MATLAB: How to do an nonuniform signal FFT

fftnonuniform signal

Hi, all,

Have anyone done FFT on an nonuniform signal without interpolation? My measured data are spaced 10 logarithmically and the measurement time is from 0.0001 ms to 10 ms, a very short period of time. When I use logarithmically spaced time, at early time, there will be more sampling points. If I use linear spacing to interpolate these data, the advantage of logarithmic spacing, early time with more data points and late time with few data points, will go away. So I am wondering if there is a method to do nonuniform signal FFT. If there is, how can I set the corresponding frequency in frequency domain?

Best Answer

You can use interp1() to interpolate the data to an evenly spaced grid, or you can use software specialized for this task:

NUFFT

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MATLAB: How to convert the units of the y-axis into miliseconds^2/Hz in power spectral density analysis of an ECG signal

The y-axis on a PSD estimate will not be in units of the time squared, it will be in amplitude squared per Hz, so for an ECG signal I assume that will be in millivolts, not milliseconds. The recorded values of the waveform are not in time.

Since you are using dspdata.psd, why not just use spectrum.periodogram?

    psdest = psd(spectrum.periodogram,ecg,'NFFT',length(ecg),'Fs',fs);
    plot(psdest)

If you want to plot without using dB (that is preferred by the way)

   plot(psdest.Frequencies,psdest.Data)

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There are a couple things you can do. Let x be your speech signal and fs your sampling frequency. The following two are power estimates.

    plot(psd(spectrum.periodogram,x,'Fs',fs));
If you do not want it in dB:
    psdest = psd(spectrum.periodogram,x,'Fs',fs);
    plot(psdest.Frequencies,psdest.Data);
    xlabel('Hz'); grid on;

Finally, just using fft()

      xdft = fft(x);
      % assume x is even length
      xdft = xdft(1:length(x)/2+1);
      freq = 0:fs/length(x):fs/2;
      plot(freq,abs(xdft));
      xlabel('Hz');

Best Answer

Related Solutions

MATLAB: How to convert the units of the y-axis into miliseconds^2/Hz in power spectral density analysis of an ECG signal

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