Prof. Madhumita Ta

mhane

DIGITAL DATA – DIGITAL SIGNAL

LINE CODES

Prof. Madhumita Ta

mhane

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.

Prof. Madhumita Ta

mhane

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,

Prof. Madhumita Ta

mhaneNon-return to zero-level (NRZ-L)

!!!!!

• 0 = no signal • 1 = signal on • The reverse is also in use.

Prof. Madhumita Ta

mhaneNon-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.

Prof. Madhumita Ta

mhaneNon-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

Prof. Madhumita Ta

mhaneNon-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.

Prof. Madhumita Ta

mhaneBipolar-Alternate Mark Inversion (AMI)!!!!!

• Uses 3 signal levels: +V, 0, -V • 0 = No line signal. • 1 = alternating +V and –V on every 1. !

Prof. Madhumita Ta

mhaneBipolar-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.

Prof. Madhumita Ta

mhanePseudo 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.

Prof. Madhumita Ta

mhaneManchester 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.

Prof. Madhumita Ta

mhaneManchester 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.

Prof. Madhumita Ta

mhaneDifferential 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.

Prof. Madhumita Ta

mhane

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.

Prof. Madhumita Ta

mhane

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 -