If you're using pyplot.specgram (i.e. "from pylab import *; specgram(...)"), note that the plot is in dB, hence the negative values. I'm surprised this fact isn't mentioned in the documentation: http://matplotlib.org/api/pyplot_api.html?highlight=specgram#matplotlib.pyplot.specgram
However, when in doubt, look at the code. On Mon, Nov 12, 2012 at 12:28 PM, Paul Anton Letnes <paul.anton.let...@gmail.com> wrote: > Heh, > > that's funny. Now then, why do my plots come out with negative values all > over the place? That's why I started digging around. After all, X * conj(X) > should be equal to the absolute square of X, right? > > Paul > > > On 12. nov. 2012, at 21:00, G Jones wrote: > >> 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
