Differential Manchester encoding

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Differential Manchester encoding (also known as CDP; Conditional DePhase encoding) is a method of encoding data in which data and clock signals are combined to form a single self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. This gives it several advantages over vanilla Manchester encoding:

• Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment.
• Because only the presence of a transition is important, polarity is not. Differential schemes will work exactly the same if the signal is inverted (wires swapped).

A '1' bit is indicated by making the first half of the signal equal to the last half of the previous bit's signal i.e. no transition at the start of the bit-time. A '0' bit is indicated by making the first half of the signal opposite to the last half of the previous bit's signal i.e. a zero bit is indicated by a transition at the beginning of the bit-time. In the middle of the bit-time there is always a transition, whether from high to low, or low to high. A reversed scheme is possible, and no advantage is given by using either scheme.

A related method is Manchester encoding in which the meaningful transitions are the mid-bit ones, and these encode data by their direction (positive-negative is one value, negative-positive is the other).

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.

Note: In differential Manchester encoding, if a "1" is represented by one transition, then a "0" is represented by two transitions and vice versa.

Source: from Federal Standard 1037C