m |
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| Line 32: | Line 32: | ||
{\mathcal X}(\omega)= \sum_{n=-\infty}^{\infty}x[n]e^{-j\omega n} | {\mathcal X}(\omega)= \sum_{n=-\infty}^{\infty}x[n]e^{-j\omega n} | ||
</math> | </math> | ||
| + | |||
| + | <span style="color:red"> --------------------------------------------- </span> | ||
set <math class="inline">\omega = \frac{2\pi k}{N}</math> and use the fact that <math>x[n]=0</math> for <math>n> N-1 </math> and for <math>n<0</math> | set <math class="inline">\omega = \frac{2\pi k}{N}</math> and use the fact that <math>x[n]=0</math> for <math>n> N-1 </math> and for <math>n<0</math> | ||
| + | |||
| + | |||
<math> | <math> | ||
{\mathcal X}(\frac{2\pi k}{N})= \sum_{n=0}^{N-1}x[n]e^{-j\frac{2\pi k}{N} n} = X_{N}[k] </math> formula(1) | {\mathcal X}(\frac{2\pi k}{N})= \sum_{n=0}^{N-1}x[n]e^{-j\frac{2\pi k}{N} n} = X_{N}[k] </math> formula(1) | ||
| + | |||
| + | <span style="color:red"> --------------------------------------------- </span> | ||
| + | : <span style="color:red"> Why do you need this part? Shouldn't you get an expression for the M point DFT of x[n] instead? </span> | ||
Now we can manipulate the DFT of y[n] | Now we can manipulate the DFT of y[n] | ||
Revision as of 09:19, 26 October 2011
Practice Problem
(This problem clarifies how zero-padding a signal changes its DFT.)
let x[n] be a signal with duration N. More precisely, assume that $ x[n]=0 $ for $ n> N-1 $ and for $ n<0 $.
Let y[n] be the zero-padding of x[n] to length M>N:
$ y[n]= \left\{ \begin{array}{ll} x[n], & 0\leq n < N,\\ 0, & N \leq n <M. \end{array} \right. $
Show that the M point DFT of y[n] satisfies
$ Y_M [k] = {\mathcal X} \left( \frac{2 \pi k }{M}\right), \text{ for } k=0,1,\ldots, M-1, $
where $ {\mathcal X} (\omega) $ is the DTFT of x[n].
You will receive feedback from your instructor and TA directly on this page. Other students are welcome to comment/discuss/point out mistakes/ask questions too!
Answer 1
$ {\mathcal X}(\omega)= \sum_{n=-\infty}^{\infty}x[n]e^{-j\omega n} $
---------------------------------------------
set $ \omega = \frac{2\pi k}{N} $ and use the fact that $ x[n]=0 $ for $ n> N-1 $ and for $ n<0 $
$ {\mathcal X}(\frac{2\pi k}{N})= \sum_{n=0}^{N-1}x[n]e^{-j\frac{2\pi k}{N} n} = X_{N}[k] $ formula(1)
---------------------------------------------
- Why do you need this part? Shouldn't you get an expression for the M point DFT of x[n] instead?
Now we can manipulate the DFT of y[n]
$ \begin{align} Y_{M}[k]&= \sum_{n=0}^{M-1}y[n]e^{-j\frac{2\pi k}{M} n} \\ &= \sum_{n=0}^{N-1}x[n]e^{-j\frac{2\pi k}{M} n} \ \ since \ y[n]=0 \ above \ N-1 \\ &= {\mathcal X}(\frac{2\pi k}{M}) \ \ by \ comparing \ to \ formula (1) \end{align} $
Answer 2
write it here.
