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From: tom jakeman on 4 Feb 2010 18:27
Rune Allnor <allnor(a)tele.ntnu.no> wrote in message <cf337b2e-c731-4ae1-bd09-37fff3b63908(a)u7g2000yqm.googlegroups.com>...
> On 9 Des, 00:05, "onemilimeter " <on...(a)example.com> wrote:
>
> > May I know why do we need to divide fft(y,NFFT) by L?
>
> First of all, it's the convention just about everybody use.
> Second, the reason why everybody use this particular
> convention is that it saves a few computations. If you
> implement a filter, say, y[n] = x[n] (*) h[n] in frequency
> domain as
>
> y = ifft(fft(x).*fft(h));
>
> there are obviously two forward DFTs and one inverse DFT.
> With this convention one scales only one transform, the IFFT,
> instead of scaling three places. It might not look like much
> of a saving these days, but once upon a time it was significant
> enough that the idea made it to the textbooks, where it first
> of all became the standard method, and second has stuck ever
> since.
>
> Rune


Hello all, interesting thread,

I'd like to know why in

plot(f,2*abs(Y(1:NFFT/2+1)))

you multiply the absolute value by two?

thanks

Tom
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