Line code: Difference between revisions

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m Common line codes: correct link
more about "DC balance" -- did I ramble on too long?
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After line coding, the signal can directly be put on a transmission line, in the form of variations of the current. The common types of line encoding are [[Unipolar encoding|unipolar]], [[Polar encoding|polar]], [[Bipolar encoding|bipolar]] and [[Manchester encoding]].
 
Unfortunately, most long-distance communication channels cannot transport a DC component.
A line coded signal must not contain a DC-component (the average signal level should be 0), because transport of a DC-component is impossible in case of long wires. An included DC-component would therefore lead to an offset at the receiving end, and thus to a higher decoding error probability.
The DC component is also called the disparity, the bias, the [[DC coefficient]].
The simplest possible line code, [[Non-return-to-zero|NRZ]], because it has unbounded DC component, gives too many errors on such systems.
Most line codes eliminate the DC component -- such codes are called "DC balanced", zero-DC, zero-bias, "DC equalized", etc.
There are 2 ways of eliminating the DC component:
* design each transmitted code such that every code that contain some positive or negative levels, also contains enough of the opposite levels, such that the average level over each code is zero. For example, [[Manchester code]] and [[Interleaved 2 of 5]].
* Use a [[paired disparity code]]. In other words, design the reciever such that every code that averages to a negative level, is paired with another code that averages to a positive level, and either code of the pair decodes to the same bits. Design the transmitter to keep track of the running DC buildup, and always pick the code that pushes the DC level back towards zero. For example, [[Alternate Mark Inversion|AMI]], [[8B10B]], [[4B3T]], etc.
 
Line coding should make it possible for the receiver to synchronise itself to the [[Phase (waves)|phase]] of the received signal. If the synchronisation is not ideal, then the signal to be decoded will not have optimal differences (in amplitude) between the various digits or symbols used in the line code. This will increase the error probability in the received data.