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FIX: Correct output of mlab._spectral_helper when scale_by_freq=False #4593

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Merged
merged 1 commit into from
Jul 16, 2015
Merged

FIX: Correct output of mlab._spectral_helper when scale_by_freq=False #4593

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9 changes: 6 additions & 3 deletions lib/matplotlib/mlab.py
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Original file line number Diff line number Diff line change
Expand Up @@ -765,9 +765,6 @@ def _spectral_helper(x, y=None, NFFT=None, Fs=None, detrend_func=None,
pass

if mode == 'psd':
# Scale the spectrum by the norm of the window to compensate for
# windowing loss; see Bendat & Piersol Sec 11.5.2.
result /= (np.abs(windowVals)**2).sum()

# Also include scaling factors for one-sided densities and dividing by
# the sampling frequency, if desired. Scale everything, except the DC
Expand All @@ -787,6 +784,12 @@ def _spectral_helper(x, y=None, NFFT=None, Fs=None, detrend_func=None,
# values. Perform the same scaling here.
if scale_by_freq:
result /= Fs
# Scale the spectrum by the norm of the window to compensate for
# windowing loss; see Bendat & Piersol Sec 11.5.2.
result /= (np.abs(windowVals)**2).sum()
else:
# In this case, preserve power in the segment, not amplitude
result /= np.abs(windowVals).sum()**2

t = np.arange(NFFT/2, len(x) - NFFT/2 + 1, NFFT - noverlap)/Fs

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12 changes: 9 additions & 3 deletions lib/matplotlib/tests/test_mlab.py
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Expand Up @@ -1805,31 +1805,37 @@ def test_psd_windowarray(self):

def test_psd_windowarray_scale_by_freq(self):
freqs = self.freqs_density
win = mlab.window_hanning(np.ones(self.NFFT_density_real))

spec, fsp = mlab.psd(x=self.y,
NFFT=self.NFFT_density,
Fs=self.Fs,
noverlap=self.nover_density,
pad_to=self.pad_to_density,
sides=self.sides)
sides=self.sides,
window=mlab.window_hanning)
spec_s, fsp_s = mlab.psd(x=self.y,
NFFT=self.NFFT_density,
Fs=self.Fs,
noverlap=self.nover_density,
pad_to=self.pad_to_density,
sides=self.sides,
window=mlab.window_hanning,
scale_by_freq=True)
spec_n, fsp_n = mlab.psd(x=self.y,
NFFT=self.NFFT_density,
Fs=self.Fs,
noverlap=self.nover_density,
pad_to=self.pad_to_density,
sides=self.sides,
window=mlab.window_hanning,
scale_by_freq=False)

assert_array_equal(fsp, fsp_s)
assert_array_equal(fsp, fsp_n)
assert_array_equal(spec, spec_s)
assert_allclose(spec_s, spec_n/self.Fs, atol=1e-08)
assert_allclose(spec_s*(win**2).sum(),
spec_n/self.Fs*win.sum()**2,
atol=1e-08)

def test_complex_spectrum(self):
freqs = self.freqs_spectrum
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