[SciPy-User] firwin behavior

Jeff Alstott jeffalstott@gmail....
Thu Aug 11 06:36:33 CDT 2011


Wow. The passing of the DC frequency is exactly the issue, and that default
behavior is clearly shown in the documentation. I see now that given a band,
the default behavior is band-stop, whereas I would expect it to be
band-pass. So, that fixed it.

What I don't understand, however, is *why* that would be default behavior.
More importantly, even if that is the default behavior, the name of the
pass_zero flag does not readily help a dumb user like me grok the
functionality. Has there been any thought to renaming it?

Thanks!

On Wed, Aug 10, 2011 at 4:39 PM, Warren Weckesser <
warren.weckesser@enthought.com> wrote:

> On Wed, Aug 10, 2011 at 8:14 AM, Jeff Alstott <jeffalstott@gmail.com>
> wrote:
> > firwin is producing unreasonable filters for me, and I'm not sure if I'm
> > misusing the code or if there is a bug. Like so:
> >
> > In [5]: from scipy.signal import firwin
> >
> > In [6]: ny = 500
> >
> > In [7]: f21f80= firwin(21, [1/ny, 80/ny]); plot(f21f80);
> > savefig('FIR21_filter80.png')
> >
> > Produces the attached file.
> >
> > In contrast, Matlab:
> >
> > Trial>> ny = 500
> >
> > ny =
> >
> >    500
> >
> > Trial>> [f20f80] = fir1(20, [1/ny, 80/ny]); figure; plot(f20f80)
> >
> > Produces the other attached file. Quite different! The filter produced by
> > the scipy function, if used with lfilter (or if taken to Matlab to use as
> a
> > filter), produces a nonsense filtering, with many high frequency
> artifacts.
> >
> > Any thoughts? This is in python3, if that matters.
>
>
> By default, firwin creates a filter that passes DC (i.e. the zero
> frequency).  To get a filter like the one produced by matlab, add the
> keyword argument pass_zero=False.
>
> Warren
>
>
> >
> > Thanks!
> >
> >
> >
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> >
> >
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