Differential Manchester encoding

From Free net encyclopedia

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 just as well.

Data is encoded by the presence or absence of a transition at the beginning of a bit period. Every bit period has at least one transition, in the middle of the period, regardless of the bit being transmitted. Generally a space (logical 0) is represented by the presence of a transition at the beginning of the period, with a mark (logical 1) represented by the lack of a transition. A reversed scheme is possible, and no advantage is given by using either scheme.

Image:Differential manchester encoding.svg

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, a "0" is represented by two transitions, and vice versa.

Source: from Federal Standard 1037Cit:Codifica Manchester differenziale