A signal

may be defined as
causal when

for all ``negative-time'' samples (
e.g., for

when

is even). Thus, the signal
![$ x=[1,2,3,0,0]\in{\bf R}^5$](http://www.dsprelated.com/josimages_new/mdft/img1239.png)
is causal while
![$ x=[1,2,3,4,0]$](http://www.dsprelated.com/josimages_new/mdft/img1240.png)
is not.
For
causal signals,
zero-padding is equivalent to simply
appending zeros to the original signal. For example,

Therefore, when we simply append zeros to the end of signal, we call it
causal zero padding.
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