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From: opto1k@jetson.uh.edu (A JETSON News User)
Newsgroups: comp.dsp
Subject: Help on FFT frequency resolution
Message-ID: <8156.278a8f4d@jetson.uh.edu>
Date: 9 Jan 91 09:34:37 GMT
Organization: University of Houston
Lines: 35

Hello,

I have a question about frequency bins from discrete FFT.  Please respond
by email.


From what I understand, bin k corresponds to frequency
	f(k) = (k * SR)/N (Hz), where k = 0, 1, ..., N/2, and
	SR is the sampling rate in Hz; N is the sample size

So, given a sample of 512 points sampled at 1024 Hz, the frequency bins are
0, 2, 4, 6, 8, 10, ..., 512 Hz.

Now, if I pad 512 zeros to the end of the data points, I have 1024 samples
sampled at 1024 Hz, and the frequency bins will be 0, 1, 2, 3, ... 512 Hz.

This seems to increase the DFFT frequency resolution.  But I remember I read
from a text book somewhere that says it isn't so (I forget which book that is);
the padding is just a way of interpolation.  Is it true?

If the padding is just an interpolation, is it acceptable to use the values
to calculate the amplitudes of the harmonics (not PSD) at 1 Hz, 3 Hz, etc.?

If it is not acceptable mathematically, is there a way to find what I want at
those frequencies?

No, this is not a class homework.  I have large amount of data from an
experiement which cannot be redone.  Any help would be appreciated.

Thanks in advance.
___
Tong Ho
University of Houston
College of Optometry
Email: opto1k@jetson.uh.edu