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4-1 DIGITAL-TO-DIGITAL CONVERSION
In this section, we see how we can represent digital
data by using digital signals. The conversion involves
three techniques: line coding, block coding, and
scrambling. Line coding is always needed; block
coding and scrambling may or may not be needed.
Topics discussed in this section:
Line Coding
Line Coding characteristics
Line Coding Schemes or Methods
4.1
LINE CODING
Line coding and decoding
Line Coding characteristics
•Signal element Vs data element
•Pulse rate Vs bit rate
•lack of synchronization
•DC Component
4.3
Figure 4.2 Signal element versus data element
4.4
Example 1
A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate
and bit rate as follows:
-3
Pulse Rate = 1/ 10 = 1000 pulses/s
Bit Rate = Pulse Rate x log2 L = 1000 x log2 2 = 1000 bps
Synchronous
Note
In synchronous transmission, we send
bits one after another without start or
stop bits or gaps. It is the responsibility
of the receiver to group the bits.
4.6
Figure 4.35 Synchronous transmission
In synchronous transmission, we send bits one after another
without start or stop bits or gaps.
It is the responsibility of the receiver to group the bits.
4.7
Figure 4.3 Effect of lack of synchronization
4.8
Figure 4.4 Line coding schemes
4.9
Unipolar NZ
•Pulse polarity is used to indicate either it is +ve or –ve
•uses only one polarity.so it is called as unipolar.
•it has 2 states 1 and 0.
•0-is used to indicate the zero voltage.
4.10
Figure 4.5 Unipolar NZ scheme
1 - indicate the +ve Edge
th
0 - indicate the 0 Edge
Dadvantage of Unipolar NZ scheme
•lack of synchronization
•DC Component
4.11
Polar schemes
RZ
NRZ-L
Polar
NRZ
NRZ - I
Manchester
BiPhase
Differential
Manchester
4.12
Figure 4.7 Polar RZ scheme
RZ
0 Indicate -ve Edge to 0th Edge
1 indicate +ve Edge to 0th Edge
4.13
Polar encoding uses two voltage levels (positive and
negative).
NRZ-L
NRZ-I
0
Indicate
+ve Edge
1 indicate –ve Edge
If next bit is 1 there is need of change in
Pulse Rate alternatively.
Figure 4.6 Polar NRZ-L and NRZ-I schemes
Polar NRZ-L and NRZ-I schemes
In NRZ-L the level of the voltage determines the value of the bit.
In NRZ-I the inversion or the lack of inversion determines the
value of the bit.
NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.
NRZ-L and NRZ-I both have a DC component problem.
4.15
Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
4.16
Manchester and differential Manchester
In Manchester and differential Manchester encoding, the
transition at the middle of the bit is used for
synchronization
The minimum bandwidth of Manchester and differential
Manchester is 2 times that of NRZ.
In bipolar encoding, we use three levels: positive, zero,
and negative.
4.17
Figure 4.9 Bipolar schemes: AMI and pseudoternary
AMI
0 - always Indicate 0th Edge
1 – Whenever the occurrence of 1’s refers to the
alternatively.
+ve Edge and -ve Edge
PSEUDOTERNARY - Alternatively +ve , Zero , -ve for all (0 and 1’s)
4.18
Note
In mBnL schemes, a pattern of m data
elements is encoded as a pattern of n
signal elements in which 2m ≤ Ln.
4.19
Figure 4.10 Multilevel: 2B1Q scheme
4.20
Figure 4.11 Multilevel: 8B6T scheme
4.21
Figure 4.13 Multitransition: MLT-3 scheme
4.22
Table 4.1 Summary of line coding schemes
4.23