drawing a fast Fourier transform

how to draw fft? Hints wanted

Answer 1

[Meg Noah]

Here's a solution

X = dlmread('ECG.csv');

Fs = 350*60;    %  [samples/min] sampling frequency 
T  = 1/Fs;      % [s] sampling period       
N  = 3000;      % [samples] Length of signal
t  = (0:N-1)*T; % [s] Time vector
deltaF = Fs/N;  % [1/min]) frequency intervalue of discrete signal

figure('color','white','position',[70 100 600 900]); 
subplot(3,1,1);
plot(60*t,X)
title({'Heartbeat data'})
xlabel('t (seconds)')
ylabel('X(t)')

% compute the fast fourier transform
Y = fft(X);

% manually shifting the FFT
Y = abs(Y/N);
Amp = [Y(ceil(end/2)+1:end)' Y(1) Y(2:ceil(end/2))']';
if (mod(N,2) == 0)
sampleIndex = -N/2:1:N/2-1; %raw index for FFT plot
else
sampleIndex = -(N-1)/2:1:(N-1)/2; %raw index for FFT plot
end
AmpSquared = Amp.^2;

subplot(3,1,2);
plot(deltaF*sampleIndex, AmpSquared);
hold on;
idx = find(AmpSquared > 15);
idx(sampleIndex(idx) < 0) = [];
plot(deltaF*sampleIndex(idx), AmpSquared(idx), '+');
for k = 1:length(idx)
    if (idx(k) > (N-1)/2 && AmpSquared(idx(k))> 15)
        h = text(deltaF*sampleIndex(idx(k)), AmpSquared(idx(k))+2,...
            ['f=' num2str(deltaF*sampleIndex(idx(k))) ' BPM']);
        set(h,'rotation',60)
    end
end
xlabel('Frequency [BPM]');
ylabel('|FFT(ECG)|^2');
title(['Power of FFT of ECG']);
xlim([0 max(deltaF*sampleIndex)/4]);

subplot(3,1,3);
half_f = deltaF*(0:(N/2));
plot(60*fftshift([half_f -fliplr(half_f(2:end+mod(N,2)-1))]), ...
    abs(fftshift(Y)/N).^2);
xlabel('Frequency [BPM]');
ylabel('|FFT(ECG)|^2');
title('Using fftshift - Displaying Full Spectrum Power');

[Meg Noah]

You'll need to change the xlim values to get to 0 to 120 BPM.