MATLAB: How to set the length of an imported series of data sets

data set length

I have 20 windaq voice files i have imported into matlab and i need to set all of the individual data sets to be the same length so i can compute the fft of all of them.

Thanks in advance

Best Answer

If you want all of them to have the same frequency resolution in the Fourier transform, zero-pad the shorter to the length of the longest. So with respect to the documentation in fft (link), ‘NFFT’ would either be the length of the longest, or 2^nextpow2(length(longest)), as you prefer.

Related Solutions

MATLAB: FFT application to Gravity data. Issues with units.

They are the same as the original units, unless you express them as power, since that squares the magnitudes. You need to divide by the length of ‘data’ to normalise the energy in the Fourier transform to get the correct magnitudes:

ftgrav = fft(data)/(length(data);

If you are doing a one-sided Fourier transform, you need to multiply the amplitude by 2 as well. See the documentation on fft (link) for a full description. (I discussed that in some detail in this recent Answer (link), so I won’t repeat it here.)

MATLAB: Amplitude Error in amplitude Spectrum of FFT

I suspect that your frequency of interest is simply not falling on a DFT bin directly. Why do you think you need to use a power of two for the DFT?

For example, if I have data sampled at 1 kHz and I have 1000 points, then frequency increment in DFT bins is 1 Hz (the bins are 1 Hz apart) so a frequency of 100 Hz for example will fall directly on a bin.

   Fs = 1000;
   t = 0:1/Fs:1-1/Fs;
   x = 1.5*cos(2*pi*100*t);
   xdft = fft(x);
   freq = 0:Fs/length(x):Fs/2;
   xdft = xdft(1:length(x)/2+1)./length(x);
   xdft(2:end) = 2*xdft(2:end);
   plot(freq,abs(xdft))

See that the magnitude is estimated exactly. Now watch what happens when I pad to 1024:

   xdft = fft(x,1024);
   freq = 0:Fs/length(xdft):Fs/2;
   xdft = xdft(1:length(xdft)/2+1)./length(x);
   xdft(2:end) = 2*xdft(2:end);
   plot(freq,abs(xdft))

by padding to 1024, I have now made the frequency of interest fall between DFT bins and that makes my amplitude estimate inaccurate.

See https://www.mathworks.com/help/signal/ug/amplitude-estimation-and-zero-padding.html

in the Signal Processing Toolbox documentation

Best Answer

Related Solutions

MATLAB: FFT application to Gravity data. Issues with units.

MATLAB: Amplitude Error in amplitude Spectrum of FFT

Related Question