{"id":209,"date":"2017-02-15T14:14:50","date_gmt":"2017-02-15T12:14:50","guid":{"rendered":"http:\/\/benediktehinger.de\/blog\/science\/?p=209"},"modified":"2017-02-15T14:14:50","modified_gmt":"2017-02-15T12:14:50","slug":"simple-filter-generation","status":"publish","type":"post","link":"https:\/\/benediktehinger.de\/blog\/science\/simple-filter-generation\/","title":{"rendered":"Simple Filter Generation"},"content":{"rendered":"

I sometimes explain concepts to my students. Then I forget the code and the next time round, I have to redo it. Take this post as an extended memory-post. In this case I showed what filter-ringing artefacts could look like. This is especially important for EEG preprocessing where filtering is a standard procedure.<\/p>\n

A good introduction to filtering can be found in this slides by andreas widmann<\/a>\u00a0or this paper by andreas widmann<\/a><\/p>\n

Impulse with noise<\/h3>\n

I simulated as simple impulse response with some additional noise. The idea is to show the student that big spikes in the EEG-data could result in filter-ringing that is quite substantial and far away from the spike.<\/p>\n

\"\"<\/a><\/p>\n

The filter<\/h3>\n

This is the filter I generated. It is a BAD filter. It shows huge passband ripples. But for educational purposes it suits me nice. I usually explain what passbands, transitionband, stopband, ripples and attenuation means.
\n\"\"<\/a><\/p>\n

The code<\/h3>\n

“`matlab
\nfs = 1000;
\nT = 2;
\ntime = 0:1\/fs:T-1\/fs;<\/p>\n

data = zeros(length(time),1);
\n% data(end\/2:end) = data(end\/2:end) + 1;
\ndata(end\/2) = data(end\/2) + 1;
\ndata = data + rand(size(data))*0.02;<\/p>\n

subplot(2,1,1)
\nplot(data)<\/p>\n

filtLow = designfilt(‘lowpassfir’,’PassbandFrequency’,100, …
\n‘StopbandFrequency’,110,’PassbandRipple’,0.5, …
\n‘StopbandAttenuation’,65,’SampleRate’,1000);<\/p>\n

subplot(2,1,2)<\/p>\n

% 0-padding to get the borders right
\ndata = padarray(data,round(filtLow.filtord));<\/p>\n

% Filter twice to get the same phase(non-causal)
\na = filter(filtLow,data);
\nb = filter(filtLow,a(end:-1:1));
\nb = b(round(filtLow.filtord)+1:end – round(filtLow.filtord));
\nplot(b)<\/p>\n

fvtool(filtLow) % to look at the filter<\/p>\n

“`<\/p>\n","protected":false},"excerpt":{"rendered":"

I sometimes explain concepts to my students. Then I forget the code and the next time round, I have to redo it. Take this post as an extended memory-post. In this case I showed what filter-ringing artefacts could look like. This is especially important for EEG preprocessing where filtering is a standard procedure. A good introduction to filtering can be found in this slides by andreas widmann\u00a0or this paper by andreas widmann Impulse with noise I simulated as simple impulse response with some additional noise. The idea is to show the student that big spikes in the EEG-data could result…<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-209","post","type-post","status-publish","format-standard","hentry","category-blog"],"_links":{"self":[{"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/posts\/209","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/comments?post=209"}],"version-history":[{"count":0,"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/posts\/209\/revisions"}],"wp:attachment":[{"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/media?parent=209"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/categories?post=209"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/benediktehinger.de\/blog\/science\/wp-json\/wp\/v2\/tags?post=209"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}