Date: Nov 17, 2012 6:10 AM
Author: Nasser Abbasi
Subject: Re: question on fft example
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