[SciPy-User] FFT Filter
Anders Harrysson
anders.harrysson@fem4fun....
Tue Sep 27 09:21:42 CDT 2011
Hi,
I have looked thou the example and obviously there are something that I
simply don't get :(. Do anyone have some good references on how to
decide the input parameters to firwin2? Furthermore, when computing he
phase delay, will this differ when using a band pass filter instead of a
low pass?
Sorry for very elementary questions...
Anders
Warren Weckesser skrev 2011-09-27 13:53:
> Anders,
>
> The attached script is a variation of
> http://www.scipy.org/Cookbook/FIRFilter. It uses firwin() to create a
> bandpass filter, and plots the frequency response and the results of
> applying the filter to a sample. It should be straightforward to
> modify it to use firwin2() instead, if desired.
>
> The script uses scipy.signal.lfilter to apply the filter, but this
> isn't the fastest way to apply a FIR filter. Take a look at an
> experiment that I did here
> http://www.scipy.org/Cookbook/ApplyFIRFilter to see a comparison of
> several way of applying a FIR filter.
>
> Regards,
>
> Warren
>
>
> On Tue, Sep 27, 2011 at 4:47 AM, Anders Harrysson
> <anders.harrysson@fem4fun.com <mailto:anders.harrysson@fem4fun.com>>
> wrote:
>
> Hi,
> Thanks a lot for the reply. I have started to look into this and
> currently I am trying to use firwin2 to generate the filter
> coefficients according to below
>
> b=signal.firwin2(512,freq,gain,nfreqs=N,nyq=nf)
>
> X2=signal.lfilter(b,[1.0],y)
>
>
> However I get strange results, when trying to filter even a very
> simple signal :(. This is probably not a scipy related question,
> but how to put the numtaps number? How is this related to the
> other inputs.
>
> Regards,
> Anders
>
> Christophe Grimault skrev 2011-09-26 18:32:
>> Hi Anders,
>>
>> Use : scipy.signals.lfilter(b, a, x)
>>
>> Where x is your signal (complex or real, it doesn't matter). As a filter
>> in the Fourier domain is basically FIR filter, you only need to pass the
>> b array (the response of the filter) and set a = [1.0].
>>
>> Chris
>>
>> On Sat, 2011-09-24 at 09:33 +0200, Anders Harrysson wrote:
>>> Dear all,
>>>
>>> I am kind of new to scipy and also new to the signal processing field
>>> that this question relates to.
>>> I am trying to do a bandpass FFT filter using python. The filter shape
>>> is symmetric around 11 Hz and is defined by the parameters ff and Hz below.
>>>
>>> x=loadtxt('file')
>>>
>>> sr = 250 # [samples/s]
>>> nf = sr/2.0 # Nyquist frequence
>>> Ns = len(tr[:,0]) # Total number of samples
>>> N=float(8192) # Fourier settings
>>>
>>> # Fourier transform
>>> X1 = fft(x,n=int(N))
>>> X1 = fftshift(X1)
>>> F1 = arange(-N/2.0,N/2.0)/N*sr
>>>
>>> # Filter
>>> ff=[0,1,1,0]
>>> Hz = [9.5, 10, 12, 12.5]
>>> k1=interp(-F1,Hz,ff)+interp(F1,Hz,ff)
>>> X1_f=X1*k1
>>> X1_f=ifftshift(X1_f)
>>> x1_f=ifft(X1_f,n=int(N))
>>>
>>> My question is now:
>>> Are ther built in functionallity for filtering in scioy and, if so, how
>>> would a similar filter looks like.
>>>
>>> Regards,
>>> Anders Harrysson
>>> _______________________________________________
>>> SciPy-User mailing list
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>>> http://mail.scipy.org/mailman/listinfo/scipy-user
>
>
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