Digital Signal Processing Reference
In-Depth Information
4
Analog to Digital and Digital
to Analog C
onversion
4.1
Brief Theory of A/D Conversion
Digital communications has proved to be a very efficient means of trans-
porting speech, music, video, and data over different kinds of media. These
media include satellite, microwave, fiber-optic, coaxial, and cellular chan-
nels. One special advantage that digital communication holds over analog
communication is in the superior handling of noise in the channel.
1
Baseband signals such as speech, music, and video are naturally occurring
analog signals. Hence, the processes of analog to digital (A/D) conversion
at the transmitter and digital to analog conversion (D/A) at the receiver are
integral sections of the entire communication system shown earlier in
Figure
1.2
. We will now discuss
pulse code modulation
(
PCM
), which is one of the
basic forms of A/D systems.
4.1.1
Pulse Code Modulation
Pulse code modulation was one of the earliest methods of A/D conversion.
2
The PCM process, as shown in
Figure 4.1
, converts an analog continuous-
time signal, such as speech or music, into a digital binary bit stream. The
three fundamental steps in the PCM process are time sampling, amplitude
quantization, and binary encoding.
4.1.1.1
Time Sampling
The first step in the PCM process is
time
sampling
, where the continuous-
time signal
x
(
t
), as shown in
Figure 4.2a
, is sampled uniformly at an interval
of
T
seconds. The output of the sampling process is the discrete-time signal
x
(
nT
) or
x
(
n
)
; n =
0, 1, 2
… N -
1, as shown in Figure 4.2b.
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