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Re: question on fft example
Posted:
Nov 17, 2012 6:10 AM


On 11/17/2012 4:49 AM, Greg Heath wrote: > "Nasser M. Abbasi" <nma@12000.org> wrote in message <k829kk$hv6$1@speranza.aioe.org>... >> On 11/15/2012 2:33 AM, oversky wrote: >>> The mathwork fft example in the following link, >>> http://bit.ly/SO9TuV >>> why do we double abs(Y(1:NFFT/2+1) when plotting the amplitude? >>> >>> plot(f,2*abs(Y(1:NFFT/2+1))) >>> >> >> This was talked about before. can google it. Here is my take on it: >> >> "Why does it multiply by 2?" >> >> http://12000.org/my_notes/on_scaling_factor_for_ftt_in_matlab/index.htm >
> To represent the same average power as the doublesided frequency representation. > > the DC and Nyquist components should NOT be doubled. > > plot((0:Fs/2), [abs(Y(1)), 2*abs(Y(2:NFFT/2)), abs(Y(NFFT/2+1)) ] ) > > Hope this helps. > > Greg >
hi Greg;
Its been a while since I looked at this myself, so I am rusty now to remember all the details (that is why I write down everything before I forget it) but there were different views on these scaling factors. Here is the same question, asked in 2009 here FYI
http://www.mathworks.com/matlabcentral/newsreader/view_thread/261034
Nasser



