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Differential Manchester encoding (DM) is a
line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...
in digital frequency modulation in which
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 interpreted ...
and
clock signal In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock sign ...
s are combined to form a single two-level self-
synchronizing Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
data stream. In various specific applications, this method is also called by various other names, including biphase mark code (CC), F2F (frequency/double frequency), Aiken biphase, and conditioned diphase.US DoD: ''Design handbook for fiber optic communications systems, Military handbook.'' Dept. of Defense, 1985, p. 65.


Definition

Differential Manchester encoding is a differential encoding technology, using the presence or absence of transitions to indicate logical value. An improvement to
Manchester coding 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 ...
which is a special case of
binary phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
, it is not necessary to know the initial polarity of the transmitted message signal, because the information is not represented by the absolute voltage levels but by their transitions. Differential Manchester encoding has the following advantages over some other line codes: * A transition is guaranteed at least once every bit, for robust clock recovery. * In a noisy environment, detecting transitions is less error-prone than comparing signal levels against a threshold. * Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. Differential coding schemes will work exactly the same if the signal is inverted (e.g. wires swapped). Other line codes with this property include NRZI,
bipolar encoding In telecommunication, bipolar encoding is a type of return-to-zero (RZ) line code, where two nonzero values are used, so that the three values are +, −, and zero. Such a signal is called a duobinary signal. Standard bipolar encodings are designed ...
,
coded mark inversion frame, CMI line coding In telecommunication, coded mark inversion (CMI) is a non-return-to-zero (NRZ) line code. It encodes ''zero'' bits as a half bit time of zero followed by a half bit time of one, and while ''one'' bits are encoded as a full b ...
, and
MLT-3 encoding MLT-3 encoding (Multi-Level Transmit) is a line code (a signaling method used in a telecommunication system for transmission purposes) that uses three voltage levels. An MLT-3 interface emits less electromagnetic interference and requires less band ...
. * If the high and low signal levels have the same magnitude with opposite polarity, the average voltage around each unconditional transition is zero. Zero
DC bias In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...
reduces the necessary transmitting power, minimizes the amount of electromagnetic noise produced by the transmission line, and eases the use of isolating transformers. These positive features are achieved at the expense of doubling the clock frequency—there are two clock ticks per bit period (marked with full and dotted lines in the figure). At every second clock tick, marked with a dotted line, there is a potential level transition conditional on the data. At the other ticks, the line state changes unconditionally to ease clock recovery. One version of the code makes a transition for 0 and no transition for 1; the other makes a transition for 1 and no transition for 0. Differential Manchester encoding is specified in the IEEE 802.5 standard for Token Ring local area networks, and is used for many other applications, including magnetic and optical storage. As Biphase Mark Code (BMC), it is used in AES3, S/PDIF, SMPTE time code,
USB PD The initial versions of the USB standard specified connectors that were easy to use and that would have acceptable life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of t ...
,
xDSL Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric di ...
and
DALI Dali or Dalí may refer to: Chinese history * Kingdom of Dali (937–1253 AD), centered in modern Yunnan * Kingdom of Nanzhao or Dali, Kingdom of Dali's predecessor state * Dali, Emperor Daizong of Tang's third and last regnal period (766–779) ...
. Many
magnetic stripe The term digital card can refer to a physical item, such as a memory card on a camera, or, increasingly since 2017, to the digital content hosted as a virtual card or cloud card, as a digital virtual representation of a physical card. They share ...
cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase, according to the ISO/IEC 7811 standard. Differential Manchester encoding is also the original modulation method used for single-density
floppy disk A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
s, followed by double-density modified frequency modulation (MFM), or Differential Manchester encoding.


See also

*
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 ...
*
McASP McASP is an acronym for Multichannel Audio Serial Port, a communication peripheral found in Texas Instruments family of digital signal processors ( DSPs) and Microcontroller Units ( MCUs). The McASP functions as a general-purpose audio serial port o ...
* Run-length limited: FM


References


Further reading

* Watkinson, John (1994) ''The Art of Digital Audio'', 2nd edition. Oxford:
Focal Press Focal Press is a publisher of creative and applied media books and it is an imprint of Routledge/Taylor & Francis. Company history The firm was founded in London in 1938 by Andor Kraszna-Krausz, a Hungarian photographer who migrated to England ...
. *
Introduction to magnetic stripe technology
* (https://www.sqa.org.uk/e-learning/NetTechDC01ECD/page_09.htm) Introduction to rudimentary biphase encoding {{Bit-encoding Line codes