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Line coding: polar RZ and bipolar AMI

Short answer

Line coding converts bits to digital signals. Polar RZ: 1 = +V for half-bit then 0, 0 = -V then 0; good sync but wide bandwidth. Bipolar AMI: 0 = 0V, 1s alternate +V/-V; zero DC, error detection, but sync lost on long zeros. For 1011001: Polar RZ: +0-0+0+0-0-0+0; AMI: +0-+00-+.

Define line coding. Explain polar RZ and bipolar AMI line coding schemes with example and compare them.

Line coding is the process of converting a sequence of binary data (bits) into a digital signal (voltage levels) that can be sent over a medium.

Polar RZ (Return to Zero)

Uses three voltage levels. A 1 is +V and a 0 is -V for the first half of the bit; the signal then returns to 0 V for the second half. Every bit therefore has a transition, which gives self-synchronisation. It needs twice the bandwidth of NRZ.

Bipolar AMI (Alternate Mark Inversion)

A 0 is 0 V. The 1s (marks) are represented by pulses of alternating polarity (+V, -V, +V, ...). Alternating polarity gives zero DC component and a simple error check (two successive pulses of the same sign is a violation). A long string of 0s gives no transitions, so synchronisation can be lost.

Example: 1 0 1 1 0 0 1

Polar RZ
      1   0   1   1   0   0   1  
+V   --|     |-| |-|         |-| 
 0     |-| |-| |-| |-| |-| |-| |-
-V       |-|         |-| |-|

Bipolar AMI
      1   0   1   1   0   0   1  
+V   ----|       |---|           
 0       |---|   |   |-------|   
-V           |---|           |---

Comparison

PointPolar RZBipolar AMI
Levels+V, -V, 0+V, -V, 0
Sending a 1+V then 0+V and -V alternately
Sending a 0-V then 00 V
SynchronisationGood for both 0 and 1Good for 1s, poor for long 0s
DC componentNear zero if 0s and 1s balanceZero
BandwidthWide (2 × NRZ)Narrower than RZ
Error detectionNoYes (violation)

✓ Answer checked by Claude Sonnet 5.5

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