as graphed in Fig.7.3(c).
In this case, can be viewed as a ``moving three-point average''
filter. Note how the corners of the rectangular pulse are ``smoothed''
by the three-point filter. Also note that the pulse is smeared to the
``right'' (forward in time) because the filter impulse response starts
at time zero. Such a filter is said to be causal (see
 for details). By shifting the impulse response left one
sample to get
(in which case
), we obtain a noncausal, smoother
symmetric about time zero so that the input signal is smoothed ``in
place'' with no added delay (imagine Fig.7.3(c) shifted left
one sample, in which case the input pulse edges align with the
midpoint of the smoother rise and fall in the output).