variance and psd of the ecg signal

Hello i am new in matlab and i have two question :

first: i can calculate the psd of ECG signal (with your help): "psdest = psd(spectrum.periodogram('Hamming'),sig,'NFFT',4096,'Fs',250);"

but i want to get percentage of the total power in a 2.7 Hz bandwidth symmetrically distributed around the dominant frequency( the frequency where the PSD is maximum), then normalized that in the 2.5–14 Hz range

and my secound question : how i can calculate the variance of ECG signal from time-domain ECG signal (without windowing)?

please help me,thank you very much for your attention.

Answer 1

[Wayne King]

Hi, how is this different than:

http://www.mathworks.com/matlabcentral/answers/16953-calculate-the-frequency-where-psd-is-maximun

If you want to normalize based on the 2.5-14 Hz band instead of 0 to the Nyquist as I did in my example, then you can do that, just use that interval in avgpower(psdest,[2.5 14])

To choose the 2.7 Hz bandwith around the maximum frequency, you just need to know the frequency spacing which you can get from

df = psdest.Frequencies(2)-psdest.Frequencies(1);

From that you can construct an interval that is maximum frequency plus or minus 2.7/2

    Fs = 250;
    t = 0:1/Fs:4-(1/Fs);
    sig = cos(2*pi*10*t)+randn(size(t));
    psdest = psd(spectrum.periodogram('Hamming'),sig,'NFFT',4096,'Fs',250);
    [mx,I] = max(psdest.Data);
    df = psdest.Frequencies(2)-psdest.Frequencies(1);
    numbins = round((2.7/2)/df);
      relperc = ...
      100*avgpower(psdest,[psdest.Frequencies(I-numbins) psdest.Frequencies(I+numbins)])/avgpower(psdest,[0 Fs/2])

To normalize on [2.5 14]

100*avgpower(psdest,[psdest.Frequencies(I-numbins) psdest.Frequencies(I+numbins)])/avgpower(psdest,[2.5 14])

The variance in the time domain is just var().

[Wayne King]

By the way, you better hope that the maximum power is in the interval [2.5, 14] Hz or normalizing on that interval won't make sense.