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Re: How to perform the 1-D signal filter? [message #58427 is a reply to message #58425]

Fri, 01 February 2008 09:16

Wox
Messages: 184
Registered: August 2006

Senior Member

On Fri, 1 Feb 2008 07:16:26 -0700, David Fanning <news@dfanning.com>
wrote:

> Wox writes:
>
>> Example below filters in time or frequency domain:
>>
>>
>> ; Time domain
>> freq1=2.
>> freq2=3.
>> freq3=4.
>> dtime=0.1
>> ntime=1000
>>
>> time=dtime*findgen(ntime)
>> signal=sin(2*!pi*freq1*time)+sin(2*!pi*freq2*time)+sin(2*!pi *freq3*time)
>>
>> ; Time domain Filter
>> f_low = 0
>> f_high = 2.5
>> timefilter = DIGITAL_FILTER(f_low*2*dtime, f_high*2*dtime, 50.,40)
>> signal=convol(signal,timefilter)
>>
>> ; Frequency domain
>> nfreq=ntime/2+1
>>
>> freq=findgen(nfreq)/(dtime*ntime)
>> fsignal=fft(signal)
>>
>> ; Frequency domain filter (instead of time domain filter)
>> if n_elements(timefilter) eq 0 then begin
>> steep=20.
>> freqfilter= 1./(1.+(freq/f_high)^steep)
>> fsignal*=freqfilter
>> endif
>>
>> plot,freq,abs(fsignal[0:nfreq-1])^2,xtitle='frequency',ytitl e='spectrum'
>
> Wonderful example, but I'm trying to understand this whole
> subject. Do you think you could flush this out with a little
> explanation of what you are doing and why you choose the terms
> you use, etc.? What kind of frequency filter are you constructing
> here? I don't necessarily see it doing any filtering of the signal,
> at least if I pass it the original signal, rather than the signal
> that had already been filtered in the time domain, as written
> in your example.
>
> Cheers,
>
> Confused

Ok, sorry for the confusion, but I was just illustrating that you can
do the same filtering in the frequency domain as in the time domain.
You do one or the other, not both at the same time. Btw, convolution
in one domain becomes multiplication in the other:

filtered = signal "convol" filter
fft(filtered) = fft(signal) x fft(filter)

But I guess you already knew all this.

The filter used is the Kaiser-Bessel filter. At least I think
digital_filter is using this. For the filter I constructed in the
fourier domain, I'm not quit sure whether it is really identical to
the KB filter, but if you plot it, it looks like a nice lowpass filter
to me :-).

I'm just typing this in a hurry... Did I answer your questions?

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[Message index]

		Re: How to perform the 1-D signal filter? By: Kenneth P. Bowman on Fri, 01 February 2008 06:58
		Re: How to perform the 1-D signal filter? By: Wox on Fri, 01 February 2008 09:16
		Re: How to perform the 1-D signal filter? By: duxiyu@gmail.com on Sat, 02 February 2008 07:14
		Re: How to perform the 1-D signal filter? By: Wox on Fri, 01 February 2008 08:57
		Re: How to perform the 1-D signal filter? By: duxiyu@gmail.com on Fri, 01 February 2008 06:54
		Re: How to perform the 1-D signal filter? By: David Fanning on Fri, 01 February 2008 06:16
		Re: How to perform the 1-D signal filter? By: Wox on Fri, 01 February 2008 05:27
		Re: How to perform the 1-D signal filter? By: duxiyu@gmail.com on Sat, 02 February 2008 07:20
		Re: How to perform the 1-D signal filter? By: David Fanning on Fri, 01 February 2008 17:07
		Re: How to perform the 1-D signal filter? By: David Fanning on Fri, 01 February 2008 10:30

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