Polar, unipolar, bipolar, Manchester; encode 101000111011

Unipolar coding

Only one polarity is used: a 1 is +V and a 0 is 0 V (NRZ), so the DC component is large and long runs of the same bit give no clock. Simple and cheap, used in short links (inside a computer).

Polar coding

Two voltages of opposite polarity are used: for polar NRZ-L, 1 = +V and 0 = -V. The DC component is lower (zero if 0s and 1s are equal in number). Variants: NRZ-I, RZ, Manchester.

Bipolar coding (AMI)

Three levels: 0 = 0 V and 1s alternate between +V and -V. There is no DC component and violations detect errors. Long strings of 0s lose synchronisation (fixed by B8ZS/HDB3).

Manchester coding

A transition occurs in the middle of every bit: low-to-high for 1 and high-to-low for 0. It is self-clocking and has no DC component, but needs twice the bandwidth. It is used in 10 Mbps Ethernet.

Encoding of 1 0 1 0 0 0 1 1 1 0 1 1

Conventions used (state these in the exam, as books differ): level before the first bit is low (-V); NRZ-L/RZ: 1 = +V, 0 = -V (unipolar: 0 = 0 V); NRZ-I: a 1 toggles the level at the start of the bit, a 0 keeps it; RZ: the pulse returns to zero in the middle of the bit; Manchester: 1 = low-to-high and 0 = high-to-low transition at mid-bit; differential Manchester: always a mid-bit transition, an extra transition at the start of the bit for a 0 and none for a 1; AMI: 0 = 0 V, 1s alternate +V and -V starting with +V.

Unipolar NRZ

      1   0   1   0   0   0   1   1   1   0   1   1  
+V   ----|   |---|           |-----------|   |-------
 0       |---|   |-----------|           |---|

Polar NRZ-L

      1   0   1   0   0   0   1   1   1   0   1   1  
+V   ----|   |---|           |-----------|   |-------
 0       |   |   |           |           |   |       
-V       |---|   |-----------|           |---|

Bipolar AMI

      1   0   1   0   0   0   1   1   1   0   1   1  
+V   ----|                   |---|   |---|       |---
 0       |---|   |-----------|   |   |   |---|   |   
-V           |---|               |---|       |---|

Manchester

      1   0   1   0   0   0   1   1   1   0   1   1  
+V     |---|   |---| |-| |-|   |-| |-| |---|   |-| |-
 0     |   |   |   | | | | |   | | | | |   |   | | | 
-V   --|   |---|   |-| |-| |---| |-| |-|   |---| |-|

AMI pulses: + 0 - 0 0 0 + - + 0 - +.