Date: Nov 17, 2012 5:49 AM
Author: Greg Heath
Subject: Re: question on fft example

"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