Digital Signal Processing Reference
In-Depth Information
distortion known as granular noise) occurs. The solution for these two problems is
to use a DM with adaptive step-size.
3.6.3.1 Digital DM System
The discrete version of the DM system operates in a similar fashion to the analog
one, and is depicted in Fig.
3.34
.
3.6.3.2 Sigma-Delta Modulation
An sigma-delta modulator (SDM) is a 1-bit processing system which is based on
the DM, but which has greater resilience to slope overload and granular noise.
Recall that slope overload occurs when successive samples are not sufficiently
correlated. Now low frequency signals tend to have low correlation between
samples, but high frequency ones do not. To reduce problems with the processing
of high frequency signals, one can simply place an integrator in front of the DM.
The integrator tends to attenuate high frequency components, so that they are more
amenable to coding with a single bit. A block diagram for the Sigma DM (SDM) is
shown in Fig.
3.35
.
Clock (
f
s
)
Integrator
e
(
n
)
1
∼
x
(
t
)
Δ
Σ
x
(
t
)
y
(
n
)
LPF
−1
Quantizer
−1
x
(
n
)
Δ
^
x
(
n
)
[ DM Decoder ]
z
−1
[ DM Encoder ]
Integrator
Fig. 3.34
First-order digital delta modulator and demodulator
Clock (
f
s
)
e
(
t
)
1
^
x
(
t
)
∫
x
(
t
)
y
(
t
)
LPF
−1
−1
Quantizer
[ SDM Decoder ]
[ SDM Encoder ]
Fig. 3.35
Sigma-delta modulation system
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