Differential Manchester code

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The differential Manchester code (also Conditioned Diphase ) is a line code for the transmission of bit sequences as a digital signal . As with the Manchester code itself, the signal is DC -free with symmetrical modulation and contains at least one edge in each bit period for the purpose of simple and robust clock recovery . In addition, it is invariant to polarity reversal, a consequence of the difference coding .

The differential Manchester code comes in two forms, biphase mark (BP-M, also called Manchester I ) and biphase space (BP-S), depending on whether the logical 1 is encoded by the presence or absence of a second edge . BP-S was chosen in the token ring standard (IEEE 802.5), BP-M in AES-3 S / PDIF (digital audio transmission) and ISO / IEC 7811 ( magnetic stripe ).

BP-M coding of the bit sequence 10100111001. The unconditional (guaranteed) signal edges are marked with solid gray lines, the coding points (potential edges) are dotted. The two line signals are mirror images of each other. Which occurs with a given bit sequence depends on the history.

The designation differential Manchester code does not mean that the sender first differentiates the bit sequence and then Manchester-encodes it, but that it (for BP-S) composes the signal from the Manchester II symbols 01 and 10 in such a way that the differentiated symbol sequence the bit sequence results in: If two consecutive symbols are the same (both 01 or both 10), a 0 is coded, otherwise a 1. The edges within the symbols are the guaranteed clock edges, while there is only an edge between two symbols if they are the same, i.e. code 0, see adjacent figure. The symbols 01 and 10 used, which each contain both levels, show that the code does not cause a DC voltage component. This enables the coded signal sequence to be transmitted via pulse transformers for the purpose of galvanic isolation .

The main advantage of differential Manchester coding is that the polarity of the coded signal is irrelevant for correct reception and decoding. With normal symmetrical signal transmission via wire pairs, interchanging the wires has no effect, which simplifies installation (you can see which wires belong together as a pair by the twisting ).

A disadvantage of the additional signal edge per bit period is the high bandwidth required , twice as high as with simple binary coding (e.g. Non Return to Zero , NRZ), which, however, does not offer reliable clock recovery without a run length limitation. See line code for various tradeoffs .

Formally, the Manchester coding (differential or not) can be identified as block code 1B2B - a data bit is encoded in two code bits. Only two of the four symbols (00 01 10 11) are used (01 10). The other two symbols (00 11) do not have a flank, where a flank is actually guaranteed in the Manchester coding. They are used in the Token Ring network to mark the beginning and end of a frame and are called J (00) and K (11).

Individual evidence

  1. ↑ Mark Wilson: The ARRL Handbook for the Radio Amateur . The League, 1987, limited preview in Google Book search
  2. ^ Jordan Isailović: Videodisc and Optical Memory Systems . Prentice Hall, 1985, limited preview in Google Book search.
  3. ^ A b Philip Miller, Michael Cummins: LAN Technologies Explained . Digital Press, 2000, ISBN 1-55558-234-6 , limited preview in Google Book Search.
  4. ^ Ulrich Reimers: DVB: The Family of International Standards for Digital Video Broadcasting . Springer, 2005, ISBN 3-540-43545-X , limited preview in the Google book search.
  5. ↑ ePanorama.net (accessed 2016-12-24).
  6. ^ M. Pieuchot: Seismic instrumentation . Geophysical Press, 1984, limited preview in Google Book search.