How do I get back filtered data using 'filter' after after getting the digital filter from bandpass?

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Ikmal Rosli
Ikmal Rosli on 11 Jun 2021
Edited: Chunru on 11 Jun 2021
Hello everyone, I'm trying to figure out the bandpass and filter function, which I am using to filter out signals in the range of 1.9 to 4.1 from my data. My code below is based on the documentation. Basically, after getting the digital filter, d, from bandpass(), I tried using filter to get back the filtered signals, but I cannot seem to get the same output as from when using bandpass. Thank you in advance!
load data
fs = 24;
figure
bandpass(data,[1.9 4.1],fs)
[~, d] = bandpass(data,[1.9 4.1],fs);
figure
plot(filter(d, data))
[~, d] = bandpass(x,[100 200],fs);
figure
plot(filter(d, data))

Accepted Answer

Chunru
Chunru on 11 Jun 2021
Edited: Chunru on 11 Jun 2021
% load data
data = randn(512, 1);
fs = 24;
figure
% design filter, filter the data, plot the results
bandpass(data,[1.9 4.1],fs)
% desing the filter only
[~, d] = bandpass(data,[1.9 4.1],fs);
% filter the data using the filter object. (bandpass use filtfilt
% internally by default)
xf = filtfilt(d, data);
% compare the origninal and the filtered signal
t =(0:length(data)-1)/fs;
figure
plot(t, data, 'b', t, xf, 'r');
legend('Original', 'Filtered');
  2 Comments
Chunru
Chunru on 11 Jun 2021
Hi. The question is not dumb. :-).
You can search document, eg. doc bandpass
Many matlab function has source code. You can edit bandpass. Then you can debug the program by stepping through.

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