[FieldTrip] does ft_freqanalysis output single or double-sided FFT amplitudes?

[Schoffelen, J.M. (Jan Mathijs)]

Hi Jack,

I don’t fully understand what you mean with single- or double-sided, but the basic output from Fieldtrip is the spectrum from 0-Nyquist, where the spectral quantities are normalised with sqrt(2/nsmp); the ‘2’ is to account for the negative frequencies. The correction step that you refer to in l.846-854 is to counteract the factor 2, because the DC and Nyquist frequency bins occur only once in the FFT output.

Best wishes,
Jan-Mathijs



On 4 May 2021, at 03:41, Jack Fogarty <jf752 at uowmail.edu.au<mailto:jf752 at uowmail.edu.au>> wrote:

Hi Fieldtrip team,

Could you please clarify, do the ‘ft_freqanalysis’ or ‘ft_specest_mtmfft’ functions output data reflecting the single or double-sided fourier spectrum? The lines 846-854 in ft_freqanalysis suggests to me it should be single-sided – is that correct?

  % correct the 0 Hz or Nyqist bin if present, scaling with a factor of 2 is only appropriate for ~0 Hz
  if ~isempty(hasdc_nyq)
    if keeprpt>1
      fourierspctrm(:,:,hasdc_nyq,:) = fourierspctrm(:,:,hasdc_nyq,:)./sqrt(2);
    else
      fourierspctrm(:,hasdc_nyq,:) = fourierspctrm(:,hasdc_nyq,:)./sqrt(2);
    end
  end
  freq.fourierspctrm = fourierspctrm;


Best,

Jack Fogarty, PhD
Postdoctoral Research Officer

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