Differential Manchester encoding

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.

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.

Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase, according to the ISO/IEC 7811 standard.

An example of Differential Manchester encoding: Gray vertical lines, full and dotted, represent the two clock ticks per bit period. In the shown variant of the encoding, 0 is represented by a transition and 1 is represented by no transition. The two line signals shown differ in their polarity; which one would occur on the line depends on the preceding line state. Example given : 1337 10 = 10100111001 2