MATLAB: How to plot microvolts values in matlab

plot eeg fft microvolts

Hi. I have eeg data that are in microvolts from emotiv.Fs=128 aquisition time=1min. When i plot the signal with the time vector t=0:1/Fs:60 i can see it.But when i do fft and plot it with the frequency vector freq=(1:length(signal))*fs/length(signal) the plot is so small i have to zoom in to see it.How can i set the axis to microvolts to prevent this?

Best Answer

You may have a significant d-c offset. Subtract the mean of the signal from the signal before you do the fft. This will have no effect on the frequency content of your signal.

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MATLAB: Remove trend and detect peaks in a photoplethysmogram(PPG) signal

If you want to eliminate the baseline wander and offset, change to a highpass filter with a different cutoff frequency:

wn=5/(fs/2); %lowpass 10Hz for ppg
[b,a] = butter(8,wn,'high'); 
ppg_head_data = filter(b,a,ppg_head);

There are much better ways to design and implement filters. Here is one:

t = linspace(0, numel(ppg_head), numel(ppg_head))/fs;
Fs = 960;                                                   % Sampling Frequency (Hz)
Fn = Fs/2;                                                  % Nyquist Frequency (Hz)
Wp = 24.5/Fn;                                               % Stopband Frequency (Normalised)
Ws = 25.0/Fn;                                               % Passband Frequency (Normalised)
Rp =   1;                                                   % Passband Ripple (dB)
Rs =  50;                                                   % Stopband Ripple (dB)
[n,Ws] = cheb2ord(Wp,Ws,Rp,Rs);                             % Filter Order
[z,p,k] = cheby2(n,Rs,Ws,'high');                           % Filter Design, Sepcify Bandstop
[sos,g] = zp2sos(z,p,k);                                    % Convert To Second-Order-Section For Stability
figure(2)
freqz(sos, 2^16, Fs)                                        % Filter Bode Plot
ppg_head_data = filtfilt(sos, g, ppg_head);                 % Filter Signal

Use the Signal Processing Toolbox findpeaks (link) function on your filtered data to locate the peaks:

[pks,locs] = findpeaks(ppg_head_data, 'MinPeakHeight',2E+5);

You can use the ‘locs’ index vector to refer to the peaks and times in the original unfiltered signal.

MATLAB: Change in frequency domain with resampling

interp1 have different options to choose from but resample has anti-aliasing filter.

Frequency vector will be different for both the ffts and can be calculated by

F=(0:L/2)*fs/L where L is the length of fft output and fs is the sampling frequency of the signal.

Check the MATLAB script for reference.

Best Answer

Related Solutions

MATLAB: Remove trend and detect peaks in a photoplethysmogram(PPG) signal

MATLAB: Change in frequency domain with resampling

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