Differential Manchester encoding
Differential Manchester Encoding (DM) is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. In various specific applications, this line code is also called by various other names, including Biphase Mark Code (CC), Frequency Modulation (FM), F2F (frequency/double frequency), Aiken Biphase, and Conditioned diphase. DM 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 represented by the absolute voltage levels but in their changes: in other words it does not matter which of the two voltage levels is received, but only whether it is the same or different from the previous one; this makes synchronization easier.
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, 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.
These positive features are achieved at the expense of doubling the bandwidth—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 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, S/PDIF, SMPTE time code, and USB PD. Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase, according to the ISO/IEC 7811 standard. Differential Manchester is also the original "frequency modulation" (FM) used on "single-density" floppy disks, followed by "double-density" modified frequency modulation (MFM), which gets its name from its relation to FM, or Differential Manchester, encoding.
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