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From: "Steve Amphlett" <Firstname.Lastname@Where-I-Work.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Understanding phase in the FFT
Date: Thu, 30 Jul 2009 07:02:01 +0000 (UTC)
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TideMan <mulgor@gmail.com> wrote in message <a4004cfa-186e-43c7-ac6f-559fd9ef78c8@12g2000pri.googlegroups.com>...
> On Jul 30, 4:50?pm, "Steve Amphlett" <Firstname.Lastn...@Where-I-
> Work.com> wrote:
> > dbd <d...@ieee.org> wrote in message <8ea06202-cfd1-44cf-8656-c6c03daa2...@q40g2000prh.googlegroups.com>...
> > > On Jul 29, 2:35 pm, "aaaa Vadimov" <a...@hotmail.com> wrote:
> > > > Hi,
> >
> > > > Here the simple code of three series, which differ only by phase. I don't understand why I am not getting phases,which I expect to. Shouldn't I get 0 (or 360), 90 and 45 ? Thanks a lot!
> >
> > > > fs = 10;
> > > > t = 0:1/fs:50-1/fs;
> >
> > > > x1 = sin(2*pi*t);
> > > > x2 = sin(2*pi*t+pi/2);
> > > > x3 = sin(2*pi*t+pi/4);
> > > > ...
> >
> > > Why do you expect a sine wave to give a phase of 0 with an fft()? The
> > > basis vectors are cos()+i*sin().
> >
> > Phase is relative. ?The phase between two signals is obtained from the cross spectrum. ?The phase of a single signal isn't that meaningful or useful.
> 
> No it's not.  It's absolute.
> In the time domain, it defines the time when the cosine is a maximum.
> In the frequency domain, it defines what proportion of the Fourier
> transform is imaginary.

Ok, phase is useful when it's relative, for example, when there is a common reference involved.