The unit sample function, often referred to as the unit impulse or delta function, is the function that
defines the idea of a unit impulse in discrete time. There are not nearly as many intricacies involved in its definition as there are in the definition of the Dirac delta function, the continuous time impulse function. The unit sample function simply takes a value of one at n=0 and a value of zero elsewhere. The impulse function is often written as
δn
δn
.
δn={1 if n=00 otherwise
δ
n
1
n
0
0
(1)
Below we will briefly list a few important properties of
the unit impulse without going into detail of their proofs.
-
δαn=1|α|δn
δ
α
n
1
α
δ
n
-
δn=δ−n
δ
n
δ
n
-
δ
(
n
)
=
u
(
n
)
-
u
(
n
-
1
)
δ
(
n
)
=
u
(
n
)
-
u
(
n
-
1
)
-
f
(
n
)
δ
(
n
)
=
f
(
0
)
δ
(
n
)
f
(
n
)
δ
(
n
)
=
f
(
0
)
δ
(
n
)
The last of these is especially important as it gives rise to the sifting property of the unit sample function, which selects the value of a function at a specific time and is especially important in studying the relationship of an operation called convolution to time domain analysis of linear time invariant systems. The sifting property is shown and derived below.
∑
n
=
-
∞
∞
f
(
n
)
δ
(
n
)
=
∑
n
=
-
∞
∞
f
(
0
)
δ
(
n
)
=
f
(
0
)
∑
n
=
-
∞
∞
δ
(
n
)
=
f
(
0
)
∑
n
=
-
∞
∞
f
(
n
)
δ
(
n
)
=
∑
n
=
-
∞
∞
f
(
0
)
δ
(
n
)
=
f
(
0
)
∑
n
=
-
∞
∞
δ
(
n
)
=
f
(
0
)
(2)
"My introduction to signal processing course at Rice University."