27-07-2011, 09:48 AM
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Overview
Introduction
Types of Line Codes
Power Spectral Density (PSD) of Line Codes
Comparison of Line Codes
Terminology
RZ Return to Zero
NRZ Non-Return to Zero
AMI Alternate Mark Inversion
PSD Power Spectral Density
The word mark, and its converse space, often appear in a description of a binary waveform. This is an historical reference to the mark and space of the telegraphist. In modern day digital terminology these have become HI and LO, or ‘1’ and ‘0’, as appropriate.
Introduction
In telecommunication, a line code (also called digital baseband signals) is a code chosen for use within a communications system for baseband transmission purposes. Line coding is often used for digital data transport.
The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission page link is called line encoding.
After line coding, the signal is put through a "physical channel", either a "transmission medium" or "data storage medium".
The most common physical channels are:
the line-coded signal can directly be put on a transmission line, in the form of variations of the voltage or current .
the line-coded signal (the "base-band signal") undergoes further pulse shaping and then modulated (to shift its frequency bandwidth) to create the "RF signal" that can be sent through free space.
the line-coded signal can be used to turn on and off a light in Free Space Optics, most commonly infrared remote control.
the line-coded signal can be printed on paper to create a bar code.
the line-coded signal can be converted to a magnetized spots on a hard drive or tape drive.
the line-coded signal can be converted to a pits on optical disc.
Goals of Line Coding
A line code is designed to meet several goals:
Self-synchronization.
The ability to recover timing from the signal itself.
Long series of ones and zeros could cause a problem.
Low probability of bit error.
The receiver needs to be able to distinguish the waveform associated with a mark from the waveform associated with a space, even if there is a considerable amount of noise and distortion in the channel.
Spectrum that is suitable for the channel.
In some cases DC components should be avoided if the channel has a DC blocking capacitance.
The transmission bandwidth should be minimized.
Categories of line codes
Symbol mapping functions
Unipolar
Polar
Bipolar
Pulse shapes .
NRZ (Nonreturn-to-zero)
RZ (Return to Zero)
Manchester (split phase)