4.1

4-1 DIGITAL-TO-DIGITAL CONVERSION4-1 DIGITAL-TO-DIGITAL CONVERSION

In this section, we see how we can represent digital In this section, we see how we can represent digital data by using digital signals. The conversion involves data by using digital signals. The conversion involves three techniques: three techniques: line codingline coding, , block codingblock coding, and , and scramblingscrambling. Line coding is always needed; block . Line coding is always needed; block coding and scrambling may or may not be needed.coding and scrambling may or may not be needed.

Line CodingLine Coding characteristicsLine Coding Schemes or Methods

Topics discussed in this section:Topics discussed in this section:

LINE CODING

Line coding and decoding

4.3

•Signal element Vs data element•Pulse rate Vs bit rate•lack of synchronization•DC Component

Line Coding characteristics

4.4

Figure 4.2 Signal element versus data element

Example 1Example 1

A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate

as follows:

Pulse Rate = 1/ 10Pulse Rate = 1/ 10-3-3

= 1000 pulses/s= 1000 pulses/s

Bit Rate = Pulse Rate x logBit Rate = Pulse Rate x log22 L = 1000 x log L = 1000 x log22 2 = 1000 bps 2 = 1000 bps

4.6

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.

Note

Synchronous

4.7

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.

4.8

Figure 4.3 Effect of lack of synchronization

4.9

Figure 4.4 Line coding schemes

4.10

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.

4.11

Figure 4.5 Unipolar NZ scheme

1 - indicate the +ve Edge

0 - indicate the 0th Edge

Dadvantage of Unipolar NZ scheme

•lack of synchronization•DC Component

4.12

Polar

RZ

NRZ

BiPhase

NRZ-L

NRZ - I

Manchester

Differential Manchester

Polar schemesPolar schemes

4.13

Figure 4.7 Polar RZ scheme

RZ RZ

0 Indicate 0 Indicate -ve Edge to 0-ve Edge to 0th th EdgeEdge

1 indicate +ve Edge to 01 indicate +ve Edge to 0th th EdgeEdge

Polar encoding uses two voltage levels (positive and negative).Polar encoding uses two voltage levels (positive and negative).

Figure 4.6 Polar NRZ-L and NRZ-I schemes

NRZ-L NRZ-L

0 0 IndicateIndicate +ve Edge +ve Edge

1 indicate –ve Edge1 indicate –ve Edge

NRZ-I NRZ-I

If next bit is 1 there is need of change in Pulse Rate If next bit is 1 there is need of change in Pulse Rate

alternatively.alternatively.

4.15

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.

Polar NRZ-L and NRZ-I schemes

4.16

Figure 4.8 Polar biphase: Manchester and differential Manchester schemes

4.17

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.

Manchester and differential Manchester

4.18

Figure 4.9 Bipolar schemes: AMI and pseudoternary

AMI AMI

0 - always Indicate 0 - always Indicate 00th th EdgeEdge

1 – Whenever the occurrence of 1’s refers to the +ve Edge and -ve1 – Whenever the occurrence of 1’s refers to the +ve Edge and -ve Edge Edge

alternatively.alternatively.

PSEUDOTERNARY PSEUDOTERNARY - Alternatively +ve , Zero , -ve for all (0 and 1’s) - Alternatively +ve , Zero , -ve for all (0 and 1’s)

4.19

In mBnL schemes, a pattern of m data elements is encoded as a pattern of n

signal elements in which 2m ≤ Ln.

Note

4.20

Figure 4.10 Multilevel: 2B1Q scheme

4.21

Figure 4.11 Multilevel: 8B6T scheme

4.22

Figure 4.13 Multitransition: MLT-3 scheme

4.23

Table 4.1 Summary of line coding schemes