Remove the 20 Hz low frequency and 95 Hz high frequency from an ecg signal with bandpass filter

close all

clear all

M=dlmread('FOETAL_ECG.dat')

v=M(:,3)

figure

subplot(4,1,1),plot(v)

Fe=250;

Te=1/Fe;

n=length(v);

t=(0:n-1)/Fe;

subplot(4,1,2),plot(t,v),

V=fft(v);

sp=abs(V);

%ff=((0:n-1)*Fe)/n

ff=(0:(n-1))/n*Fe;

subplot(4,1,3),

plot(ff(1:length(ff)/2),sp(1:length(sp)/2));

N=4;

Wn= [19 96];

filtru=fir1(N,Wn/(Fe/2),'bandpass')

semnalfiltrat_fir1=filter(filtru,1,v/100);

subplot(4,1,4),plot(t,semnalfiltrat_fir1),

figure

subplot(2,1,1),plot(t,v),

subplot(2,1,2),plot(t,semnalfiltrat_fir1),

H_fir1=abs(a);

figure

plot(b,H_fir1),

[Star Strider]

Removing those specific frequencies requires either an IIR bandstop filter for each, or a FIR filter that can remove both at the same time. The problem is that filtering out the 20 Hz signal will also filter out a significant part of the EKG record (usually considerd to span 0 Hz to 100 Hz).

Since I seriously doubt that this would ever be a real-world situation, I assume this is a homework problem.

We give hints for homework problems, so I have here.

.

Answer 1

[Ashutosh Singh Baghel]

Hi Denis,

I understand that you wish to remove several freq peaks from a signal, this can be achived by using two bandstop filters centered at the frequencies to be removed. Follow the folloing example for a better understanding. A signal having multiple frequency contents is passed through a bandstop filter of center frequency as 95Hz. the output of filter 'X_out' is then plotted later.

Here

fs = 500;                   %Sampling Frequency
t = 0:1/fs:4-1/fs;          %time axis

x1 = sin(2*pi*t*20);        %signal one with 20Hz
x2 = sin(2*pi*t*40);        %signal two with 40Hz
x3 = sin(2*pi*t*95);        %signal three with 95Hz
x4 = sin(2*pi*t*45);        %signal four with 45Hz

X = x1+x2+x3+x4;             %signal containing all

[pxx,fxx] = pspectrum(X,fs);
plot(fxx,pow2db(pxx))

N_1      = 10;  % Order
Fstop1_1 = 94;  % First Stopband Frequency
Fstop2_1 = 96;  % Second Stopband Frequency
Astop_1  = 80;  % Stopband Attenuation (dB)

% Construct an FDESIGN object and call its CHEBY2 method.
h_1  = fdesign.bandstop('N,Fst1,Fst2,Ast', N_1, Fstop1_1, Fstop2_1, Astop_1, fs);
Hd_95 = design(h_1, 'cheby2');

X_ans = filter(Hd_95,X);
[pxx_out,fxx_out] = pspectrum(X_ans,fs);
plot(fxx_out,pow2db(pxx_out))

Similarly, a 20Hz bandstop filter can be designed. Follow the MATLAB documentation link on filterDesign, and how to filter data.