I think I've got it! I only filtered out the pos freq and not the negative! (At the far end of s). I will check now.
It would be nice to forget about that bit s[n/2:n]. But I get if I get rid of it all, then when I back transform into time domain I imagine I'll get artifacts. Bill On Sat, Nov 28, 2009 at 9:53 AM, William Simpson <william.a.simp...@gmail.com> wrote: > I am puzzled by a filtering problem using fft(). I don't blame R. > > I have a waveform y consisting of the sum of 2 sinewaves having freqs f1 and > f2. > I do s = fft() of y. > Remove s's spike at freq=f2 > Do inverse fft on s. > The resulting waveform still has a lot of f2 in it! But the filtering > should have removed it all. > What is going on, and how to fix?? > > Thanks very much for any help. > Bill > > > Below is code illustrating the problem. > > n<-1024 > #f1=2, f2=7 > y<- 9*cos(2*pi*2.0*(1:n)/n) + 3*cos(2*pi*7.0*(1:n)/n) > plot(y,type='l') > > s<-2*Re(fft(y))/n > plot(s[1:20], type='l') > > #note freq[i] is s[i+1]; e.g. f=7.0 peak is at s[8] > coef<- 1-(((1:n)>8-1) & ((1:n)<8+1)) > s2<- fft(y)*coef # I have now removed f2 > > plot(2*Re(s2[1:20])/n, type='l') #f2 is removed from spectrum > > y2<-Re(fft(s2,inverse=TRUE)/n) #now get back to time domain > > plot(y,type='l') #original waveform f1+f2 > lines(y2,type='l',col='blue' ) #filtered waveform > lines(9*cos(2*pi*2.0*(1:n)/n), type='l',col='red' ) #what it should > look like -- f2 removed > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
