Line code: Difference between revisions

Revision as of 23:29, 7 December 2006

Revision as of 20:25, 9 December 2006

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In [[telecommunication]], a '''line code''' (also called '''digital baseband modulation''') is a [[code]] chosen for use within a [[communications system]] for [[transmission (telecommunications)|transmission]] purposes.

For digital data transport '''line coding''' is often used. Line coding consists of representing the [[digital signal]] to be transported, by an amplitude- and time-discrete signal, that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The [[waveform]] pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called '''''line encoding'''''.

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]].

After line coding, the signal is put through a "physical channel" or "medium". Sometimes the characteristics of 2 very different-seeming channels are similar enough that the same line code is used for them. 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") is further [[modulation|modulated]] 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 [[barcode]].

* the line coded signal can be converted to magnetic fields on a [[hard drive]] or [[tape drive]].

* the line coded signal can be converted to spots on [[optical disc]].

Unfortunately, most long-distance communication channels cannot transport a DC component.

The DC component is also called the disparity, the bias, the [[DC coefficient]].

The simplest possible line code, [[Non-return-to-zeroUnipolar encoding|NRZunipolar]], 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: