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Re: How to perform the 1-D signal filter? [message #58434 is a reply to message #58425]

Fri, 01 February 2008 06:54

duxiyu@gmail.com
Messages: 88
Registered: March 2007

Member

Thank you very much.
But I still have some questions about the parameters in the procedure
Digital_Filter.

Result = DIGITAL_FILTER( Flow, Fhigh, A, Nterms [, /DOUBLE] )

In the example, you set A=50 and Nterms=40 and I do not know where
their values come from and how I get them.

Seondly, I want to know the relation between the filter in time and
frequency domain.
if the following command is excuted, I think newfsignal is exactly
equal to fft(newsignal), isn't it?
"newsignal=convol(signal,timefilter)
fsignal=fft(signal)
newfsignal=fsignal*freqfilter"
And I also do not konw why you set steep=20.

Du

On Feb 1, 9:27 pm, Wox <nom...@hotmail.com> wrote:
> On Fri, 1 Feb 2008 02:20:52 -0800 (PST), "dux...@gmail.com"
>
>
>
> <dux...@gmail.com> wrote:
>> Dear all,
>
>> Here I give a signal example and hope somone can show me how to
>> perferm the frequency filter on it.
>
>> ;creat a signal data with two peaks in frequency domain at 2 and 3 Hz.
>> t=findgen(1000)/10.
>> data=sin(2*!pi*2*t)+sin(2*!pi*3*t)
>
>> freq=findgen(501)/100.
>> v=fft(data)
>> plot,freq,abs(v[0:500])^2,xtitle='frequency',ytitle='spectru m'
>
>> I want to filter the signal with the frequency higher than 2.5 Hz. How
>> do I do this?
>
>> I have read the help files about Digital_Filter and Convol, but I do
>> not know how to select the parameters for Signal_Filter.
>
>> Du
>
> 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'

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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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