I have an EMG-signal and would like to get the mean value of the amplitude in a defined frequency space. I tried making FFT, but i don't get a vector with amplitudes and frequencies. I am new to Matlab - please help.
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1 Answers
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Here is how you would get power (i.e., amplitude) and frequency from the fft function in Matlab:
fs = 1024; % sampling rate of signal
time = 1; % time in sec of simulated signal
xn = randn (fs*time,1); % creating random signal
nfft = time*fs; % size of window on which to perform FFT
Y = fft (xn, nfft);
% Convert value to obtain the power of the signal at
% each frequency
Pyy = Y .* conj(Y) / nfft;
% Create a frequency axis for ploting
fy = fs/nfft * (0:(nfft/2) -1);
plot (fy, Pyy(1:nfft/2))
ylabel ('power (AU)^2') % arbitrary units
xlabel ('frequency (Hz)')
xlim ([0 512])
However, keep in mind that EMG is not a stationary signal. When doing spectral analysis on EMG signals we are trying to approximate the true spectrum. This is why EMG is usually seperated into windows of data. FFT is performed on each of these windows and the averaged.
Rather than using the fft function, you might want to consider using the pwelch function in the Matlab Signal Processing Toolbox. It allows you to set the window size, amount of overlap of the windows, etc.
Alternatively you could use the very popular Neurospec Matlab toolbox writen by David Halliday. It has a PDF that describes all the functions and examples you can use. It has additional functionality (e.g., coherence), but you can ignore it if you like and simply extract the spectral data.
