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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
<pau...@gm...> 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
>> <pau...@gm...> 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
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>

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
<pau...@gm...> 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ドル for 25 servers or applications!
> http://p.sf.net/sfu/zoho_dev2dev_nov
> _______________________________________________
> Matplotlib-users mailing list
> Mat...@li...
> https://lists.sourceforge.net/lists/listinfo/matplotlib-users

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

On 29 October 2012 14:50, Daryl Herzmann <ak...@gm...> wrote:
> I've been attempting to get basemap to clip a contourf display. I have not
> had any luck! Attached is a self contained example. Could somebody kindly
> point out what I am doing wrong!?!
>
Hi Daryl,
You were almost there. Remove the call to mask_outside_polygon and replace
it with
for collection in cs.collections:
 collection.set_clip_path(patch)
Attached is your corrected example and the output produced.
Ian

On Mon, Nov 12, 2012 at 11:43 AM, Nils Wagner <ni...@go...>wrote:
> Hi all,
>
> how can I hide ticks and/or labels in the presence of sharex=ax. Only
> the last subplot 313 should have ticks and labels.
>
> import matplotlib.pyplot as plt
> fig=plt.figure(0,figsize=(16,24))
>
> ax = fig.add_subplot(311)
> ax.set_xticks([])
> ax.set_xticklabels('')
>
> ax1 = fig.add_subplot(312,sharex=ax)
> ax1.set_xticks([])
> ax1.set_xticklabels('')
>
> ax2 = fig.add_subplot(313,sharex=ax)
> ax2.set_xticks(ind+width)
> ax2.set_xticklabels( contname,rotation='90',fontsize=8)
>
> Nils
>
>
ax.label_outer() will set the appropriate visibility settings for the
particular subaxes depending on where it is in the grid. Just call it for
each subplot being shared and you are good to go.
Cheers!
Ben Root

Hi all,
how can I hide ticks and/or labels in the presence of sharex=ax. Only
the last subplot 313 should have ticks and labels.
import matplotlib.pyplot as plt
fig=plt.figure(0,figsize=(16,24))
ax = fig.add_subplot(311)
ax.set_xticks([])
ax.set_xticklabels('')
ax1 = fig.add_subplot(312,sharex=ax)
ax1.set_xticks([])
ax1.set_xticklabels('')
ax2 = fig.add_subplot(313,sharex=ax)
ax2.set_xticks(ind+width)
ax2.set_xticklabels( contname,rotation='90',fontsize=8)
Nils