[Matplotlib-users] mlab: computing power spectra

[Antonino I. <tri...@gm...>]

Hi to all,

I'm computing some spectrograms using the convenient functions defined in
matplotlib.mlab.

I found something that I don't completely understand. In computing the PSD
the onesided power spectrum is scaled by a factor of 2 except for the first
and the last bin:

https://github.com/matplotlib/matplotlib/blob/v1.4.2/lib/matplotlib/mlab.py#L776

in principle this should account for the contributions that in a two-sided
spectrum is spread over negative and positive frequencies. However the lack
of scaling in the highest frequency seems to cause a systematic
underestimation on that frequency bin.

Is it possible that the power spectrum should  be scaled, in the case of
even-length signal, up to the last bin (so that -1 in the slicing should be
removed)?

With correct scaling a signal comprised of two sinusoids of same power, one
being at the Nyquist frequency, should have the same amplitude (excluding
scallopping losses). I don't have time to test it at the moment, but maybe
someone more expert can already tell me if I'm missing something or the
matplotlib scaling is wrong.

Thanks,
Antonio