1. 4.1 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. Line Coding Line Coding characteristics Line Coding Schemes or Methods Topics discussed in this section:

2. LINE CODING Line coding and decoding

3. 4.3 Signal element Vs data element Pulse rate Vs bit rate lack of synchronization DC Component Line Coding characteristics

4. 4.4 Figure 4.2 Signal element versus data element

5. 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: Pulse Rate = 1/ 10 -3 = 1000 pulses/s Bit Rate = Pulse Rate x log 2 L = 1000 x log 2 2 = 1000 bps

6. 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

7. 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.

8. 4.8 Figure 4.3 Effect of lack of synchronization

9. 4.9 Figure 4.4 Line coding schemes

10. 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.

11. 4.11 Figure 4.5 Unipolar NZ scheme 1 - indicate the +ve Edge 0 - indicate the 0 th Edge Dadvantage of Unipolar NZ scheme lack of synchronization DC Component

12. 4.12 Polar RZ NRZ BiPhase NRZ-L NRZ - I Manchester Differential Manchester Polar schemes

13. 4.13 Figure 4.7 Polar RZ scheme RZ 0 Indicate -ve Edge to 0 th Edge 0 Indicate -ve Edge to 0 th Edge 1 indicate +ve Edge to 0 th Edge 1 indicate +ve Edge to 0 th Edge

14. Polar encoding uses two voltage levels (positive and negative). Figure 4.6 Polar NRZ-L and NRZ-I schemes NRZ-L 0 Indicate +ve Edge 0 Indicate +ve Edge 1 indicate –ve Edge 1 indicate –ve EdgeNRZ-I If next bit is 1 there is need of change in Pulse Rate alternatively. If next bit is 1 there is need of change in Pulse Rate alternatively.

15. 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

16. 4.16 Figure 4.8 Polar biphase: Manchester and differential Manchester schemes

17. 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

18. 4.18 Figure 4.9 Bipolar schemes: AMI and pseudoternary AMI 0 - always Indicate 0 th Edge 0 - always Indicate 0 th Edge 1 – Whenever the occurrence of 1’s refers to the +ve Edge and -ve Edge alternatively. 1 – Whenever the occurrence of 1’s refers to the +ve Edge and -ve Edge alternatively. PSEUDOTERNARY - Alternatively +ve, Zero, -ve for all (0 and 1’s)

19. 4.19 In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2 m ≤ L n. Note

20. 4.20 Figure 4.10 Multilevel: 2B1Q scheme

21. 4.21 Figure 4.11 Multilevel: 8B6T scheme

22. 4.22 Figure 4.13 Multitransition: MLT-3 scheme

23. 4.23 Table 4.1 Summary of line coding schemes