How do I filter correctly ?
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Hi,
I have two small signals sampled both on 15360Hz:
1)"almost" a SIN, with fundamental frequency on 60Hz. 2) a noise signal (that would represent a SIN too), with fundamental frequency on 60Hz too.
I want to smooth the second signal, so, I did a 2. order butterworth LP filter, cutting over 200Hz.
So, plotting the 2 signal, I got the noised signal smoothed like I want.
The problem is,
If I took the same filter (LP) on the 1 signal, it will change its magnitude. Why (the first signal just have signals down the 200Hz) ??? Am I missing some theory ??
My butterworth:
function Hd=filtsignal
Fs=15360;
N=2; % order
Fc = 200; % cutoff
h=fdesign.lowpass('N,F3db', N, Fc, Fs);
Hd=design(h,'butter');
On my main code:
% y is the 1 signal
y=filter(filtsignal,y);
% y2 is the 2 signal
y2=filter(filtsignal,y2);
Andre
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Accepted Answer
Star Strider
on 22 May 2014
I don’t completely understand. However if you filter out noise, the magnitude of the output signal from the filter will decrease because you’ve removed some of the energy in the signal.
Also, I suggest using filtfilt rather than filter. It eliminates phase distortion.
4 Comments
Star Strider
on 22 May 2014
You might be able to reduce the attenuation by using a higher-order filter, and using tf2sos and then filtfilt (with the second-order-section syntax) instead of filter. The filtfilt function will eliminate phase distortion. Other filter designs (Chebyshev, elliptic) have different passband characteristics that might work better in your application. The only way to find out which is best is to experiment.
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