A digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary data '0' and '1' are transmitted over digital channel by encoding each data bit into signal elements. Encoding scheme is mapping from data bits to signal elements. Line coding is done to prevent DC wandering and loss of synchronisation on long strings of '0' and '1'. It may give some amount of error detection as in AMT.

1. Prof. Madhumita Tamhane
DIGITAL DATA – DIGITAL SIGNAL
LINE CODES

2. Prof. Madhumita Tamhane
2
Line Codes
•Line Coding is the process for converting
digital data into digital signal. .
•Digital data is found in binary format.
•It is represented (stored) internally as series
of 1s and 0s.
•Digital signal is denoted by discreet signal,
which represents digital data.
•There are three types of line encoding
schemes available:
•Unipolar encoding: single voltage level to
represent data.
•Binary 1, high voltage
•Binary 0, no voltage is transmitted.
•E.g. Unipolar-Non-return-to-zero.

3. Prof. Madhumita Tamhane
3
Line Codes
•Polar encoding: Polar encoding scheme uses
multiple voltage levels to represent binary
values.
•E.g. polar NRZ, RZ, Manchester, Differential
Manchester.
•Bipolar encoding: Bipolar encoding scheme
uses three voltage levels to represent binary
values, positive, negative and zero.
•Zero voltage represents binary 0 and bit 1 is
represented by altering positive and negative
voltages.
•E.g. AMI,

4. Prof. Madhumita Tamhane
Non-return to zero-level (NRZ-L)
!
!
!
!
!
• 0 = no signal
• 1 = signal on
• The reverse is also in use.

5. Prof. Madhumita Tamhane
Non-return to zero-level (NRZ-L)
• Advantage:
• Easy to generate.
• Disadvantage:
• DC component development during long strings
of 0 or 1 result in “baseline wander”.
• Loss of synchronization during long strings of 0
or 1, as no transition available.

6. Prof. Madhumita Tamhane
Non-return to zero-Invert on 1 (NRZ-I)
!
!
• 0 = No transition .
• 1 = Transition at beginning of interval
• Can be generated by F/F in toggle mode.
• Can be decoded by comparing adjacent
bits.
EX-OR
Delay T

7. Prof. Madhumita Tamhane
Non-return to zero-Invert on 1 (NRZ-I)
• Advantage:
• No DC component during long strings of 1.
• No loss of synchronization for long strings of 1.
• More reliable to detect a transition in presence of
noise than the level
• Disadvantage:
• Presence of DC component resulting in “baseline
wander” during long strings of 0.
• Loss of synchronization during long strings of 0
as no transition available.

8. Prof. Madhumita Tamhane
Bipolar-Alternate Mark Inversion (AMI)
!
!
!
!
!
• Uses 3 signal levels: +V, 0, -V
• 0 = No line signal.
• 1 = alternating +V and –V on every 1.
!

9. Prof. Madhumita Tamhane
Bipolar-Alternate Mark Inversion (AMI)
• Advantage:
• No DC voltage for long strings of 1or 0.
• No loss of synchronization for long strings of 1.
• Alternate +V and –V on 1 provide simple means
of error detection.
• Disadvantage:
• Loss of synchronization during long strings of 0
as no transition available.

10. Prof. Madhumita Tamhane
Pseudo ternary
!
• Opposite to AMI.
• 0 = Alternating +V and –V on every 0.
• 1 = No line signal.
• Uses 3 signal levels: +V, 0, -V
• Disadvantageous for long strings of 1.

11. Prof. Madhumita Tamhane
Manchester Coding
!
!
!
!
!
• Always transition in middle of bit period.
• 0 = low-to-high transition.
• 1 = high-to-low transition.
• IEEE802.3 for Ethernet follows the opposite.
• 1 = low-to-high transition.
• 0 = high-to-low transition.
• Both have same advantages.

12. Prof. Madhumita Tamhane
Manchester Coding
• Advantage:
• No loss of sync for long strings of 0 or 1.
• Transition in middle of bit period provided
synchronization.
• Called self clocking codes.
• No DC component.
• Error detection if noise hampers transition.
• Disadvantage:
• Bandwidth is doubled.

13. Prof. Madhumita Tamhane
Differential Manchester Coding
!
!
!
!
• Always transition in middle of bit period.
• 0 = transition at beginning of bit.
• 1 = No transition at beginning of bit.
• More reliable to detect a transition in presence of
noise than the level as in Manchester coding.
• Other advantages and disadvantages are same as
Manchester coding.

14. Prof. Madhumita Tamhane
Bipolar with 8 – zero substitution
(B8ZS)!
!
!
!
• Done on AMI to avoid sync loss for long string of 0.
• 8 continuous zeros replaced by new sequence.
• If the immediate preceding pulse is of (-) polarity,
then 8 zeros replaced as 000 - + 0 + - .
• If the immediate preceding pulse is of (z) polarity,
then 8 zeros replaced as 000 + - 0 - + .
• Violation indicates presence of replacement.

15. Prof. Madhumita Tamhane
High Density Bipolar – 3 zeros
( HDB3)
• Done on AMI to avoid sync loss for long string of 0.
• 4 continuous zeros replaced by new sequence.
• Successive violation should be of alternating polarity
for no additional DC.
• Violations should be self balancing.
Preceding pulse No of 1’s since last substitution
ODD EVEN
- ve 0 0 0 - + 0 0 +
+ ve 0 0 0 + - 0 0 -