[SciPy-User] Signal to noise ratio

[Warren Weckesser]

Ariel Rokem wrote:
> Hi Warren,
>
> thanks for this example!
>
> I am getting the following error:
>
> Library/Frameworks/Python.framework/Versions/6.0.0/lib/python2.6/site-packages/scipy/signal/filter_design.py:224: 
> BadCoefficients: Badly conditionned filter coefficients (numerator): 
> the results may be meaningless
>   "results may be meaningless", BadCoefficients)
>
> For any attempt to filter a low-pass below 12 Hz in this example (so - 
> I don't get the plot you got - instead I get a flat line on the bottom 
> subplot). Do you (or anyone?) have any idea why that is?
>

Nils Wagner reported the same behavior a week or so ago, and I see the 
same behavior now ("BadCoefficients" and a flat line in the last plot).  
Try lowering the order of the Butterworth filter to order=6.

Warren


> Cheers,
>
> Ariel 
>
> On Wed, Feb 24, 2010 at 7:51 AM, Warren Weckesser 
> <warren.weckesser at enthought.com 
> <mailto:warren.weckesser at enthought.com>> wrote:
>
>     Nils Wagner wrote:
>     > Hi all,
>     >
>     > I have two questions concerning signal processing
>     >
>     > I have used scipy.stats.signaltonoise to compute the
>     > signal-to-noise ratio.
>     > The value is 0.0447.
>     > How can I judge it ?
>     >
>     > How can I filter out high frequencies using scipy ?
>     >
>
>     I posted an example low-pass filtering using 'butter' and
>     'lfilter' from
>     scipy.signal here:
>
>        http://mail.scipy.org/pipermail/scipy-user/2010-January/024032.html
>
>     Warren
>
>     > How can I eliminate noise from the signal ?
>     >
>     > Nils
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>
> -- 
> Ariel Rokem
> Helen Wills Neuroscience Institute
> University of California, Berkeley
> http://argentum.ucbso.berkeley.edu/ariel
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