Digital signal encoding techniques_Digital-3.pptx

Digital signal encoding techniques

1. Line Coding Contents: Waveform representation of binary digits PCM waveform type Why so many PCM waveform? Selection of PCM waveforms

2. Waveform representation of binary digits

3. Waveform types On-off / Unipolar waveform Polar waveform Bipolar waveform Polar Bipolar Unipolar V/2 -V/2

4. Merits and Demerits of Different waveforms The on-off pulse is attractive from the point of view of simplicity of terminal apparatus It has several disadvantages For a given transmitted power it is less immune to noise than the polar scheme It has a non zero PSD at DC so ac coupling is required during transmission Transmission bandwidth is excessive On-off signaling has no error detection or correction capability It is not transparent, i.e. long string of zero creates problem in timing extraction

5. Merits and Demerits of Different waveforms Advantages of polar signaling Polar signaling is more efficient than on-off signaling For a given transmitted power polar signaling is more efficient It is transparent Disadvantages No discrete clock frequency component in a polar signaling

6. Merits and Demerits of Different waveforms Advantages of bipolar signaling Its spectrum has a DC null Its bandwidth is not so excessive It has single error detection capability It has discrete component of clock frequency when it is rectified Disadvantages A bipolar signaling requires twice as much power as that required for a polar signaling It is not transparent i.e. long string of zero creates problem in timing extraction

7. PCM Waveform Type/Line Coding NRZ Phase Encoded NRZ-L Dicode RZ Dicode NRZ RZ Multilevel Binary NRZ-M NRZ-S Unipolar RZ Bipolar RZ RZ-AMI Bi-  -L Bi-  -M Bi-  -S DM Line Coding

8. Various PCM Waveform Changes level from ‘1’ ’0’ or ‘0’ ’1’ ‘1’ change in level ’0’ no change in level Differential coding ‘1’ no change in level ’0’ change in level ‘1’ half period wide pulse ’0’ absence of pulse ‘1’ one-half-bit wide + ve pulse ’0’ one-half-bit wide - ve pulse ‘1’ equal magnitude alternating pulses, ’0’ absence of pulses

9. Various PCM Waveform ‘1’ half-bit-wide pulse positioned at the first half of the bit interval ‘0’  positioned at opposite side Manchester coding Transition at the beginning ‘1’  no second transition ‘0’ second transition one- half bit interval later Transition at the beginning ‘1’ second transition one- half bit interval later ‘0’  no second transition ‘1’  Transition at the mid point of the bit interval 0’  no transition unless it follows by another zero Miller coding ‘1’  ‘0’ or ‘ 0’ ’1’ data transition changes the pulse polarity, without data transition the ‘0’ level is sent Duobinary

10. Why so many PCM waveform DC component null Self clocking Error detection Bandwidth Compression Differential encoding Noise immunity

11. DC component null

12. Self Clocking

13. Error detection Duobinary

14. Bandwidth Compression

15. Differential encoding Differential coding

16. Noise immunity Threshold level Threshold level

17. High Density Bipolar (HDB) Signaling The problem of nontransparancy in bipolar signaling is eliminated by adding pulses when no. of consecutive ‘0’s exceeds n. Such a modified coding is designated as high density bipolar coding, HDBn , where n can take on any value 1, 2, 3 ….and so on. The most important of the HDB codes is HDB3. The basic idea is: When no. of binary “ 0s ” are more than n n+1 “ 0s ” are replaced by one of the special sequences (in case of n=3) “ 000V ” or “ 100V ”, depending on no. of “ 1 ” before the n+1 0s When no. of “ 1 ” is odd, the sequence is “ 000V ” and when the no. of “ 1 ” is even, the sequence is “ 100V ” The “ V ” bit is encoded by a pulse of such a polarity as to violate the bipolar rule “ 1 ” bit in “ 100V ” is encoded by a pulse of polarity following the bipolar rule

18. High Density Bipolar (HDB) Signaling Input Digits: 1 1 0 1 1 1 0 0 0 0 1 0 1 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 1 1 0 1 0 1 0 0 0 0 1 Coded Digits: 1 1 0 1 1 1 0 0 0 V 1 0 1 1 0 1 1 0 0 V 1 0 0 V 0 0 1 0 1 1 0 1 0 1 0 0 0 V 1 Transmitted Waveforms Due to bipolar violation, HDB signaling retains error detection capability DC null is obtained as bipolar signaling

19. Digital Modulation Contents: Digital Modulation Techniques Bandwidth requirements Transmitter and Receiver Operations