Digital Signal Processing Reference
In-Depth Information
shown in Figure 6.44(c). Note that no bandpass filters are required in the
demodulation of pulse-amplitude-modulated signals. Note also that from Figure 6.43,
each signal must be bandlimited such that
The telephone system in the United States uses time-division multiplexing. As
shown in Figure 6.45, the rate at which each conversation is sampled is 8 kHz. Audio
signals (human conversations) have a spectrum that runs to approximately 20 kHz.
To ensure proper separation of the pulse-modulated signal in the frequency do-
main, these conversations are passed through a bandpass filter with a lower cutoff
frequency of 200 Hz and an upper cutoff frequency of 3.2 kHz. This filtering de-
grades the audio quality of telephone conversations. However, the frequency limi-
tation allows the information content of the voice signal to be used to amplitude
modulate a train of rectangular pulses with a relatively long period. The PAM signal
can then be interleaved in time with PAM signals from several other conversations
for transmission over a single communication circuit.
v
M
6 v
c/2
.
Thus far, we have produced the PAM signal by multiplying a carrier signal made up
of a train of rectangular pulses by an analog message signal. We now study another
process for producing a PAM signal. In this case, we use discrete sampled values of
the message signal to modulate the carrier signal. The result will be an amplitude-
modulated train of rectangular pulses known as
flat-top
PAM. Practically, a
flat-top
Voice
signals
Low-pass
filters
Electronic
switches
x
1
(
t
)
y
1
(
t
)
m
1
(
t
)
H
1
( )
s
1
(
t
)
m
2
(
t
)
x
2
(
t
)
y
2
(
t
)
y
tm
(
t
)
H
2
( )
s
2
(
t
)
x
n
(
t
)
y
n
(
t
)
m
n
(
t
)
H
n
( )
s
n
(
t
)
H
i
( )
1
s
i
(
t
)
6.4
0.4
0.4
6.4
t
i
t
i
125
t
(krad / s)
( s)
(
i
1)
n
t
i
t
a
(125 s)
Figure 6.45
Pulse-amplitude modulation with TDM.
