>>>
>>> Hi Tim,
>>>
>>>> I was wondering whether anyone knew the exact filter specifications of
>>>
>>> the
>>>>
>>>> Basic FIR and Short IIR filters implemented in eeglab.
>>>
>>>
>>> eegfilt() uses firls() or fir1() to compute the filter coefficients for a
>>> given order.
>>>
>>> iirfilt() designs an elliptic filter and automatically selects the order
>>> to meet the
>>> filter specs (high cutoff, low cutoff, passband ripple and stopband
>>> attenuation).
>>>
>>>> Also - and this is probably a question about filters in general - I was
>>>
>>> hoping someone could tell me how to specify a 24 dB/oct roll-off (or I
>>> suppose 12 dB/oct given that the script uses filtfilt). What order would
>>> that correspond to in an FIR filter?
>>>
>>> Depends on which filter design technique you use. If you want to go the
>>> manual way,
>>> first pick a design technique (windowing, Kaiser window, optimum filters
>>> via
>>> Parks-McClellan, ...), provide the desired specifications, and the
>>> corresponding
>>> parameters (including filter order) will pop out.
>>>
>>> You can play around with MATLAB's filter design tools if you have MATLAB
>>> and the
>>> Signal Processing Toolbox. You can use fdesign or filterbuilder, or
>>> fdatool. This
>>> will let you fully customize your filter.
>>>
>>> If you want to use such a custom FIR filter with EEGLAB, I guess you
>>> would
>>> have to
>>> write a short plugin or filter outside of EEGLAB (i.e. working with
>>> EEG.data
>>> directly).
>>>
>>> HTH
>>> Clemens
>>>
>>> Tim
>>>
>>>> Dear Tim,
>>>>
>>>> I'm also interested in knowing this. Arno?
>>>>
>>>> Makoto
>>>>
>>>> 2012/6/4 Tim Schoof<t.schoof at ucl.ac.uk>:
>>>>>
>>>>> Dear eeglab list,
>>>>> I was wondering whether anyone knew the exact filter specifications of
>>>>> the
>>>>> Basic FIR and Short IIR filters implemented in eeglab.
>>>>> Also - and this is probably a question about filters in general - I was
>>>
>>> hoping someone could tell me how to specify a 24 dB/oct roll-off (or I
>>> suppose 12 dB/oct given that the script uses filtfilt). What order
>>> would
>>>>>
>>>>> that correspond to in an FIR filter?
>>>>> Many thanks,
>>>>> Tim
>>>>> _______________________________________________
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>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Makoto Miyakoshi
>>>> JSPS Postdoctral Fellow for Research Abroad
>>>> Swartz Center for Computational Neuroscience
>>>> Institute for Neural Computation, University of California San Diego
>>>>
>>>
>>>
>>>
>>
>>
>>
>

--
Makoto Miyakoshi
JSPS Postdoctral Fellow for Research Abroad
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
```