Hi, If you trace back into the code further, you will see that the Pxx is computed as X = fft(x), Pxx = X * conj(X) which is real, but the data type will be complex with a ~0 imaginary part (up to floating point precision). Thus the Pxx.real is just to ensure that the resulting data type is real instead of complex to save memory. Glenn
On Mon, Nov 12, 2012 at 11:42 AM, Paul Anton Letnes <paul.anton.let...@gmail.com> wrote: > Hi, > > not 100% sure this is a bug, but here goes: > > In file matplotlib/lib/matplotlib/mlab.py, the functions psd (power spectral > density) and specgram returns the real part of the fourier transform. > % grep -n Pxx.real mlab.py > 390: return Pxx.real,freqs > 470: Pxx = Pxx.real #Needed since helper implements generically > (git version 4f902fac1c5bf267e3fdeb4c2045926d7498e85a, cloned from github > today) > > This all means that the specgram plot routine yields the real part of the > Fourier transform, rather than its absolute square (forgetting normalization > for simplicity of discussion). The definition of the PSD is that it is the > absolute square of the Fourier transform: > https://en.wikipedia.org/wiki/Power_spectral_density#Energy_spectral_density > > Hence, I believe this is a bug which should be fixed. > > Cheers > Paul > ------------------------------------------------------------------------------ > Monitor your physical, virtual and cloud infrastructure from a single > web console. Get in-depth insight into apps, servers, databases, vmware, > SAP, cloud infrastructure, etc. Download 30-day Free Trial. > Pricing starts from $795 for 25 servers or applications! > http://p.sf.net/sfu/zoho_dev2dev_nov > _______________________________________________ > Matplotlib-users mailing list > Matplotlib-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/matplotlib-users ------------------------------------------------------------------------------ Monitor your physical, virtual and cloud infrastructure from a single web console. Get in-depth insight into apps, servers, databases, vmware, SAP, cloud infrastructure, etc. Download 30-day Free Trial. Pricing starts from $795 for 25 servers or applications! http://p.sf.net/sfu/zoho_dev2dev_nov _______________________________________________ Matplotlib-users mailing list Matplotlib-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-users
