Differential Manchester encoding

<h2 id="definition">Definition</h2>
<p>Differential Manchester encoding is a <a href="/facts/Differential_encoding/AquYPwpu">differential encoding</a> technology, using the presence or absence of transitions to indicate <a href="/facts/Logical_value/SY4e2w8l">logical value</a>. An improvement to <a href="/facts/Manchester_coding/0nb5SVmP">Manchester coding</a> which is a special case of <a href="/facts/Binary_phase-shift_keying/TFnpYe64">binary phase-shift keying</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.
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<p>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, marked with full lines, the line state changes unconditionally to ease clock recovery.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> 
</p><p>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. 
</p><p>Differential Manchester encoding has the following advantages:
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<ul><li>A transition is guaranteed at least once every bit, for robust <a href="/facts/Clock_recovery/hrTr6rlS">clock recovery</a>.</li>
<li>In a noisy environment, detecting transitions is less error-prone than comparing signal levels against a threshold.</li>
<li>Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. <a href="/facts/Differential_coding/AquYPwpu">Differential coding</a> schemes will work exactly the same if the signal is inverted (e.g. wires swapped). Other line codes with this property include <a href="/facts/NRZI/8f3cfm9r">NRZI</a>, <a href="/facts/Bipolar_encoding/I84wDjmR">bipolar encoding</a>, <a href="/facts/Coded_mark_inversion/YjBd9gY0">coded mark inversion</a>, and <a href="/facts/MLT-3_encoding/7qGmBQBx">MLT-3 encoding</a>.</li>
<li>If the high and low signal levels have the same magnitude with opposite polarity, the average voltage around each unconditional transition is zero. Zero <a href="/facts/DC_bias/crU5EHzl">DC bias</a> reduces the necessary transmitting power, minimizes the amount of electromagnetic noise produced by the transmission line, and eases the use of isolating transformers.</li></ul>
<p>These positive features are achieved at the expense of doubling the clock frequency of the encoded data stream.
</p><p>Differential Manchester encoding is specified in the <a href="/facts/IEEE_802.5/mfQj6GSx">IEEE 802.5</a> 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 <a href="/facts/AES3/Il4EdVYj">AES3</a>, <a href="/facts/S/PDIF/7esUguQA">S/PDIF</a>, <a href="/facts/SMPTE_time_code/atPpXSrI">SMPTE time code</a>, <a href="/facts/USB/XcIKWLyg">USB PD</a>, <a href="/facts/XDSL/3pY5osov">xDSL</a> and <a href="/facts/Digital_Addressable_Lighting_Interface/2wpLWnbT">DALI</a>. Many <a href="/facts/Magnetic_stripe/1mcYEQoT">magnetic stripe</a> cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase, according to the <a href="/facts/ISO/IEC_7811/Fom07R9x">ISO/IEC 7811</a> standard.  Differential Manchester encoding is also the original modulation method used for single-density <a href="/facts/Floppy_disk/rShxfuxa">floppy disks</a>, followed by double-density <a href="/facts/Modified_frequency_modulation/XLIItN1f">modified frequency modulation</a> (MFM).
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<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Manchester_code/0nb5SVmP">Manchester code</a></li>
<li><a href="/facts/McASP/0B9oI2nz">McASP</a></li>
<li><a href="/facts/Run-length_limited/WkGWpAGR">Run-length limited: FM</a></li></ul>

<p> This article incorporates <a href="/facts/Copyright_status_of_works_by_the_federal_government_of_the_United_States/Q1jvr96g">public domain material</a> from <a href="https://web.archive.org/web/20220122224547/https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm"><i>Federal Standard 1037C</i></a>. <a href="/facts/General_Services_Administration/Np3opv9x">General Services Administration</a>. Archived from <a href="https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm">the original</a> on 2022-01-22.
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<h2 id="further-reading">Further reading</h2>
<ul><li>Watkinson, John (1994) <i>The Art of Digital Audio</i>, 2nd edition. Oxford: <a href="/facts/Focal_Press/pQEAhNVv">Focal Press</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-240-51320-7</li>
<li>Savard, John J. G. (2018) [2006]. <a href="http://www.quadibloc.com/comp/tapeint.htm">"Digital Magnetic Tape Recording"</a>. <i>quadibloc</i>. <a href="https://web.archive.org/web/20180702234956/http://www.quadibloc.com/comp/tapeint.htm">Archived</a> from the original on 2018-07-02. Retrieved 2018-07-16.</li>
<li><a href="http://www.hightechaid.com/tech/card/intro_ms.htm">Introduction to magnetic stripe technology</a> <a href="https://web.archive.org/web/20150220232154/http://www.hightechaid.com/tech/card/intro_ms.htm">Archived</a> 2015-02-20 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>US DoD: Design handbook for fiber optic communications systems, Military handbook. Dept. of Defense, 1985, p. 65. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Horowitz, Paul (2015). "14.7.10 Biphase coding". The Art of Electronics (Third ed.). New York, NY, USA. p. 1041. ISBN 978-0-521-80926-9.{{cite book}}:  CS1 maint: location missing publisher (link) <a href="978-0-521-80926-9" target="_blank">978-0-521-80926-9</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
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Differential Manchester encoding open-in-new

Differential Manchester encoding