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From: Debora on 9 Aug 2010 12:10
Thanks Wayne. Your suggestion works, but it removes more signal instead of all the signal of that peak.

Is there a way of removing all frequencies under 5 and having zero signal until then?

Thank,
Debora

"Wayne King" <wmkingty(a)gmail.com> wrote in message <i3p181$p3l$1(a)fred.mathworks.com>...
> "Debora " <debi_can(a)yahoo.com.br> wrote in message <i3otib$rjd$1(a)fred.mathworks.com>...
> > Sorry, let me try to explain further.
> >
> > First, yes, I have used the filter design toolbox for the butterworth filter.
> >
> > As for the signal, I may not be using the appropriate terms, so I will try to write about it. My data has frequencies up to 1000 Hz. I am considering, from literature and previous works, that emg frequencies are from 20 to 500 Hz. Those are the frequencies I've used. Under 20 Hz, close to 0 Hz, I have this peak. This always happens with the equipment I've used, so I am sure this is noise and must be removed.
> >
> > Thanks, again,
> > Debora
>
> Hi Debora, Assuming that you really want to remove this peak (I'm speaking about Jan's post). Then:
> 1.) Identify the frequency corresponding to the peak.
> 2.) Use iirnotch() from the Filter Design Toolbox to remove the oscillation.
>
> For example, just creating some data here:
>
> Fs = 1000;
> t = linspace(0,1,1000);
> % just simulating a 50-Hz component
> x = cos(2*pi*50*t)+randn(size(t));
> % identifying the frequency
> h = spectrum.periodogram;
> msspec = msspectrum(h,x,'Fs',1000,'NFFT',length(x));
> [y,index] = max(msspec.Data);
> freq = msspec.Frequencies(index);
> % convert that frequency to normalized frequency
> W = (2*freq)/Fs;
> BW = W/35;
> % get the notch filter
> [b,a] = iirnotch(W,BW);
> y = filtfilt(b,a,x);
> msfilt = msspectrum(h,y,'Fs',1000,'NFFT',length(x));
> subplot(211);
> plot(msspec); title('Before Notch Filtering');
> subplot(212);
> plot(msfilt); title('After Notch Filtering');
>
> Hope that helps,
> Wayne
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