Natasha asked . 2021-01-29

Filtering ECG signal with stopband filter using Butterworth filter method

 clear;
Data = csvread('ecg_HF_noise.csv',1,0); 
signal = Data(:,3)/100000;
N = length(signal);
FreqS = 94.3;                                                           % Sampling frequency
nyqFreq = FreqS/2;                                                      % Nyquist frequency

 fft_signal = abs(fft(signal));

 figure(1);                                                             % Plot single-sided magnitude spectrum (normalized)
x1 = 0:1/(N/2 -1):1;
y1 = fft_signal(1:N/2);
p1 = line(x1,y1);
grid on;
ax1 = gca; % current axes
axis([0 1 0 1]);
xlabel('Normalised frequency [\pi rads/sample]');
ylabel('Magnitude');
[b a] = butter(8, [0.49 0.55], 'stop');           
H = freqz(b,a,N/2);
hold on;
x2 = linspace(0,nyqFreq,500);
y2 = abs(H);
ax1_pos = ax1.Position;
ax2 = axes('Position',ax1_pos,'XAxisLocation','top','YAxisLocation','right','Color','none');
ax2.XLabel.String = 'Frequency [Hz]';
p2 = line(x2,y2,'Parent',ax2,'Color','r');                              % Plot frequency response
title('Magnitude spectrum of signal and frequency response of filter');
axis([0 nyqFreq 0 1]);
legend([p1 p2],'magnitude spectrum','frequency response');
x_filtered = filter(b,a,signal);
hold off;

An here the filtered signal vs. the noisy signal:

 

 

 

Figure 2:

I don't understand why by rejecting the ca. 24Hz peak, the signal is filtered. What about the noisy part between 0 to 0.4 and the mysterious large peak at the beginning? -When I apply the filter over that region the signal is not filtered. How can that be explained? I had my PC adapter (50Hz) near the sensor during the time of capture of the data.

 

What might that peak at the beginning with the large amplitude be? The unfiltered signal at the beginning of figure 3 looks normal though.

matlab , signal , signal processing