B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

300 SAMPLING AND ANALOG-TO-DIGITAL CONVERSION
Figure 6.33
(a) Conventional
delta modulator.
(b) I:-
modulator.
(c) Simpler I:-
modulator.
Comparator
Integrator
Sampler
& pulse
generator
Integrator
n n n n
_JLJ LJ U I
L2J
Receiver
Transmitter
(a)
Integrator
Comparator
Sampler
& pulse
generator
_J7J7f7
. .
Integrator
Receiver
Transmitter
(b)
m(t)
+
-----I
Integrator
Comparator
Sampler
& pulse
generator
_Jlflf7
--
Receiver
Transmitter
(c)
Adaptive Delta Modulation (ADM)
The DM discussed so far suffers from one serious disadvantage. The dynamic range of amplitudes
is too small because of the threshold and overload effects discussed earlier. To address
this problem, some type of signal compression is necessary. In DM, a suitable method appears
to be the adaptation of the step value E according to the level of the input signal derivative.
For example, in Fig. 6.31, when the signal m(t) is falling rapidly, slope overload occurs. If we
can increase the step size during this period, the overload could be avoided. On the other hand,
if the slope of m(t) is small, a reduction of step size will reduce the threshold level as well as
the granular noise. The slope overload causes d q
[k] to have several pulses of the same polarity
in succession. This calls for increased step size. Similarly, pulses in d q
[k l alternating continuously
in polarity indicates small-amplitude variations, requiring a reduction in step size. In
ADM we detect such pulse patterns and automatically adjust the step size. 14 This results in a
much larger dynamic range for DM.
6.8 VOCODERS AND VIDEO COMPRESSION
PCM, DPCM, ADPCM, DM, and l;-D.M are all examples of what are known as waveform
source encoders. Basically, waveform encoders do not take into consideration how the signals
for digitization are generated. Hence, the amount of compression achievable by waveform
encoders is highly limited by the degree of correlation between successive signal samples.