
130 Chapter 3
Basic Digital Operation
Differential Encoding
For a bit-by-bit illustration of the encoding process, see the following illustration.
How Differential Encoding Works
Differential encoding employs offsets in the symbol table to encode user-defined modulation
schemes. The Differential State Map table editor is used to introduce symbol table offset
values which in turn cause transitions through the I/Q State Map based on their associated
data value. Whenever a data value is modulated, the offset value stored in the Differential
State Map is used to encode the data by transitioning through the I/Q State Map in a
direction and distance defined by the symbol table offset value.
Entering a value of +1 will cause a 1-state forward transition through the I/Q State Map, as
shown in the following illustration.
NOTE The following I/Q State Map illustrations show all of the possible state
transitions using a particular symbol table offset value. The actual
state-to-state transition would depend upon the state in which the modulation
had started.
As an example, consider the following data/symbol table offset values.
Data Offset Value
00000000 +1
00000001
−1
00000010 +2
1
01 00 1 100
1
1
0
raw (unencoded) data
differentially encoded data
11
11
1
1
1
111
0
0
0
change =
no change =
0
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