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Concept
Line code
Summary
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained code in data storage systems.
Some signals are more prone to error than others as the physics of the communication channel or storage medium constrains the repertoire of signals that can be used reliably.
Common line encodings are unipolar, polar, bipolar, and Manchester code.
After line coding, the signal is put through a physical communication 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 (often using differential signaling).
the line-coded signal (the baseband signal) undergoes further pulse shaping (to reduce its frequency bandwidth) and then is modulated (to shift its frequency) to create an RF signal that can be sent through free space.
the line-coded signal can be used to turn on and off a light source in free-space optical communication, most commonly used in an 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 magnetized spots on a hard drive or tape drive.
the line-coded signal can be converted to pits on an optical disc.
Some of the more common binary line codes include:
Each line code has advantages and disadvantages. Line codes are chosen to meet one or more of the following criteria:
Minimize transmission hardware
Facilitate synchronization
Ease error detection and correction
Achieve a target spectral density
Eliminate a DC component
Most long-distance communication channels cannot reliably transport a DC component. The DC component is also called the disparity, the bias, or the DC coefficient. The disparity of a bit pattern is the difference in the number of one bits vs the number of zero bits.
Official source
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