How to plot microvolts values in matlab?

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?

Answer 1

[Star Strider]

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.

[vaggelis papasot]

Thank you very much for your answer!I had a suspicion about the dc offset,but reading the emotiv epoc specifications pdf https://emotiv.com/product-specs/Emotiv%2520EPOC%2520Specifications%25202014.pdf it says it has 50-60hz notch filter and thought it was to cut line noise.Now plotting a channel AF3 in time domain before cutting of the dc has its first value at 500uV,then cutting dc by AF3=AF3-mean(channel1) centers the signal at 0.Also plotting the signal with dc off at frequency domain gives a clear view of the frequency plot and is more visible that the signal is filtered between 0.2-45Hz.I hope that if anyone else is having the same problem with eeg data from emotiv or emokit will benefit from this question.Thanks again for your answer!

[Star Strider]

As always, my pleasure!

My own experience with biomedical signal processing led me to hypothesise the d-c offset problem. Thank you for your detailed analysis.

Answer 2

[John D'Errico]

help axis