Digital Signal Processing Reference
In-Depth Information
Magnitude of zero-order hold frequency response
1.2
1
ideal lowpass
filter
0.8
zero-order
hold
0.6
0.4
0.2
0
3
2.5
2
1.5
0
Frequency (multiples of sampling frequency)
1
0.5
0.5
1
1.5
2
2.5
3
Figure 6.30
Magnitude frequency response of a zero-order hold.
Ideal
sampler
Zero-order
hold
X
()
X
s
()
X
p
(
)
e
j T
1
Figure 6.31
Transform models for a
sampler and data hold.
j
T
x(nT
S
), n = 0, ;1, ;2, ;3,
The signal can be reconstructed as shown in
Figure 6.27(b). The device that reconstructs sampled signals in this manner is called a
zero-order hold. Other types of data holds can be constructed [2] and are equally valid
for the modeling process. The only change in Figure 6.31 is in the transfer function for
the data hold. Further discussion of these data holds is beyond the scope of this topic.
Á .
6.5
SINUSOIDAL AMPLITUDE MODULATION
Modulation is the process used to shift the frequency of an information signal so
that the resulting signal is in the desired frequency band. There are several reasons
why it is important to do so. One reason is that the human voice is dominated by fre-
quency components of less than 1 kHz. If we were to attempt to transmit a human
