Differential Manchester encoding

Differential Manchester encoding (DM) is a

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.^[2]

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.
  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

• Manchester code
• McASP
• Run-length limited: FM

References