[Matplotlib-users] Possible error in psd, specgram and company

[Luis M. García-C. G. <lui...@gm...>]

Hello!

I have a doubt about the way matplotlib computes (and plots) a psd.  I have 
the following example code (I've imported pylab for convenience here):

  from pylab import *

  A = 10. ** (10. / 20.) / sqrt(2.) * 2. # This amplitude should give 10 dB
  frequency = 5.
  sampling_frequency = 1000.
  t_max = 12.
  t = arange(0., t_max, 1. / sampling_frequency)
  x = A * cos(2. * pi * frequency * t)

  Pxx, freqs = mlab.psd(x, Fs=sampling_frequency, NFFT=len(x),
      window=mlab.window_none)

  dB_actual = 20. * log10(rms_flat(x)) # dB_actual = 10., as expected
  dB_psd = 10. * log10(Pxx.max()) # dB_psd = 20.791812460476248


As I understand, this should not happen this way.  dB_psd should be equal to 
10, as dB_actual.  I can get it to compute the correct value with this:

  Pxx, freqs = mlab.psd(x / sqrt(t_max), Fs=sampling_frequency, NFFT=len(x),
      window=mlab.window_none)

(i.e., dividing the variable by the square root of the time). Is this really 
the expected behavior or this is a bug?

If it does matter, I'm using using matplotlib 1.3.1.

Thanks for your help!

-- 
Luis Miguel García-Cuevas González