Distortion at end of signal after low pass filtering

Hi,

I have created a low pass filter with the following properties to smooth out my signal:

function Hd = lpfilt
%LPFILT Returns a discrete-time filter object.

% MATLAB Code
% Generated by MATLAB(R) 9.5 and Signal Processing Toolbox 8.1.
% Generated on: 13-Dec-2019 17:10:56

% Butterworth Lowpass filter designed using FDESIGN.LOWPASS.

% All frequency values are in Hz.
Fs = 10;  % Sampling Frequency

Fpass = 0.45;        % Passband Frequency
Fstop = 0.55;        % Stopband Frequency
Apass = 1;           % Passband Ripple (dB)
Astop = 60;          % Stopband Attenuation (dB)
match = 'stopband';  % Band to match exactly

% Construct an FDESIGN object and call its BUTTER method.
h  = fdesign.lowpass(Fpass, Fstop, Apass, Astop, Fs);
Hd = design(h, 'butter', 'MatchExactly', match);

% [EOF]

I then apply my filter to my signal using the following syntax:

y = filtfilt(SOS,G,x)  % SOS and G are filter parameters

This is what the result looks like (see figure below - black - original unfiltered signal; red - filtered signal). The filter is smoothing the signal fine except for the start and end of the signal where it doesn't quite follow the original. Its worse at the end - appears to be distorted i.e. going upwards - is there a way to correct for this?

Thanks!

Answer 1

[Piyush Dubey]

When filtering a signal the output signal is shifted in time with respect to the input and sometimes the filter delays some frequency components more than others. This link describes on how to compensate for delay and distortion.

[Impala]

Dear Piyush,

Many thanks for your answer - I have already applied the filtfilt function as described in the link you kindly posted. However, it doesn't seem to have any effects on the distortion at the start and end of the signal - it still doesn't follow the original.

I have attached my data - posY (original), h (the filter coefficients) and posY_filt (the filtered version after applying the filter using filtfilt).

Maybe I'm not applying it correctly. Any help would be much appreciated.

Thanks!

[Piyush Dubey]

Can you try increasing the order of the filter?

d1 = designfilt('lowpassiir','FilterOrder',12, ...
    'HalfPowerFrequency',0.15,'DesignMethod','butter');
y_new = filtfilt(d1,posY);
p1 = plot(y_new);
hold('on');
p2 = plot(posY);
p3 = plot(posY_filt);
legend([p1; p2; p3], ["New filter"; "Actual signal"; "Old Filter"])

I was able to reduce the distortion with the above snippet.

[Impala]

Excellent! It's worked! Many thanks once again. Much appreciated :)