question on fft example

[oversky]

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)))

[Nasser M. Abbasi]

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

--Nasser

[oversky]

Thank you. I found a lot of useful material in your website.

[Greg Heath]

"Nasser M. Abbasi" <n...@12000.org> wrote in message <k829kk$hv6$1...@speranza.aioe.org>...

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

[Nasser M. Abbasi]

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

[Greg Heath]

"Nasser M. Abbasi" <n...@12000.org> wrote in message <k87rb1$edu$1...@speranza.aioe.org>...

Oh yeah.

1. In single-sided power and amplitude spectra, DC and Nyquist are never doubled
2. Other power spectrum components are doubled
3. Other amplitude spectrum components should be multiplied by sqrt(2)
because when they are squared they should represent the power spectrum.

Greg