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From: kk KKsingh on 26 May 2010 15:28
How can i calculate the Energy of the signal in Matlab

When i try to calculate for positive half i am getting Energy at right frequency ! But some thing is wrong when i do it for whole signal i am not getting at -30 and +30...
N=1024;
dt=.001;
t=0:dt*dt*(N-1);
y=sin(2*pi*30*t);
e=abs(fft(y))/(N);
energy=e.^2;
k=1/dt/N*[-N/2:N/2-1]
plot(k,e)

I am not able to get where i am doing wrong ! If i do it for positive half using energy=e.^2(1:N/2)..I am getting at Energy30 Hz..but if i use for full then i am not getting at -30 and +30 but getting some where near 470..

Thanks for your help

kk
From: Matt J on 26 May 2010 15:37

plot(k,fftshift(e))
From: kk KKsingh on 26 May 2010 16:07
"Matt J " <mattjacREMOVE(a)THISieee.spam> wrote in message <htjt91$1qa$1(a)fred.mathworks.com>...
>
> plot(k,fftshift(e))

Thanks ! I tried this before too but didnt worked ! But this time it did my fault !
From: kk KKsingh on 27 May 2010 18:15
"kk KKsingh" <akikumar1983(a)gmail.com> wrote in message <htjsok$rl6$1(a)fred.mathworks.com>...
> How can i calculate the Energy of the signal in Matlab
>
> When i try to calculate for positive half i am getting Energy at right frequency ! But some thing is wrong when i do it for whole signal i am not getting at -30 and +30...
> N=1024;
> dt=.001;
> t=0:dt*dt*(N-1);
> y=sin(2*pi*30*t);
> e=abs(fft(y))/(N);
> energy=e.^2;
> k=1/dt/N*[-N/2:N/2-1]
> plot(k,e)
>
> I am not able to get where i am doing wrong ! If i do it for positive half using energy=e.^2(1:N/2)..I am getting at Energy30 Hz..but if i use for full then i am not getting at -30 and +30 but getting some where near 470..
>
> Thanks for your help
>
> kk


Is there any difference between the energy if i calculate it in time domain and in frequency domain !

Suppose my signal is x=[1 2 3 4 5 6]

So its energy will be x^2...

and in Frequency domain

its (fft(x))^2

As, far as i know there is no change in Energy if signal in both domains!
From: Wayne King on 27 May 2010 18:39
"kk KKsingh" <akikumar1983(a)gmail.com> wrote in message <htmqt9$8u9$1(a)fred.mathworks.com>...
> "kk KKsingh" <akikumar1983(a)gmail.com> wrote in message <htjsok$rl6$1(a)fred.mathworks.com>...
> > How can i calculate the Energy of the signal in Matlab
> >
> > When i try to calculate for positive half i am getting Energy at right frequency ! But some thing is wrong when i do it for whole signal i am not getting at -30 and +30...
> > N=1024;
> > dt=.001;
> > t=0:dt*dt*(N-1);
> > y=sin(2*pi*30*t);
> > e=abs(fft(y))/(N);
> > energy=e.^2;
> > k=1/dt/N*[-N/2:N/2-1]
> > plot(k,e)
> >
> > I am not able to get where i am doing wrong ! If i do it for positive half using energy=e.^2(1:N/2)..I am getting at Energy30 Hz..but if i use for full then i am not getting at -30 and +30 but getting some where near 470..
> >
> > Thanks for your help
> >
> > kk
>
>
> Is there any difference between the energy if i calculate it in time domain and in frequency domain !
>
> Suppose my signal is x=[1 2 3 4 5 6]
>
> So its energy will be x^2...
>
> and in Frequency domain
>
> its (fft(x))^2
>
> As, far as i know there is no change in Energy if signal in both domains!

Hi, it depends on how you define the Fourier transform whether it is a unitary operator or not (energy preserving). The way the DFT is defined in Matlab, it is not energy preserving:

x = [1 2 3 4 5 6];
xDFT = fft(x);

Note that
norm(x,2)
is not equal to
norm(xDFT,2)

But that
norm(xDFT,2)/sqrt(length(x))

equals the norm of the time domain signal.

Wayne
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