Line code

From Wikipedia, the free encyclopedia

An example of coding a binary signal using rectangular pulse amplitude modulation with polar non-return-to-zero code

An example of Bipolar encoding, or AMI.

Encoding of 11011000100 in Manchester encoding

An example of Differential Manchester encoding

An example of Biphase Mark Code

An example of MLT-3 encoding.

In telecommunication, a line code (also called digital baseband modulation) is a code chosen for use within a communications system for 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. The common types of line encoding are unipolar, polar, 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 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, unipolar, 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, AMI, 8B10B, 4B3T, etc.

Line coding should make it possible for the receiver to synchronise itself to the 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.

It is also preferred for the line code to have a structure that will enable error detection.

Note that the line coded signal and a signal produced at a terminal may differ, thus requiring translation.

A line code will typically reflect technical requirements of the transmission medium, such as optical fiber or shielded twisted pair. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and loss of amplitude.