In telecommunication, a line code 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). After line coding, the signal can directly be put on a transmission line, in the form of variations of the (electrical) current.
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 therefor at the receiving end lead to an offset, and thus to a higher decoding error probability.
Secondly, line coding should make possible that the receiver can synchronise itself on (the phase of) the received signal. If the synchronisation is not ideal, then the actual decoding is not performed with signal values at the instances where in the received signal the differences (in amplitude) between the symbols or digits used in the line code are optimal. And this will increase the error probability.
Thirdly it is preferred if the line code has a structure that will enable error detection.
Note 1: A line code may differ from the code generated at a user terminal, and thus may require translation.
Note 2: A line code may, for example, reflect a requirement of the transmission medium, e.g. , optical fiber versus shielded twisted pair.
known line codes
Also see:
To be deleted 'Source: from Federal Standard 1037C and from MIL-STD-188'