Re: The points of FFT transform


"jia" <jia.qinghua@xxxxxxxxx> wrote in message
news:1149068748.138082.66680@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi, here

A basic problem of DSP make me confused. When we want to the spectrum
character of a signal sequence, we can use the function "fft( )" of
Matlab.

fft(X,n) returns the n-point DFT.
1) If the length of X is less than n, X is padded with trailing zeros
to length n.
2) If the length of X is greater than n, the sequence X is truncated.

If we want to do a DFT of a sequence of discrete signal X, with the
length of N.

What is the proper points for DFT? just "n=N" is enought, or the
bigger, the better?

If n>N, then padded zeros, it is equal to interpolation. While, if "n"
is infinite, it maybe equal to the fourier transform of continuous
signal. So, I am confused.

What is the proper points for DFT?

The simplest thing to do is to use fft(X).

Anything you do is "proper" if it meets your intent.

It appears you understand what happens if n>N. Unless you need this, it's
extra computation that provides no additional information - just more
points.

Fred


.

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