[Matplotlib-devel] PSD Scaling (matplotlib version 1.4.X versus 1.5.X)

[Thomas Caswell]

I suspect that this is related to
https://github.com/matplotlib/matplotlib/issues/4328 which was fixed by
https://github.com/matplotlib/matplotlib/pull/4593 and was an intentional
change.

Tom

On Thu, May 12, 2016 at 3:13 PM plotalot <dr369 at nau.edu> wrote:

> Hello,
>
> I noticed an inconsistency with the pyplot.psd function when upgrading
> from matplotlib version 1.4.0 to 1.5.3
>
> I have written some code (below) that will show what I'm talking about
> when run with the different matplotlib versions. In 1.4.0, the line "PX2
> /= fft_size" was needed, but gives the wrong answer (P_ave is not equal
> to 1, and Pxw.max() is -36dB instead of 0dB) if it is included when
> using 1.5.3
>
> I am mainly concerned with knowing if this change was intentional; will
> it behave like this from now on (no longer needing division by fft_size)?
>
> import matplotlib.pyplot as plt
> import matplotlib.mlab as mlab
> import numpy as np
>
> fft_size = 4096
> fs = 2500.00
>
> gram_lines = 100;
> t = np.arange(1.0,gram_lines*fft_size,1.0);
> t = t / fs;
> del_f = fs/fft_size;
> base_freq = 10.0              # in Hz
> bin_err = 0.0
> f = (int(base_freq / del_f) + bin_err) * del_f # ensure f is in center
> of Fourier band
> A = np.sqrt(2.0);  # if the amplitude is sqrt(2) then the total power =
> 1.0 linear and 0.0 in dB
> dat = A * np.sin(2 * np.pi * f * t);
>
> (Px2, F) = plt.psd(dat, NFFT = fft_size, Fs=fs, Fc=0,
> detrend=mlab.detrend_none, window=mlab.window_none, noverlap=0, pad_to =
> None, sides='onesided', scale_by_freq = False);
> Px2 /= fft_size;   #comment this line out if using 1.5.1??
> P_ave = sum(Px2);
> Px2 = 10.0*np.log10(Px2);
>
> #print results to compare actual with expected
> print "Sum of psd Px2 = %f, This should equal 1.0."  %P_ave
> print "Max psd Px2 = %f dB. This should equal 0.0 dB if freq is a
> multiple of freq resolution.\n" %(Px2.max())
>
>
>
>
> --
> View this message in context:
> http://matplotlib.1069221.n5.nabble.com/PSD-Scaling-matplotlib-version-1-4-X-versus-1-5-X-tp47064.html
> Sent from the matplotlib - devel mailing list archive at Nabble.com.
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