Philipp,
What you want to do is apply a "notch" filter to your signal, which is the
converse of a "band pass" filter.
Filtering can be applied in either the time or freq domain. They are
equivalent in terms of linear transformations.
Your attempt to filter by cutting frequencies out of an FFT is in the
frequency domain. At first glance this seems ideal, because you seem to be
able to zero out any arbitrary band of frequencies. But this is an
illusion. You are only constraining the sampled points in the FFT. The
points in between are not constrained to zero, so this won't work
"perfectly", as you might have imagined. You can make it "better" by
increasing the FFT size, but perfection would demand an infinite number of
points.
In the time domain, you achieve the same results by convolving a filter
against the sampled points. Again, a perfect filter does not exist, but is
the limit as the filter length goes to infinity. A filter to "notch" out a
single frequency would have to be rather long and it will spill over
somewhat and attenuate neighboring frequencies. You will have to determine
how "steep" the walls of your filter will have to be.
Practical time-domain filters can be built with the Remez algorithm. A
message was posted in Oct detailing an Octave version of this algorithm,
under the subject:"An octave port of Remez algorithm to design linear
phase FIR filters".
HTH,
John Day
Computer Science Innovations
At 05:34 PM 12/7/01 +0100, Philipp Schwaha wrote:
hi!
what i need to do is fiter a signal to remove noise. what i wanted to try
first was a fft then cut all frequencies except the the important ones (e.g
100Hz, 200Hz ...), but when i did the fft i did not know how to do this,
since there are no frequencies associated with the values the function fft
returns.
this is the first time i try to do something using an fft.
then i discovered the function fftfilter, but i could not make it produce the
results i wanted.
how can i remove all frequencies from a certain signal, except a few selected
ones?
thanks
philipp
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Octave is freely available under the terms of the GNU GPL.
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Subscription information: http://www.octave.org/archive.html
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