[SciPy-user] scipy.signal.firwin

[Tom K.]

Bugzilla from ndbecker2 at gmail.com wrote:
> 
> Jan Rauberg wrote:
> 
>> Lev Givon <lev <at> columbia.edu> writes:
>> 
>>> 
> 
>>> If you want to create a high-pass filter, just specify the cutoff to
>>> firwin and flip the signs of the obtained coefficients, i.e.,
>>> 
>>> b = -firwin(N,cutoff)
>>> 
>>> L.G.
>>> 
>> 
>> 
>> Thank you for the fast response. But in the way as you described I get an
>> inverted low pass figure. That's not a high pass and no solution for my
>> problem too. Perhaps there is another solution? J.R.
> 
> A trivial (not ideal) solution is G(z) = 1-H(z)
> 
> 

Simple inversion won't do the trick - you can invert the spectrum as Jan
suggests, or shift the entire spectrum by pi radians.

Example:

from scipy import signal
import numpy as np

N=11
h=signal.firwin(11, .3)
n=np.arange(-(N/2), (N/2)+1)

1) spectrum inversion G(z) = 1-H(z)
How: Subtract original filter from a kronecker delta function.
g=(n==1)-h

2) shift entire spectrum by pi radians
How: Elementwise multiply by (-1)**n
g1=(-1)**n*h

The resulting highpass filters will have different characteristics:
 g is complementary filter to h (bandwidth is pi-bandwidth(h))
 g1 has same bandwidth as h

These techniques won't work for even length filters, but you probably won't
want an even length highpass filter anyway because of the null at pi
radians.

  - Tom K.


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