Line Coding: Polar

Line Coding: Polar 7

Polar Line Coding – uses two non-zero voltage level for represent.
of two data levels - one positive & one negative

• • “DC-problem” alleviated
• • 4 main types of polar coding:

 

in NRZ there are two types :NRZ-level   NRZ-invert
(1) Nonreturn to Zero
(NRZ)

• • NRZ-level: signal level represents particular
• bit, (e.g.) 0 = positive volt. , 1 = negative volt.
•  poor synchronizat. for long series of 1-s & 0-s 
• • NRZ-invert: inversion of voltage level = bit 1,
  no voltage = bit 0
• 1s in data streams enable synchronization
•  long sequence of 0-s still a problem

Line Coding: Unipolar

Line Coding: Unipolar 6
Unipolar Line Coding – uses only one non-zero and one zero
voltage level
• (e.g.) 0 = zero level, 1 = non-zero level
• simple to implement, but obsolete due to
two main problems:

•  DC component present 
•  lack of synchronization for long series of 1-s or

line coging

Line Coding: Design Consideration 3
Line Coding – process of converting binary data (sequence of
bits) to a digital signal
• digital signal depends ‘linearly’ on information bits - bits
are transmitted ‘one-by-one’ - different from block coding


Data vs. Signal Level
• data levels – number of values / levels used
to represent data (typically only two: 0 and 1)
• signal levels – number of values / levels allowed in a particular signal DC Two signal levels, two data levels. Three signal levels, two data level


DC Component
in Line Coding
– some line coding schemes have a residual (DC)
component, which is generally undesirable

 transformers do not allow passage of DC component
DC component ⇒ extra energy – useless!
Self-Synchronization
(Clocking)
– to correctly interpret signal received from
sender receiver’s bit interval must exactly
correspond to sender’s bit intervals
• if receiver clock is faster/slower, bit intervals
not matched ⇒ receiver misinterprets signal
• self-synchronizing digital signals include
timing information in itself, to indicate the
beginning & end of each pulse (see pp. 8-10)