[SciPy-dev] RFR: #902 firwin upgrades

Tom K. tpk@kraussfamily....
Mon May 11 20:44:32 CDT 2009

Charles R Harris <charlesr.harris <at> gmail.com> writes:

> On Sun, May 10, 2009 at 8:41 PM, Tom K. <tpk <at> kraussfamily.org> wrote:
> (I sent this to scipy-dev <at> scipy.org, thinking it would appear here,
> I am horribly confused - sorry for the duplication 
> Link to Trac ticket:  http://projects.scipy.org/scipy/ticket/902
> This is adding a highpass, bandpass, bandstop, and multiband capability to
> signal.firwin.  I submitted a patch attached to the ticket to 
> filter_design.py.
> I also added a test class in tests/test_filter_design.py.
> Let me know if there are any problems with the patch as this is my first patch
> to scipy and I am new to SVN.
> I think class BandPass is too general a name for a method based on windowing.
> Have you looked at the remez filter design? You can design the type of 
> filters you are talking about using it also.
> Chuck 

For PassBand, which of these options sounds best:
 1) rename to _PassBand, or FirwinPassBand
 2) make local to firwin function
 3) remove class altogether and just go with a list of (left, right) tuples

I hadn't seen remez as the wrapper is in signaltools, not filter_design, but
thanks for the pointer.  See this thread 
http://www.nabble.com/firwin-upgrades-td23246480.html for discussion about
the interface to firwin.

The question I have, what is the right way to provide a convenient shorthand
for describing a multiband filter with just a vector?  The Signal Processing
Toolbox has this divergence which maybe we want to avoid.  E.g. fir1 takes a 
list of band edges and you leave 0 and 1 off (they are present implicitly)
and the magnitudes - which are not required to be specified by the user - 
alternate between 1 and 0; the stop, high, and DC-0 options
change the magnitude to start at 0 instead (whereas pass, low, DC-1 start
with the default magnitude of 1).  Remez and firls take only band edges
in pairs and require the same for the magnitude - this allows for 
a piece-wise linear desired function with unspecified regions in between.
I haven't looked at the newly submitted fir2 (which is sitting in a trac issue)
but the Signal Processing Toolbox has a list of edges and magnitudes with
0 and 1 implicit again IIRC.  fir2 doesn't require paired bands, you just 
specify a piecewise linear function and that gets interpolated over the entire 
band and inverted to the impulse domain.

The more I think about it, the functions seem slightly different in the
ways that the algorithms work and so it makes some sense that they each
accept arguments that are tailored to be the most natural for that

It looks like remez in scipy takes only constant bands.  Hence it may make
sense to unify it with firwin.  firwin would then allow varying amplitudes
(the feature I added has them all the same).  The cost is some simplicity,
but perhaps some "smarts" could be added that make the magnitude vector
implicit if unspecified (which could work for remez in the same way).
Is that what you would propose? 

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