buttor band pass filter

I build a bandpass filter with buttor for 0.8Hz to 3Hz for AC part. there are 3000 line data. and each 128 point in 1 second. So I choose the sample rate is 128. But why is it only look fine after 2 sec. form 0 to 2 it look so terrible why?

num = xlsread('Untitled 12.xls');

time = num(:,1);

signal = num(:,2);

[b, a]=butter(2, [0.8 3]/(128/2),'bandpass');

signal_disposed =filter(b,a,signal);

plot(time(128:end),signal_disposed(128:end));

Answer 1

[Star Strider]

First, I would use filtfilt here:

signal_disposed =filtfilt(b,a,signal);

Second, your signal does not look horrible. It looks as though you have about a 1 Hz signal that according to your filter design is being passed appropriately.

[xiang kunyu]

yeah, I use for PPG signal AC. The AC of the PPG look like that. But I only not dont know why 0-2s signal like that. because the delay of filter?

[Star Strider]

You didn’t include your entire signal, only from 128:end. I don’t know what your entire original or filtered signal looks like.

See if using the filtfilt function gives a different result.