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From: Matt Fetterman on 12 Jan 2010 12:48
"Jason " <syu.3(a)osu.edu> wrote in message <hiib9i$iu4$1(a)fred.mathworks.com>...
> "Jason " <syu.3(a)osu.edu> wrote in message <hii9dg$g2b$1(a)fred.mathworks.com>...
> > Hello all,
> > I am running into a small problem here with Matlab. I have a radar waveform that contains doppler content, and I am trying to plot it in a spectogram.
> > I am able to plot it on to a spectogram using the >spectogram command, but what I really wanted to do is to change the doppler frequency of the spectogram into the velocity such as m/s on the spectogram?
> > I am thinking that if I can get the frequencies of the spectogram, then I can just scale it into velocity, because I know the radar frequency and its wavelength.
> > But how excatly can I do that to get the frequency and then the velocity??? Please help??
>
> Thanks for the responses.
> I am trying to plot a spectrogram that shows velocity vs time instead of frequency vs time.
> So I was hoping to change the doppler frequency content of the spectrogram to velocity using the following formula:
> v = (1/2)*(Fd/Fc)*c
> v = velocity of moving object
> Fd = doppler freq
> Fc = transmitted freq of radar
> c = RF wave speed of light
>
> My code:
> S=spectrogram(A, Win, Nlap, 'yaxis');
> [m,n]=size(S);
> imagesc(20.*log10(abs((S(1:m,:)))));
> shading flat;
> axis xy; %cartesian
>
> and my waveform is in A, its a digitally sampled signal at 22KHz.
>
> Is there any way to extract that frequency from S, which I am guess is the complex number resulted from stft of A? But exactly how to work it?
> Thanks

Spectrogram gives you an image, with plots time on the x-axis and frequency on the y-axis. At each point on the x-axis (time), you can find the max value of the spectrogram. This will give you the central Doppler frequency at this time.
So with this approach you will convert the two-dimensional spectrogram into a one dimensional plot of Doppler frequency vs time.
Regards! Matt
From: Matt Fetterman on 12 Jan 2010 12:54
"Matt Fetterman" <mattinjersey(a)yahoo.com> wrote in message <hiickk$hvh$1(a)fred.mathworks.com>...
> "Jason " <syu.3(a)osu.edu> wrote in message <hiib9i$iu4$1(a)fred.mathworks.com>...
> > "Jason " <syu.3(a)osu.edu> wrote in message <hii9dg$g2b$1(a)fred.mathworks.com>...
> > > Hello all,
> > > I am running into a small problem here with Matlab. I have a radar waveform that contains doppler content, and I am trying to plot it in a spectogram.
> > > I am able to plot it on to a spectogram using the >spectogram command, but what I really wanted to do is to change the doppler frequency of the spectogram into the velocity such as m/s on the spectogram?
> > > I am thinking that if I can get the frequencies of the spectogram, then I can just scale it into velocity, because I know the radar frequency and its wavelength.
> > > But how excatly can I do that to get the frequency and then the velocity??? Please help??
> >
> > Thanks for the responses.
> > I am trying to plot a spectrogram that shows velocity vs time instead of frequency vs time.
> > So I was hoping to change the doppler frequency content of the spectrogram to velocity using the following formula:
> > v = (1/2)*(Fd/Fc)*c
> > v = velocity of moving object
> > Fd = doppler freq
> > Fc = transmitted freq of radar
> > c = RF wave speed of light
> >
> > My code:
> > S=spectrogram(A, Win, Nlap, 'yaxis');
> > [m,n]=size(S);
> > imagesc(20.*log10(abs((S(1:m,:)))));
> > shading flat;
> > axis xy; %cartesian
> >
> > and my waveform is in A, its a digitally sampled signal at 22KHz.
> >
> > Is there any way to extract that frequency from S, which I am guess is the complex number resulted from stft of A? But exactly how to work it?
> > Thanks
>
> Spectrogram gives you an image, with plots time on the x-axis and frequency on the y-axis. At each point on the x-axis (time), you can find the max value of the spectrogram. This will give you the central Doppler frequency at this time.
> So with this approach you will convert the two-dimensional spectrogram into a one dimensional plot of Doppler frequency vs time.
> Regards! Matt

Actually sorry. My last note is interesting but it does not answer your question that you asked. There is a linear mapping between Doppler frequency and velocity though, check Wikipedia.
Regards Matt
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