short term fourier tranform.....

hi...i want to plot the STFT of a signal in matlab.....the signal is
the return signal(echo)...from the rotating blades of a hovering
helicopter....can...anyone...give me a method...to calculate STFT of
this in matlab..i tried with the specgram..function....but..iam
unable..to set it's parameters accordingly..so iam not getting the
expected graph.....plz...help......

On May 23, 10:45 pm, pks.n...@gmail.com wrote:
> hi...i want to plot the STFT of a signal in matlab.....the signal is
> the return signal(echo)...from the rotating blades of a hovering
> helicopter....can...anyone...give me a method...to calculate STFT of
> this in matlab..i tried with the specgram..function....but..iam
> unable..to set it's parameters accordingly..so iam not getting the
> expected graph.....plz...help......

The crystal ball is fuzzy.  How can anybody possible tell
what the problem is if you don't give any meaningful parameter?

What do you expect, and what do you have in your experiment?

To the original poster:  please continue the discussion here
instead of emailing me directly.

Julius

On 24 May, 05:45, pks.n...@gmail.com wrote:
> hi...i want to plot the STFT of a signal in matlab.....the signal is
> the return signal(echo)...from the rotating blades of a hovering
> helicopter....can...anyone...give me a method...to calculate STFT of
> this in matlab..i tried with the specgram..function....but..iam
> unable..to set it's parameters accordingly..so iam not getting the
> expected graph.....plz...help......

The 'spectrogram' function in matlab is the correct function to
use (it might be called 'specgram', though). Helicopter sounds
have base frequencies around 10-20 Hz, and I would assume you
need something like 0.5 -1 Hz bin width, maximum. These are
numbers I guess based on previous experience with this sort
of analysis; you will not find them anywhere.

To get 1Hz bin width select your window length as round(1/Fs)
where Fs is your sampling frequency. I would suggest somewhere
between 50% and 75% overlap between successive frames.

These are not fixed numbers, but a starting point for you to
start playing around.

Rune