Differential Manchester encoding is a
differential encoding technology, using the presence or absence of transitions to indicate
logical value. An improvement to
Manchester coding which is a special case of
binary phase-shift keying, 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. 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. 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 has the following advantages: • 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 clock frequency of the encoded data stream. 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,
xDSL and
DALI. 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 encoding is also the original modulation method used for single-density
floppy disks, followed by double-density
modified frequency modulation (MFM). == See also ==