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Re: Butterworth filter

• To: mathgroup at smc.vnet.net
• Subject: [mg108507] Re: Butterworth filter
• From: "Kevin J. McCann" <kjm at KevinMcCann.com>
• Date: Sun, 21 Mar 2010 02:04:57 -0500 (EST)
• References: <hnkfa8$po0$1@smc.vnet.net> <hnl0ln$6uq$1@smc.vnet.net> <201003190746.CAA08409@smc.vnet.net> <ho1uha$hoe$1@smc.vnet.net>

This is not a very good way to filter a signal. The resulting time 
series will "ring" due to the "sharp edges" in the filter. A much better 
way to filter the signal is with a Hamming or Hanning weighted bandpass 
or lowpass (whichever is appropriate) filter. This gives a much better 
response without the ring. This ringing is 13dB down from the peak, and 
can be significant, but with a Hamming filter the ringing is around 60dB 
down from the peak.

Kevin

Sseziwa Mukasa wrote:
> 
> Perhaps you're not getting many responses because your question is  
> somewhat unclear.  A Butterworth filter is typically used for analog  
> signal processing, but your data is digitized so you'd have to use a  
> digital filter.  One can digitize Butterworth filters but they don't  
> have all the properties of an analog Butterworth filter, furthermore,  
> it is trivial to implement an ideal low pass filter with superior  
> performance to a Butterworth for digitized data: Fourier transform  
> the signal, zero out all values greater than the desired cut off  
> frequency, Inverse Fourier transform to get the filtered signal.   
> Without further information about your data whether this is  
> appropriate or not, but if the goal is a low pass filter why insist  
> on a Butterworth?
> 

• Follow-Ups:
  • Re: Re: Butterworth filter
    • From: Sseziwa Mukasa <mukasa@jeol.com>

• References:
  • Re: Butterworth filter
    • From: Paul Floyd <root@127.0.0.1>