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From: Greg Heath on 2 Jul 2010 07:26
On Jul 1, 10:01 am, "Agus Nugroho" <agus_nugroh...(a)yahoo.com> wrote:
> Dear comrade in arms,
>
> I would like to ask you concerning the problem that I am facing now and I hope you do not mind to help me.
>
> I have tested a sinusoidal signal to find it's auto spectrum with algorithms (X- axis represents frequency and Y-axis represents Amplitude). The X-axis (frequency) of the curve is correct, however, I am not really sure about the Y-Axis (amplitude). Could you help me by seeing my algorithm below whether that is correct to find its amplitude? :
>
> -------------------------------------------------------------------------
> Fs=20;
> Ts=1/Fs;
> t=0:Ts:12.75;
> n=length(t);
> y=sin(2*pi*0.3515*t); %generate sinusoidal wave with freq 0.3515
> figure(1)
> plot(t,y)
>
> [YfreqD,freqRng]=positiveFFT(y,Fs);
>
> figure(2)
> YfreqDb=2*abs(YfreqD);      % amplitude scale
>
> Auto_sp=2*(conj(YfreqDb).*YfreqDb);           %auto spectrum signal
>
> Auto_scale=sqrt(2*Auto_sp)*(2/256);               %Auto spectrum scale, 256 is nfft
>
> plot(freqRng, Auto_scale)
>
> ---------------------------------------------------------------------------­--------
>
> the algorithm for function :
>
> function[YfreqD,freqRng]=positiveFFT(windowedSignal,Fs)
> N=length(windowedSignal);
> k=0:N-1;
> T=N/Fs;
> freqRng=k/T;
> YfreqD=fft(windowedSignal)/N;
> cutOff=ceil(N/2);
> YfreqD=YfreqD(1:cutOff);
> freqRng=freqRng(1:cutOff);
> -------------------------------------------------------------------
>
> Thank you very much for your help.
>
> Kindest regards,
>
> Agus

Im general,

cutoff = ceil((N-1)/2)

(Just in case you run into examples with N odd).

Hope this helps.

Greg
From: Agus Nugroho on 2 Jul 2010 07:42
Dear Dr. Grade,

Thank you for your correction.

Regards,

Agus
From: Greg Heath on 2 Jul 2010 08:51
On Jul 2, 7:42 am, "Agus Nugroho" <agus_nugroh...(a)yahoo.com> wrote:
> Dear Dr. Grade,
>
> Thank you for your correction.
>
> Regards,
>
> Agus

I've been called a lot of things; but never that!

Greg
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