remove a specific frequency from emg signal
in [Matlab]
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From: Wayne King on 9 Aug 2010 12:46 "Debora " <debi_can(a)yahoo.com.br> wrote in message <i3p99f$6p1$1(a)fred.mathworks.com>... > Thanks Wayne. Your suggestion works, but it removes more signal instead of all the signal of that peak. > > Is there a way of removing all frequencies under 5 and having zero signal until then? > > Thank, > Debora > > "Wayne King" <wmkingty(a)gmail.com> wrote in message <i3p181$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 Hi Debora, Yes, you can highpass filter the signal, removing everything below 5 Hz. What is your sampling frequency? Wayne
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