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mlab.psd behaviour has changed in 1.5.x, psd depends on window size NFFT? #7237
Description
Issue seen in matplotlib 1.5.3, Linux, installed with pip.
Retesting software that was originally written in matplotlib 1.4 times, I am noticing a huge discrepancy in psd plots.
It looks like recent matplotlib mlab.psd fails to properly normalize the psd wrt the window size NFFT (at least when scale_by_freq is set to False).
As a result psd plots (in dB) are shifted up and down when the NFFT parameter is changed.
It is unclear to me if this is an intended result. In any case, it does not seem to be documented.
Activity
efiring commentedon Oct 8, 2016
This is intended--it is the way the "power spectrum" works, as opposed to the "power density spectrum". The former is what you get with
scale_by_freq=False, the latter withTrue. It is illustrated in the second plot here: http://currents.soest.hawaii.edu/ocn_data_analysis/_static/Spectrum.html. As NFFT changes, the power spectrum preserves the height of a spectral peak from a single sinusoid, but not the level of a broad-bandwidth background. The reverse is true of the power spectral density.See #4328 and #4593.
callegar commentedon Oct 10, 2016
I was expecting that. My point was actually the change in behavior wrt previous matplotlib versions that I did not find in the changelog and that ended up breaking backwards compatibility. Possibly I missed that.
efiring commentedon Oct 10, 2016
No, I think we missed it. It looks like it should be in http://matplotlib.org/api/api_changes.html, but isn't.
jklymak commentedon Jan 28, 2023
This is really old history - closing...