Path: news.mathworks.com!not-for-mail
From: <HIDDEN>
Newsgroups: comp.soft-sys.matlab
Subject: fft for beginners
Date: Sat, 4 Jul 2009 06:44:02 +0000 (UTC)
Organization: Sevilla
Lines: 15
Message-ID: <h2mtni$phl$1@fred.mathworks.com>
Reply-To: <HIDDEN>
NNTP-Posting-Host: webapp-05-blr.mathworks.com
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit
X-Trace: fred.mathworks.com 1246689842 26165 172.30.248.35 (4 Jul 2009 06:44:02 GMT)
X-Complaints-To: news@mathworks.com
NNTP-Posting-Date: Sat, 4 Jul 2009 06:44:02 +0000 (UTC)
X-Newsreader: MATLAB Central Newsreader 1896218
Xref: news.mathworks.com comp.soft-sys.matlab:552747


Hi all,

I am just starting doing fft with MatLab. No previous experience.

I am working with 2^19 experimental data points extending over some 50hours arranged as a vector X whose length is 2^19. I am getting on the fft since I need to study correlations and convutions in sets of different experimental data (so ifft will be also needed)

Since the rate of time in the experimental data is known I can control easily what the index of X means: say, from X(1,i) to X(1,i+1) there are 0.11ms while, of course, the first datapoint is t=0.

Now, I got: W=fft(X), which is again a vector whose length is 2^19.

My question is how is the index of this vector W running in the frequency domain? Say, low index means low frequency? frequencies are equally spaced? on what basis?

Also, I would like to know what would I be doing if I fftshift the vector W.

Thanks in advance.