Databases Reference
In-Depth Information
Slope overload
region
Granular region
F I GU R E 11 . 13
A source output sampled and coded using delta modulation.
Slope overload
region
Granular region
F I GU R E 11 . 14
A source output sampled and coded using adaptive delta modula-
tion.
One way to avoid this impasse is to adapt the step size to the characteristics of the input, as
shown in Figure
11.14
. In quasi-constant regions, make the step size small in order to reduce
the granular error. In regions of rapid change, increase the step size in order to reduce overload
error. There are various ways of adapting the delta modulator to the local characteristics of
the source output. We describe two of the more popular ways here.
11.6.1 Constant Factor Adaptive Delta Modulation
(CFDM)
The objective of adaptive delta modulation is clear: increase the step size in overload regions
and decrease it in granular regions. The problem lies in knowing when the system is in each
of these regions. Looking at Figure 11.13, we see that in the granular region the output of the
quantizer changes sign with almost every input sample; in the overload region, the sign of the
quantizer output is the same for a string of input samples. Therefore, we can define an overload
or granular condition based on whether the output of the quantizer has been changing signs.
A very simple system [
176
] uses a history of one sample to decide whether the system is in
overload or granular condition and whether to expand or contract the step size. If
s
n
denotes
