Differential Manchester encoding

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Differential Manchester encoding Scheme is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It is not necessary to know the polarity of the sent signal since the information is not kept in the actual values of the voltage but in their change: in other words it does not matter whether a positive or negative voltage is received, but only whether the polarity is the same or different from the previous one; this makes synchronization easier.

This encoding scheme is frequently called Differential Manchester since one way to describe it is applying Manchester encoding to differentiated data. Other names are biphase mark code (BMC) and Conditioned diphase.[1]

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.
  • Detecting transitions is less error-prone than comparing signal levels against a threshold in a noisy environment.
  • 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 (wires swapped). Other line codes with this property include NRZI, bipolar encoding, coded mark inversion, and MLT-3 encoding.
  • 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 reduces the necessary transmitting power, minimizes the amount of electromagnetic noise produced by the transmission line, and eases the use of isolating transformers.
An example of Differential Manchester encoding: Gray vertical lines, full and dotted, represent the two clock tics per bit period. In the shown variant of the encoding, 0 is represented by a transition and 1 is represented by no transition. The two line signals shown differ in their polarity; which one would occur on the line depends on the preceding line state.

These positive features are achieved at the expense of doubling the bandwidth -- there are two clock tics per bit period (marked with full and dotted lines in the figure). Every other clock tic, marked with dotted lines, there is a potential level transition conditional on the data. At the other tics, 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 is specified in the IEEE 802.5 standard for token ring LANs, and is used for many other applications, including magnetic and optical storage. As Biphase Mark Code (BMC), it is used in AES3 and S/PDIF. Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase. That standard is described in ISO/IEC 7811. SMPTE time code also uses BMC.

BMC is also the original "frequency modulation" used on single-density floppy disks, before being replaced by "double-density" modified frequency modulation.

References[edit]

  1. ^ US DoD: Design handbook for fiber optic communications systems, Military handbook. Dept. of Defense, 1985, p. 65.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

See also[edit]