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Topic: question on fft example
Replies: 5   Last Post: Nov 17, 2012 11:02 PM

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Nasser Abbasi

Posts: 5,645
Registered: 2/7/05
Re: question on fft example
Posted: Nov 17, 2012 6:10 AM
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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




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