In
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...
, a line code is a pattern of voltage, current, or photons used to represent digital data
transmitted down a
communication channel
A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for inform ...
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
Bipolar may refer to:
Astronomy
* Bipolar nebula, a distinctive nebular formation
* Bipolar outflow, two continuous flows of gas from the poles of a star
Mathematics
* Bipolar coordinates, a two-dimensional orthogonal coordinate system
* Bipolar ...
, and
Manchester code
In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no DC ...
.
Transmission and storage
After line coding, the signal is put through a physical communication channel, either a
transmission medium or
data storage medium.
[Karl Paulsen]
"Coding for Magnetic Storage Mediums"
.2007. The most common physical channels are:
* the line-coded signal can directly be put on a
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
, in the form of variations of the voltage or current (often using
differential signaling
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conduc ...
).
* the line-coded signal (the ''
baseband
In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into ...
signal'') undergoes further
pulse shaping In electronics and telecommunications, pulse shaping is the process of changing the waveform of transmitted pulses to optimize the signal for its intended purpose or the communication channel. This is often done by limiting the bandwidth of the tra ...
(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
In electronics, a remote control (also known as a remote or clicker) is an electronic device used to operate another device from a distance, usually wirelessly. In consumer electronics, a remote control can be used to operate devices such as ...
.
* 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
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...
or
tape drive
A tape drive is a data storage device that reads and writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and a long archival stability.
...
.
* the line-coded signal can be converted to pits on an
optical disc
In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of pits and lands on a special material, often aluminum, on one of its flat surfaces. ...
.
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
Disparity
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. The ''running disparity'' is the
running total {{Unreferenced, date=June 2019, bot=noref (GreenC bot)
A running total or rolling total is the summation of a sequence of numbers which is updated each time a new number is added to the sequence, by adding the value of the new number to the previous ...
of the disparity of all previously transmitted bits. The simplest possible line code,
unipolar, gives too many errors on such systems, because it has an unbounded DC component.
Most line codes eliminate the DC component such codes are called
DC-balanced, zero-DC, or DC-free. There are three ways of eliminating the DC component:
* Use a
constant-weight code. Each transmitted
code word
In communication, a code word is an element of a standardized code or Communications protocol, protocol. Each code word is assembled in accordance with the specific rules of the code and assigned a unique meaning. Code words are typically used for ...
in a constant-weight code is designed such that every code word that contains some positive or negative levels also contains enough of the opposite levels, such that the average level over each code word is zero. Examples of constant-weight codes include
Manchester code
In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no DC ...
and
Interleaved 2 of 5.
* Use a
paired disparity code In telecommunication, a paired disparity code is a line code in which at least one of the data characters is represented by two codewords of opposite disparity that are used in sequence so as to minimize the total disparity of a longer sequence of ...
. Each code word in a paired disparity code that averages to a negative level is paired with another code word that averages to a positive level. The transmitter keeps track of the running DC buildup, and picks the code word that pushes the DC level back towards zero. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples of paired disparity codes include
alternate mark inversion,
8b/10b and
4B3T.
* Use a
scrambler. For example, the scrambler specified in for
64b/66b encoding
In data networking and transmission, 64b/66b is a line code that transforms 64-bit data to 66-bit line code to provide enough state changes to allow reasonable clock recovery and alignment of the data stream at the receiver. It was defined by ...
.
Polarity
Bipolar line codes have two polarities, are generally implemented as RZ, and have a radix of three since there are three distinct output levels (negative, positive and zero). One of the principle advantages of this type of code is that it can eliminate any DC component. This is important if the signal must pass through a transformer or a long transmission line.
Unfortunately, several long-distance communication channels have polarity ambiguity. Polarity-insensitive line codes compensate in these channels.
There are three ways of providing unambiguous reception of 0 and 1 bits over such channels:
* Pair each code word with the polarity-inverse of that code word. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples include
alternate mark inversion,
Differential Manchester encoding
Differential Manchester encoding (DM) is a line code in digital frequency modulation in which data and clock signals are combined to form a single two-level self-synchronizing data stream. In various specific applications, this method is also call ...
,
coded mark inversion and
Miller encoding.
*
differential coding In digital communications, differential coding is a technique used to provide ''unambiguous'' signal reception when using some types of modulation. It makes data to be transmitted to depend not only on the current signal state (or symbol), but also ...
each symbol relative to the previous symbol. Examples include
MLT-3 encoding and
NRZI.
* Invert the whole stream when inverted
syncwords are detected, perhaps using
polarity switching
Run-length limited codes
For reliable
clock recovery at the receiver, a
run-length limitation may be imposed on the generated channel sequence, i.e., the maximum number of consecutive ones or zeros is bounded to a reasonable number. A clock period is recovered by observing transitions in the received sequence, so that a maximum run length guarantees sufficient transitions to assure clock recovery quality.
RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m''/''n'', refer to the rate of the code, while the remaining two specify the minimal ''d'' and maximal ''k'' number of zeroes between consecutive ones. This is used in both
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...
and storage systems that move a medium past a fixed
recording head.
Specifically, RLL bounds the length of stretches (runs) of repeated bits during which the signal does not change. If the runs are too long, clock recovery is difficult; if they are too short, the high frequencies might be attenuated by the communications channel. By
modulating
In music, modulation is the change from one tonality ( tonic, or tonal center) to another. This may or may not be accompanied by a change in key signature (a key change). Modulations articulate or create the structure or form of many pieces, as ...
the
data
In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpret ...
, RLL reduces the timing uncertainty in decoding the stored data, which would lead to the possible erroneous insertion or removal of bits when reading the data back. This mechanism ensures that the boundaries between bits can always be accurately found (preventing
bit slip
In digital transmission, bit slip is the loss or gain of a bit or bits, caused by clock drift – variations in the respective clock rates of the transmitting and receiving devices.
One cause of bit slippage is overflow of a receive buffer that ...
), while efficiently using the media to reliably store the maximal amount of data in a given space.
Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, followed by RLL (1,3) MFM code which were widely used in
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
s until the mid-1980s and are still used in digital optical discs such as
CD,
DVD,
MD,
Hi-MD and
Blu-ray
The Blu-ray Disc (BD), often known simply as Blu-ray, is a digital optical disc data storage format. It was invented and developed in 2005 and released on June 20, 2006 worldwide. It is designed to supersede the DVD format, and capable of s ...
using
EFM and
EFMPLus codes. Higher density RLL (2,7) and RLL (1,7) codes became the
de facto standards for hard disks by the early 1990s.
Synchronization
Line coding should make it possible for the receiver to synchronize itself to the
phase of the received signal. If the clock recovery is not ideal, then the signal to be decoded will not be sampled at the optimal times. This will increase the probability of error in the received data.
Biphase line codes require at least one transition per bit time. This makes it easier to synchronize the transceivers and detect errors, however, the baud rate is greater than that of NRZ codes.
Other considerations
A line code will typically reflect technical requirements of the transmission medium, such as
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
or
shielded twisted pair. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and attenuation.
Common line codes
*
2B1Q
*
4B3T
*
4B5B
*
6b/8b encoding
*
8b/10b encoding
In telecommunications, 8b/10b is a line code that maps 8-bit words to 10-bit symbols to achieve DC balance and bounded disparity, and at the same time provide enough state changes to allow reasonable clock recovery. This means that the dif ...
*
64b/66b encoding
In data networking and transmission, 64b/66b is a line code that transforms 64-bit data to 66-bit line code to provide enough state changes to allow reasonable clock recovery and alignment of the data stream at the receiver. It was defined by ...
*
128b/130b encoding
*
Alternate mark inversion (AMI)
*
Coded mark inversion (CMI)
*
EFMPlus, used in
DVDs
*
Eight-to-fourteen modulation (EFM), used in
compact disc
The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...
s
*
Hamming code
In computer science and telecommunication, Hamming codes are a family of linear error-correcting codes. Hamming codes can detect one-bit and two-bit errors, or correct one-bit errors without detection of uncorrected errors. By contrast, the s ...
*
Hybrid ternary code
*
Manchester code
In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no DC ...
and
differential Manchester
Differential Manchester encoding (DM) is a line code in digital frequency modulation in which data and clock signals are combined to form a single two-level self-synchronizing data stream. In various specific applications, this method is also cal ...
*
Mark and space
Mark and space are terms used in telecommunications to describe two different signal states of a communications signal, generally at the physical layer of a communications system. The terms derive from the early days of the electric telegraph sys ...
*
MLT-3 encoding
*
Modified AMI code
Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, u ...
s: B8ZS, B6ZS, B3ZS, HDB3
*
Modified frequency modulation, Miller encoding and delay encoding
*
Non-return-to-zero (NRZ)
*
Non-return-to-zero, inverted (NRZI)
*
Pulse-position modulation
*
Return-to-zero (RZ)
*
TC-PAM
Optical line codes
*
Alternate-Phase Return-to-Zero (APRZ)
*
Carrier-Suppressed Return-to-Zero (CSRZ)
*
Three of Six, Fiber Optical (TS-FO)
See also
*
Physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
Th ...
*
Self-synchronizing code and bit synchronization
References
*
External links
Line Coding Lecture No. 9
Line Coding in Digital CommunicationCodSim 2.0: Open source simulator for Digital Data Communications Model at the University of Malaga written in HTML
{{Bit-encoding
*
Physical layer protocols
Coding theory